Sliding in the Farmington Siding Landslide Complex, Davis County, Utah

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Hylland, Lowe LIQUEFACTION - INDUCED LANDSLIDING, FARMINGTON SIDING LANDSLIDE COMPLEX, DAVIS COUNTY, UTAH UGS Special Study 95

CHARACTERISTICS,CHARACTERISTICS, TIMING,TIMING, ANDAND HAZARDHAZARD POTENTIALPOTENTIAL OFOF LIQUEFACTION-INDUCEDLIQUEFACTION-INDUCED LANDSLIDINGLANDSLIDING ININ THETHE FARMINGTONFARMINGTON SIDINGSIDING LANDSLIDELANDSLIDE COMPLEX,COMPLEX, DAVISDAVIS COUNTY,COUNTY, UTAHUTAH

by Michael D. Hylland and Mike Lowe

Aerial view looking northwest along Interstate 15, showing hummocky landslide terrain on northern part of Farmington Siding landslide complex.

Backhoe trench excavated on a hummock sideslope within the Farmington Siding landslide complex, with the Wasatch Range Trench exposure of liquefied sand and in the background. Block of organic soil incorporated into disrupted silt interbed. landslide deposits during landsliding.

SPECIAL STUDY 95 1998 UTAH GEOLOGICAL SURVEY a division of Utah Department of Natural Resources

CHARACTERISTICS, TIMING, AND HAZARD POTENTIAL OF LIQUEFACTION-INDUCED LANDSLIDING IN THE FARMINGTON SIDING LANDSLIDE COMPLEX, DAVIS COUNTY, UTAH

by Michael D. Hylland and Mike Lowe

Research supported by the U.S. Geological Survey (USGS), Department of the Interior, under USGS award number 1434-94-G-2498. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government.

SPECIAL STUDY 95 1998 UTAH GEOLOGICAL SURVEY a division of Utah Department of Natural Resources CONTENTS

Abstract ...... 1 Introduction ...... 2 Previous Work ...... 2 Geology and Geomorphology ...... 3 Trench Descriptions ...... 4 Trench FST1 ...... 5 Trench FST2 ...... 7 Trench FST3 ...... 7 Trench FST4 ...... 7 Trench FST5 ...... 11 Slope-Failure Modes and Extent of Internal Deformation ...... 11 Geotechnical Properties ...... 11 Geomorphology ...... 14 Structure ...... 14 Landslide Timing ...... 16 Geomorphic Expression of Landslide Features ...... 16 Great Salt Lake Shorelines ...... 16 Soil-Profile Development ...... 16 Radiocarbon Ages ...... 17 Episodes of Landsliding ...... 19 Geologic/Hydrologic Conditions During Landsliding ...... 20 Seismic Considerations ...... 20 Earthquake Magnitude-Distance Relations ...... 21 Peak Ground Accelerations and Critical Accelerations ...... 21 Liquefaction Severity Index ...... 23 Estimated Newmark Landslide Displacements ...... 24 Fault-Zone Paleoseismicity ...... 26 Hazard Potential ...... 27 Conclusions and Recommendations ...... 28 Acknowledgments ...... 29 References ...... 30 Appendix A: Descriptions of Trench Units ...... 34 Appendix B: Radiocarbon Analyses and Calibrations ...... 38

ILLUSTRATIONS

Figure 1. Location of the Farmington Siding landslide complex ...... 2 Figure 2. Simplified geologic map of the Farmington Siding landslide complex ...... 3 Figure 3. Pleistocene lacustrine deposits of Lake Bonneville ...... 4 Figure 4. Hummocky landslide terrain on northern part of Farmington Siding landslide complex ...... 4 Figure 5. Locations of trenches excavated on the Farmington Siding landslide complex ...... 5 Figure 6. Trench FST1, excavated on a hummock sideslope ...... 6 Figure 7. Log of trench FST1 ...... 6 Figure 8. Trench FST2, excavated on the landslide scarp ...... 8 Figure 9. Log of trench FST2 ...... 8 Figure 10. Trench FST3, excavated between two high points on a large hummock ...... 9 Figure 11. Log of trench FST3 ...... 9 Figure 12. Liquefied sand injected along fault plane in trench FST3 ...... 9 Figure 13. Trench FST4, excavated on a hummock sideslope ...... 10 Figure 14. Log of trench FST4 ...... 10 Figure 15. Shallow ground water in test pit C of composite trench FST5 ...... 12 Figure 16. Log of trench FST5 ...... 12 Figure 17. Styles of deformation in landslide deposits in the Farmington Siding landslide complex ...... 15 Figure 18. Folded lacustrine strata exposed in a road cut along Main Street ...... 15 Figure 19. High-angle fault involving organic soil unit in trench FST3 ...... 16 Figure 20. Generalized areas of landsliding within the Farmington Siding landslide complex during four events ...... 19 Figure 21. Fault zones in the vicinity of the Farmington Siding landslide complex with evidence for Holocene movement ...... 21 Figure 22. Empirically derived curve of maximum distance from fault-rupture zone to lateral spreads or flows ...... 22 Figure 23. Comparison of the timing of landslide events at the Farmington Siding landslide complex with Wasatch-fault-zone paleoseismicity and Great Salt Lake fluctuations ...... 26

TABLES

Table 1. Subsurface geotechnical properties ...... 13 Table 2. Radiocarbon age estimates ...... 18 Table 3. Opportunity for lateral spread or flow based on empirical earthquake magnitude-distance relations ...... 22 Table 4. Opportunity for liquefaction-induced ground failure based on peak ground acceleration and critical acceleration ...... 23 Table 5. Liquefaction severity index ...... 24 Table 6. Estimated Newmark landslide displacements ...... 25 Table 7. Maximum distance at which earthquake can generate 5 cm of Newmark displacement ...... 25 Table 8. Relationships between ground displacement and damage to structures ...... 27 CHARACTERISTICS, TIMING, AND HAZARD POTENTIAL OF LIQUEFACTION-INDUCED LAND- SLIDING IN THE FARMINGTON SIDING LANDSLIDE COMPLEX, DAVIS COUNTY, UTAH by Michael D. Hylland and Mike Lowe

ABSTRACT and associated high ground-water levels, as well as with the timing of documented large earthquakes on the The Farmington Siding landslide complex covers ap- Wasatch fault zone. Thus, relatively major landsliding is proximately 19.5 square kilometers (7.5 mi2) in Davis likely associated with large earthquakes coincident with County, Utah. The landslide complex consists of liquefac - high lake and ground-water levels. tion-induced landslides that show evidence of recurrent Numerous earthquake source zones have been active movement during latest Pleistocene and Holocene time. in northern Utah during the Holocene. Empirical earth - This study qualitatively assesses the hazard associated quake magnitude-distance relations indicate that liquefac - with future liquefaction-induced landsliding by evaluating tion-induced landsliding at the Farmington Siding land- slope-failure modes and extent of internal deformation slide complex could have been triggered by large earth - within the landslide complex, inferring the geologic and quakes on the East Cache, East Great Salt Lake, West Val - hydrologic conditions under which landsliding occurred, ley, and Oquirrh fault zones, as well as the Brigham City, determining the timing of landsliding, and evaluating the Weber, Salt Lake City, Provo, and possibly Nephi seg - relative likelihood of various earthquake source zones to ments of the Wasatch fault zone. However, comparison of trigger liquefaction-induced landsliding at the Farmington expected peak horizontal ground accelerations with calcu - Siding landslide complex. lated critical accelerations, as well as quantitative esti - The landslide deposits comprise fine-grained, late mates of liquefaction severity index and Newmark land- Pleistocene to Holocene lacustrine sediments of Lake slide displacements, point to large earthquakes on the Bonneville and Great Salt Lake. Geotechnical borehole nearby Weber segment as being the most likely to trigger data confirm the presence of liquefiable sand and silt in significant liquefaction-induced landsliding at the Farm - the shallow subsurface within the landslide complex, and ington Siding landslide complex. geologic evidence for liquefaction includes injected sand, The susceptibility to liquefaction-induced landsliding attenuation and disruption of silt and clay interbeds within in the vicinity of the Farmington Siding landslide complex sand beds, and failure of very gentle slopes not otherwise may presently be less than at other times during the susceptible to landsliding. Both lateral spread and flow Holocene, given the lower average lake and associated have been important slope-failure modes, but flow has had ground-water levels during historical time as compared to a dominant influence on the morphology of the complex. those that characterized much of the Holocene. However, Relative and absolute timing information indicates at a higher potential for liquefaction-induced landsliding least three, and possibly four, landslide events: the first would exist if the area experienced strong ground shaking sometime between 14,500 and 10,900 14 C yr B.P.; the during a time of increased soil pore-water pressures asso - second just prior to 7,310 ± 60 14 C yr B.P. (8,100 [+250, ciated with abnormally high lake and/or ground-water lev - -200] cal yr B.P.); the third(?) sometime prior to 5,280 ± els. Based on geologic conditions and the pattern of 60 14 C yr B.P. (6,000 [+300, -250] cal yr B.P.); and the previous landsliding, the relative hazard associated with fourth between 2,340 ± 60 and 2,440 ± 70 14 C yr B.P. liquefaction-induced landsliding is higher in the northern (2,750 and 2,150 cal yr B.P.). The landslide events gener - part of the landslide complex and in the crown area adja - ally progressed from south to north, and the southern part cent to the north and northeast margins of the complex, of the complex has remained relatively stable during the and is lower in the southern part of the landslide complex late Holocene. The timing of these landslide events corre - and in the flank and crown areas adjacent to the northwest, sponds well with the timing of Great Salt Lake highstands east, and southeast margins of the complex. Given the rel - 2 Utah Geological Survey ative likelihood of a large earthquake in this part of Utah landslide complex is in a largely rural area, but state and in the near future and the possible consequences of large- interstate highways, railroads, petroleum and natural-gas displacement slope failure involving lateral spread or pipelines, and other lifelines cross the complex. Contin - flow, special consideration of the potential for liquefac - ued population growth along the Wasatch Front increases tion-induced landsliding in the northern part of the com - the likelihood of urban development within and adjacent plex and in the crown area north and northeast of the to the landslide complex. Development along the Wa- complex is warranted in land-use planning. satch Front has proceeded with little consideration of hazards associated with liquefaction-induced landslides. Slope-failure mechanisms, extent of internal deformation, INTRODUCTION and timing of landslide events are poorly understood, and these factors must be evaluated to enable local govern - The Farmington Siding landslide complex is in Davis ments to effectively plan for develop ment and implement County, Utah, about 25 kilometers (15 mi) north of Salt hazard-reduction strategies as needed. Lake City (figure 1). The landslide complex covers ap- The purpose of this study is to assess the hazard asso - proximately 19.5 square kilometers (7.5 mi 2) and is one of ciated with future liquefaction-induced landsliding within 13 late Pleistocene/Holocene features along the Wasatch and adjacent to the Farmington Siding landslide complex Front mapped by previous investigators as possible lique - by evaluating slope-failure modes and extent of internal faction-induced lateral spreads. The Farmington Siding deformation within the complex, inferring the geologic and hydrologic conditions under which landsliding occur- Ogden (12 miles) red, determining the timing of landsliding, and evaluating R1W R1E the relative likelihood of various earthquake source zones to trigger liquefaction-induced landsliding. We chose the T4N Farmington Siding landslide complex for this study because of the distinctiveness of geomorphic features on the W northern part of the complex and the presence of landslide

A deposits that are clearly of different ages. Furthermore,

S because much of the area is rural, appropriate land-use A planning measures can still be implemented to protect T future development. C

H This study was sponsored jointly by the Utah Geolog - ical Survey and U.S. Geological Survey, with partial funding provided through the National Earthquake Haz - T3N ards Reduction Program (NEHRP). Results of the study FARMINGTON were originally presented in a final technical report to the SIDING U.S. Geological Survey (Hylland and Lowe, 1995). R ? LANDSLIDE A

COMPLEX N PREVIOUS WORK G

E The Farmington Siding landslide complex was first identified by Van Horn (1975). He recognized two ages of ? landsliding in the complex, based in part on differences in T2N soil development on the landslide deposits. He also noted that the younger (northern) landslide disrupts the Gilbert shoreline of Great Salt Lake, but was unable to determine the relation between the older (southern) landslide and the Gilbert shoreline. Van Horn (1975) assigned an age of 2,000 years or less to the younger landslide and 2,000 to 5,000 years to the older landslide based on soil develop - ment and his assumed age of the Gilbert shoreline. The landslide complex and adjacent areas were later mapped Salt Lake City (10 miles) by Miller (1980), Anderson and others (1982), and Nelson Salt Lake City 01 2 3 4 5 km and Personius (1993). Miller (1980) and Anderson and

0 1 2 3 mi others (1982) mapped two landslides of different ages in the complex after Van Horn (1975). Both of these maps Figure 1 . Location of the Farmington Siding landslide complex. Base from U.S. Geological indicate the younger landslide truncates the Gilbert shore - Survey Ogden and Salt Lake City 30' x 60' quad - line. The maps differ, however, in that Anderson and oth - rangle maps. ers (1982) mapped the Gilbert shoreline across and ad- Farmington Siding landslide complex, Davis County, Utah 3

jacent to the southern margin of the older landslide, 11155' Qlo whereas Miller (1980) did not. Nelson and Personius 1 (1993) mapped the complex as three separate landslides Qlo Qaf and note (p. 13) that the younger, northern landslide N 2 Qaf “...clearly postdates the Gilbert shoreline...,” but they did Qaf Qaf not map the Gilbert shoreline across either of the two 4 3 Qlo older landslides in the southern part of the complex. Qmy 4100' 89 Anderson and others (1982) summarize the results of Qlo 5 subsurface geotechnical investigations for numerous sites 15 G Qlo

within the Farmington Siding landslide complex. Addi - 6 x 14 tionally, Miller and others (1981) drilled two test holes Qsm 13 Qaf within the landslide complex, and Chen and Associates x Qly 15 (1988) investigated a site near the middle of the complex 16 FARMINGTON 7 Qmy G 17 23 for the Davis County Criminal Justice Complex. The toe x Qsm 18 area of the northern part of the landslide complex may Qlo 24 x 19 Qsm Qmo have been encountered during a drilling project in Farm - 8 G ington Bay to test foundation conditions for a proposed 20 Qaf Qmo G 9 21 Qmo water-storage reservoir (Everitt, 1991). Inclined and de- F x Qsm Qaf

A x x

R x x x formed bedding in lacustrine sediments, attributed to land - M x G x 22

I x x N x x sliding, was encountered in drill holes to a maximum of G x x x x T x x

x G O Qaf x x about 21 meters (70 ft) below the bottom of the bay. An N x B Qmo Qlo organic clay layer immediately overlying the landslide A 10 4057'30'' 14 Y deposit yielded a radiocarbon age of 2,930 ± 70 C yr B.P. x 15 Qly 12 (Everitt, 1991). 11 Detailed mapping and limited trenching of the Farm - 0 1 2mi ington Siding landslide complex were completed as part 0 123km of a study of possible liquefaction-induced landslides along the Wasatch Front (Harty and others, 1993; Lowe and others, 1995). Harty and others (1993) concluded that EXPLANATION both lateral spread and flow have occurred within the Map Units landslide complex. They mapped two landslides of differ - ent ages within the complex, and recognized the Gilbert Qmy Younger mass-movement (lateral spread and flow) shoreline as preserved across the southern landslide. Cal - deposits endar-calibrated radiocarbon age estimates for soils ob- Qmo Older mass-movement (lateral spread and flow) tained by Harty and others (1993) from the northern deposits landslide indicate movement sometime after 4,530 ± 300 Qaf Alluvial-fan and debris-flow deposits cal yr B.P. but before 2,730 ± 370 cal yr B.P. Harty and Qsm Marsh deposits others (1993) believe the northern landslide formed closer Qly Younger lacustrine (Great to the younger date, possibly during either the penultimate Salt Lake) deposits Qlo Older lacustrine (Lake or antepenultimate surface-faulting earthquake on the Bonneville) deposits nearby Weber segment of the Wasatch fault zone. Map Symbols GEOLOGY AND GEOMORPHOLOGY Landslide scarp Normal fault; ball The Farmington Siding landslide complex is in a gen - on downthrown side tly sloping area underlain primarily by fine-grained, strati - Lineament fied, late Pleistocene to Holocene lacustrine deposits of G G Lake Bonneville and Great Salt Lake (Van Horn, 1975; Gilbert shoreline

Miller, 1980; Anderson and others, 1982; Harty and oth - x x Other shorelines of ers, 1993; Lowe and Harty, 1993; Nelson and Personius, Great Salt Lake 1993; Lowe and others, 1995) (figures 2 and 3). The 16 Geotechnical borehole crown is underlain primarily by Lake Bonneville sand and site from Anderson and 12 others (1982); arrow silt deposits and is at an elevation of about 1,340 meters indicates site off map (4,400 ft) in the vicinity of the city of Farmington. Holocene marsh deposits are present in topographically Figure 2. Simplified geologic map of the Farmington Siding landslide low areas, especially near Farmington Bay at an elevation complex (modified from Harty and others, 1993), and borehole sites of about 1,281 meters (4,200 ft). Post-Lake Bonneville from which geotechnical data summarized in table 1 were obtained. 4 Utah Geological Survey

ers. Hummocks on the southern part of the complex are morphologically subtle, generally having less than about 2 meters (6 ft) of relief. The hummocks are typically elongate and parallel to the main scarp in the north - western part of the complex, but become more randomly oriented with increasing distance from the head (Van Horn, 1975; Harty and others, 1993). Subtle transverse lineaments are present in the central part of the complex (Harty and others, 1993).

TRENCH DESCRIPTIONS As a follow-up to the mapping and trenching of Harty and others (1993), we excavated five backhoe trenches within the Farmington Sid - Figure 3 . Pleistocene lacustrine deposits of Lake Bonneville consisting of thin-bedded sand and silt, exposed in a road cut along Swinton Lane in the landslide main scarp (see figure 5 for loca - ing landslide complex between Dec- tion). Trowel for scale. ember 1994 and March 1995 to further evaluate slope-failure mode and extent of internal deformation, and to obtain alluvial-fan and debris-flow deposits overlie the lacustrine datable soil samples to refine landslide-timing estimates. deposits in places along the eastern and northern margins Existing development and land use, property ownership, of the landslide complex. Streams flowing westward from and shallow ground water placed limitations on trench the Wasatch Range have locally dissected the landslide locations. Four trenches (FST1, FST2, FST3, and FST4) main scarp, which is clearly visible along the northwest to were within the northern (younger) landslide of Harty and northeast margin of the complex. others (1993), and one (FST5) was across the boundary Ground slopes within the landslide complex range between their younger and older landslides (figure 5). We from about 0.4 to 0.8 percent (0.3-0.5 degree). Unfailed logged geologic and soil units on a planimetric base con - slopes adjacent to the complex range from about 1 to 2 structed on a 1-meter by 1-meter grid using level lines. percent (0.6-1 degree) along the flanks and 6 to 11 percent Refer to appendix A for detailed unit descriptions. Soil- (3-6 degrees) in the crown areas east of the complex. The head region in the northern and eastern parts of the landslide complex generally displays negative topographic relief (zone of depletion), whereas the distal region in the western part of the complex generally displays slight positive topographic relief (zone of accumulation). Landslide geomorphology includes scarps, hummocks, closed depressions, and transverse lineaments. Well-pre -

I served lateral and main scarps in the - 1 northern part of the complex range in 5 height from about 3 to 12 meters (10- 40 ft). Hummocks and closed depres - sions are present over most of the complex, but are more common in the northern part (Harty and others, 1993) (figure 4). Hummocks on the north - ern part are morphologically distinct, having as much as about 6 meters (20 Figure 4 . Aerial view toward the northwest showng hummocky landslide terrain on northern part ft) of relief and lateral dimensions of Farmington Siding landslide complex. Approximate position of landslide main scarp indicated ranging from tens to hundreds of met- by dashed line with hachures. Hummocks appear as irregularly shaped lighter areas. Farmington Siding landslide complex, Davis County, Utah 5 horizon nomenclature follows the U.S. Soil Conservation Trench FST1 Service description system (Soil Survey Staff, 1981; We excavated trench FST1 on the Oakridge Country Guthrie and Witty, 1982), and pedogenic carbonate hori - 1 zons were classified using the system developed by Gile Club golf course in the NE /4 section 14, T. 3 N., R. 1 W., and others (1966) and Machette (1985) and modified by Salt Lake Base Line and Meridian (SLBM) (figure 5), on Birkeland and others (1991). Radiocarbon ages of organic the sideslope of a hummock interpreted to be a landslide soils encountered in the trenches, and implications for block (figure 6). The trench was approximately 15.5 met- landslide-timing estimates, are discussed below under “Land- ers (51 ft) long and averaged 2 meters (7 ft) deep (figure slide Timing.” 7). The trench exposed landslide deposits derived from

R. 1 W. R. 1 E.

Swinton Lane exposure

NFJET Main Street NFJWT exposure

FSLC boundary

FST1 .

FST2 N 3 . T FST3 FST4

younger landslide FST5

FST

older landslide

0 1 mile 0 1 kilometer

Figure 5 . Locations of trenches excavated on the Farmington Siding landslide complex during this study (FST1 through FST5), trenches excavated by Harty and others (1993) (NFJET, NFJWT, and FST), and Swinton Lane and Main Street exposures discussed in text. Boundary of landslide complex (solid line) and margin between younger and older landslides (dashed line) from Harty and others (1993). Note topographic expression of hummocks and depressions on younger landslide. Base from U.S. Geological Survey Farmington and Kaysville 7.5' quadrangle maps.

6 Utah Geological Survey lacustrine sediments (units 1, 2, and 3) consisting of interbedded well-sorted fine sand and clayey silt. The strata are generally inclined and/or gently folded with apparent dips as steep as 34 degrees, presumably due at least in part to backtilting of the block during landsliding. The strata have been disrupted along several discrete low- and high- angle faults. Although we observed no identifiable marker beds, the lack of matching strata across the dominant low-angle fault indicates at least 3 meters (10 ft) of appar -

ent displacement along this structure. The low-angle fault

is offset by a high-angle fault with approximately 30 cen - timeters (1 ft) of apparent dip slip. Conformable contacts between silt and sand beds in unit 2 near this high-angle

fault are also offset along small-displacement (less than 3 centimeters [1 in]) faults. A sand bed in unit 3 near the upslope end of the trench appeared to have a well-devel - Figure 6. Trench FST1, excavated on a hummock sideslope. View is to- oped boudinage structure perhaps associated with lique - ward the northwest. faction, although this could also be a loading-related depositional feature.

Trench FST1 Mapped by M.D. Hylland, C.E. Bishop, and G.E. Christenson December 1994 Planimetric base constructed on a 1 m x 1 m grid Computer drafted by B.H. Mayes

EXPLANATION

1,2 UNIT DESCRIPTIONS SYMBOLS Modern soil: A horizon Geologic contact,

S1(A) A horizon a Unstratifieda sand dashed where indistinct S1(Bk) Bk horizon (stage I+) Laminae in clay and silt Landslide deposits derived from lacustrine sediment: Intra-unit contact 3 Brown to yellowish-brown interbedded clayey silt and sand Soil-unit boundary Fault, dashed where 2 Brown to yellowish-brown interbedded sand and clayey silt indistinct (arrows show 1 Olive-brown to brown interbedded sand and clayey silt relative movement)

0 N29W

) S1(A) a S aa aa a R E a aa a a a a a T 1 a 3 aa a a E aa aa a a a a a a a a a a M S1(Bk) ( a a a a a

a a 3 a a a a a E a aa a a a a a a a C a a N a aa a aaa a a a a a a a 2 aa a a a a a A a a a a T aa aa

S 2 a a a a aaa a a a a a I a a a

3 a a

D 3 1 a a a a

2 a a a a a a a 1 2 2 1

3 0 1 23 4 5 6 DISTANCE (METERS)

Footnotes: 1 See appendix A for detailed descriptions. 2 Units with same number on log are lithologically similar but not necessarily stratigraphically equivalent. Figure 7 . Log of trench FST1. Farmington Siding landslide complex, Davis County, Utah 7

1 A modern soil profile comprising A and Bk horizons SE /4 section 14, T. 3 N., R. 1 W., SLBM (figure 5). The is developed to a maximum depth of 80 centimeters (32 trench extended across a broad topographic depression be- in) in FST1. The Bk horizon displayed stage I+ carbonate tween two high points on a large hummock (figure 10). morphology. The trench was approximately 38 meters (125 ft) long and averaged 2 to 2.5 meters (7-8 ft) deep (figure 11). The trench exposed landslide deposits derived from lacustrine Trench FST2 sediments (units 1, 2, and 3) similar to those exposed in 1 We excavated trench FST2 in a hay field in the NW /4 FST1. High-angle faulting and gentle folding character - section 14, T. 3 N., R. 1 W., SLBM (figure 5), on and ap- ized the majority of deformation, although strongly folded proximately perpendicular to the trend of the landslide beds were present locally. Marker beds indicated 3 to 7 scarp (figure 8). The trench, which extended from the centimeters (1-3 in) of apparent dip slip on individual base of the scarp to approximately three-fourths of the faults within unit 1 at the north end of the trench. Al- way to the top of the scarp, was approximately 35 meters though these faults appear imbricated on the trench log, (115 ft) long and averaged 1.5 to 2 meters (5-7 ft) deep their measured strikes (determined by direct measurement (figure 9). The base of the trench exposed lacustrine of fault surfaces and by correlating faults on opposite sedi ments consisting of unstratified, well-sorted fine sand with trench walls) vary by 76 degrees, indicating the fault thin, disrupted silt interbeds (unit 1), overlain by lami - planes intersect over relatively small lateral distances. nated clayey silt (unit 2) locally cross-cut by sand dikes 1 Units 1 and 2 contain several small sand dikes 1 to 7 cen - to 4 centimeters (0.4-1.6 in) thick. The disruption of the timeters (0.4-2.8 in) thick, some of which were injected silt interbeds and injection of the sand into the overlying along fault planes (figure 12). Unit 3 consists of scattered clayey silt indicate liquefaction of unit 1. A Bk horizon blocks of structureless to brecciated clayey silt that likely consisting of units S1(Bk), S1(Bkb), and S2(Bk) com - represents material from units 1 and 2 that was reworked prises a paleosol on units 1 and 2. This Bk horizon dis - during landsliding. played stage II carbonate morphology. Unit 2 and the Bk Several large, irregular blocks of silty, organic-rich horizon have been offset at four locations by high-angle soil comprising unit S1(A) were present near the middle faults, presumably during landslide-scarp formation or of trench FST3. One of these blocks was truncated by a modification. Offset of geologic and soil-unit boundaries fault bounding a distinctive horst. These blocks are inter - at two of these faults indicated 85 centimeters (34 in) of preted to be fragments of one or more soil A horizons that cumulative vertical displacement. Vertical displacement were incorporated into the landslide deposits during land - could not be determined at the other two faults because of sliding. the lack of matching strata across the faults, but appeared A modern soil profile comprising A, Bk, Bt, and Bw to total at least 60 centimeters (24 in). horizons is developed to an approximate average depth of A wedge-shaped unit of probable colluvial origin (unit 1.3 meters (4 ft) in FST3. A lower Bk horizon consisting 3) overlies unit S1(Bkb). Unit 3 consists of unstratified, of unit S2(Bk2) displayed stage II carbonate morphology organic silty sand with clay and fine gravel, and displayed and grades upward into a laterally variable B horizon con - a pervasive pinhole (vesicular) soil texture. A weakly sisting of units S2(Bt), S2(Bk1), and S2(Bw). Unit S2(Bt) developed pedogenic carbonate morphology (stage I) is is a weakly developed argillic horizon in the middle and developed in this unit. Unit 3 is juxtaposed against units 2 topographically lowest part of the trench. This unit grades and S1(Bk) along a sharp, high-angle contact interpreted laterally into unit S2(Bk1), which displayed stage I car - to be a buried scarp free-face. Unit 3, therefore, appar - bonate morphology. Near the north end of the trench, unit ently represents landslide-scarp-derived colluvium that S2(Bk1) also grades laterally into unit S2(Bw), which is a was deposited on the surface of the downdropped land - weakly developed cambic horizon. A-horizon thickness slide block shortly after scarp formation. No evidence for ranges from about 20 to 60 centimeters (8-24 in). a buried A horizon underlying unit 3 was observed. How - ever, the presence of a paleosol A horizon could be masked Trench FST4 by accumulated secondary calcium carbonate, especially if the A horizon was thin and/or poorly developed. We excavated trench FST4 on the flank of the south - A modern soil profile comprising A and Bk horizons eastern extension of the same large hummock on which is developed to a maximum depth of 1.5 meters (5 ft) in FST3 was excavated (figures 5 and 13). Trench FST4 was FST2. The Bk horizon, consisting of unit S2(Bk), dis - approximately 20 meters (66 ft) long and averaged 1.5 played stage I to stage II carbonate morphology, the latter meters (5 ft) deep (figure 14). The trench exposed land - where the unit overprints unit S1(Bkb) at the east end of slide deposits derived from lacustrine sediments (units 1, the trench. A-horizon thickness ranges from 25 to 100 2, and 3) similar to those exposed in FST1 and FST3. The centimeters (10-40 in). deposits are dominated by strongly folded, laminated sandy silt and clay (unit 1). Inclined strata consisting of Trench FST3 interbedded clay, silt, and sand (unit 2) are juxtaposed against unit 1 along a high-angle fault of uncertain dis - We excavated trench FST3 in a horse pasture in the placement. A fault-bounded blocky deposit (unit 3) con -

8 Utah Geological Survey

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Farmington Siding landslide complex, Davis County, Utah 9

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0 1 0

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r r (METERS) DISTANCE e S u u g g g r i i a F F l

10 Utah Geological Survey d . r s a n w o i o t t

a r s i b

i l 5 w

1 a e c i

V d

n . a e

p s 2 o e l

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d 0

1 5 1 a h x d 3 i C c c + e

t R d l x - 0 i n a 4 e 5 a

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W T a

S 1 o e 1 f d F

n t A

h a 1

h x c i t 2 i n d

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1 ( B a p

e ( 1 . a t t 1 a 0

S S

e o 3 1 e e n h 1 t t e

e o e u S S o

r o 2 1 F s u

g e i h F t 9 2 ) t n e s s w n e i ) 1 r

o S r p e r

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( t o E

f c e M e l ( n

i b p t E

s ) C s i m t r N t d a n l a i A s n e i

T e s

S y n w I m 7 e

d o o r D e d r n b e y r v e r r a S t t h

d u o a c a L a y n b w c r a d

E a l O t a

o l b n d i 5 i s a n e B l c

u 7 e d

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e b s

i n Y a t n

l r l i f t a i e i a i o S 6 t a e n

d o a l r z n

a d i u d

s , n e r r - u i - t n t e l - v w l o a l k l s e i ) l u r m a i l o h t f k r n a o u

a a a h s B n n ( I S L G U A I d c B s F i 1 e r S y g a a m 5

M N

1 3

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1 B d M N ( e

n 1 d v . a S A a a 4 a : n 4 H t s r 4 n L .

n a g T 9 o d

B P e

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e n A F b , a r E d t ( t

l 1 e a i e c

d d l S 1 s s y b l u n

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a a i

e 3 l H r r n n h O

d c t c t I .

i d o s n e e

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D e P c t t e . I l l e T t d i i a l s R d s s . u

a e 4 C

p y y m b b T S e y d r o

m r y S n E e c f b o t i )

a 2 a

I l F r D n

t s

C i c d

e e h T e n I n n v g e c i r w m a N w w n p i t e o o e o U s n r r p r d r (

1 t b a b b

n l a - - s - n f t o h i h h P o n o z s s s M

s z i i i 1 o i i r o g r z w w i o w p o o r o o h o : e l l h l l

o l L l l i

d h e

e k w e o

. Y e Y s Y B B A

4 d i ) n 1 l ) r

s ) k w e e d A B B r d ( ( ( n 0 o u 1 1 1 a g M S 1 2 3 L S S i

3 2 F

1 0 DISTANCE (METERS) DISTANCE Farmington Siding landslide complex, Davis County, Utah 11 sisting of clayey silt with sand lenses and stringers and SLOPE-FAILURE MODES AND EXTENT scattered gravel is enclosed within unit 1. OF INTERNAL DEFORMATION A modern soil profile comprising A, Bw, and Bk hori - zons is developed to depths ranging from 40 to 125 cen - Determining slope-failure modes and the extent of timeters (16-50 in) in FST4. The Bk horizon, consisting internal deformation within the Farmington Siding land - of unit S1(Bk), displayed stage I carbonate morphology slide complex requires evaluating the geotechnical prop - and grades upward into a weakly developed cambic hori - erties of subsurface materials as well as the geomorph- zon consisting of unit S1(Bw). A-horizon thickness ology and structure of the complex and adjacent areas. ranges from about 13 to 60 centimeters (5-24 in). Specific factors that must be addressed include the lique - faction susceptibility of subsurface materials, the vertical Trench FST5 and lateral distribution of materials susceptible to lique - faction, landslide geomorphology, topographic slope, and We excavated trench FST5 in an abandoned field in 1 style(s) of deformation. the NW /4 section 24, T. 3 N., R. 1 W., SLBM (figure 5). FST5 was a composite trench made up of five test pits (A through E) in the area of the boundary between the older Geotechnical Properties and younger landslides identified by Harty and others Because the scope of this project included no subsur - (1993). The water table was at or just below the ground face exploration other than relatively shallow trenching, surface in this area at the time the test pits were excavated we relied heavily on geotechnical data compiled by (figure 15), and water had to be pumped out of each test Anderson and others (1982) for their liquefaction poten - pit prior to logging. We used test pits, rather than a single tial map of Davis County. Anderson and others (1982) continuous trench, to reduce the volume of water that had compiled logs of geotechnical boreholes at sites between to be pumped. Although the surface expression of the the Wasatch Range and Great Salt Lake, 24 of which are landslide boundary has been obscured by agricultural located on or within 1.6 kilometers (1 mi) of the Farming - activities, it is relatively distinct on 1952 1:12,000-scale ton Siding landslide complex (figure 2). We reviewed the aerial photographs. We established a 25-meter- (80-ft-) original logs and observed that the deposits involved in wide zone, within which the landslide boundary should be slope failure within the landslide complex generally con - located, relative to cultural features evident on the aerial sist of laterally discontinuous layers of clay, silt, sand, and photographs. The test pits, which were excavated within gravel. Based on ground-water depth, grain-size distribu - and on either side of this zone, were each 3.5 to 9 meters tion, and standard-penetration-test (SPT) data, Anderson (11-30 ft) long (figure 16). The maximum depth of the and others (1982) identified liquefiable deposits in the test pits averaged about 1 meter (3 ft) and was controlled shallow subsurface consisting of very loose to medium by the depth of caving sands. dense sand, silty sand, and silt (table 1). These deposits Landslide deposits (units 1 and 2) derived from lacus - are generally thin bedded, but liquefiable layers of varying trine sediment were exposed in test pits B, C, D, and E thickness are present throughout the shallow subsurface (figure 16). These deposits consist of interbedded clay, within the landslide complex and in unfailed areas near sand, and silty sand. The exposure in test pit E indicated the margins of the complex. that the strata are inclined at least locally. A paleosol con - Subsurface data indicate that the depth to a basal fail - sisting of A and weakly developed Bt horizons is present ure surface or liquefiable layer may vary considerably. on these landslide deposits. The upper Bt horizon, consist - However, data from the eastern and central parts of the ing of unit S1(Bt1), is slightly gleyed as a result of reduc - complex may help to locally constrain the depth of a pos - ing conditions associated with the shallow ground water. sible failure surface. Anderson and others’ (1982) data in Younger landslide deposits (unit 3) overlap the older land - the eastern part of the complex indicate that relatively slide deposits and paleosol in test pit B. Unit 3 consists of fine-grained deposits consisting of silty sand, silt, and clay silty fine sand that was unstratified in test pit A, but dis - with sand interbeds extend to depths of about 9 to 12 played poorly developed horizontal bedding in test pit B. meters (30-40 ft) below the ground surface, and are under - We interpret this unit to be sand that liquefied and flowed lain by sand and gravel. Likewise, Miller and others over the surface of pre-existing landslide deposits. The (1981) and Chen and Associates (1988) indicate a similar exposure of this unit in test pit B included numerous, vari - sequence in the middle part of the complex, where the ously oriented blocks of dark, organic-rich soil that may upper, fine-grained deposits extend to depths ranging from be either soil A-horizon fragments incorporated into the about 4 to 6 meters (14-20 ft). The data also indicate that landslide deposits or infilled animal burrows. the upper deposits contain organic matter and have rela - A modern A horizon 25 to 30 centimeters (10-12 in) tive densities (based on SPT blow counts) ranging from thick was present in all of the test pits. This horizon very loose to medium dense, whereas the lower sand and appeared to extend slightly deeper in test pit A. However, gravel is medium dense to very dense. A similar sequence the generally uniform depth of the soil and the presence of is also apparent in unfailed strata east of the eastern mar - nearby furrows indicate soil disturbance associated with gin of the complex, where Anderson and others’ (1982) cultivation. data indicate that loose, fine-grained deposits extend as

12 Utah Geological Survey

a : . t s

: s n

y e

n t n l T i

e o m i s i S 2

t t m ) d i t c d

A e a d n n n a

i s r e t

E e a

b d s s s e i

d n n e n l i 1 d

e i r n

a r ) a n n S N t p a e i s i

0 r c s s g N

9 e

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( d c I I o 5 u

e 2

f a w w n - c l 1 T e h

T e l

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s + s M a )

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a a d

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2 0 ) S d p

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n u e e s . e

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b P s p i s k h y . o a t p e a s m r t t a p ) ) a

0 ) o

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p a 5 t t e N n M t

A t 3 o

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9 S n ) . 2 1 F m S S S ) E a 9 S 0

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1 4

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p D c a M 0 d 2 h l ( . t 0 ) C

A u l 5 0 )

) C r h 3 E t 0 1 - a t p A 3 y f

T S C 0 R 5 s t + s A C - a 1 n h t N

( i 2 H i R r 3 n B S c e

A R 2 c 0 p P d - r .

T r o s E 5

S t

0 t

0 F S c

c r a s 0 T T I t t

2 D e 5 , e

e D 5 s . i t t S E e M t

6 T i 6 f e F ( s h , u s M o S M

T a 3 ( o p P c

F ( w b 0 p

n e m E 1 i t c m b v o i e C

r o r n t C N c s d a

) e l T A A e e 0 P e 0 T m h 0 i p ) t

S 0 3 0 0 0

n A I T p DISTANCE (METERS) DISTANCE - B p ( 3 5 1 2 , a l 1 D a u

a a 3 0 2

S P 0

e C M 4 k

R a + a -

)

a ) a a b p

0 m a 1

5 a t 5 A t ( a T B 3 a 2 a ( , a a S h 1 S 2 t a F ( 1

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e 2 0 aa

t n a 2 2 e i f ) 1 - t a , C i a 0 o s C

s 0 s a 0 R s ) a 0 a o R - 2 a 2 - n a 1 a t p b p a a 3 o b

+ 5 B r i a m a

( 5 t 0 e T a a 1 o 0 T a 5 v a S c 5 a S c 9 S a o a 4 F ( f

o F ( a l o a 5

1 g - C 3 n e

i t t 2 i w a t p o

s t h r s s d e

e t e t r w n

g e I e n n v

i i i r o v h

o c t r

i h n e a s t w a

e l

r a y ) t ? p b A 0 s

0 a d d 1 e n -

1 n , a e

C u 0 h 5 o 0 t R r

- T 2 a a g a d a S +

5 F a

0 w T 5 ) h ) o w S l aa a c p 7 A l o F ( ( t n A 3 a 2 2

( e s h 2 S r i t S S a a

A f

T w

. o S

g E

1 V DISTANCE (METERS) DISTANCE o .

0 1

e L d . r

n 5 . u u 6 T g o i 1 S r

F F g e r u g i F Farmington Siding landslide complex, Davis County, Utah 13

Table 1. Subsurface geotechnical properties within and near the Farmington Siding landslide complex.

Liquefiable Deposits

1 Site No. Borehole 2 Ground-Water Depth (m) Depth (m) USCS Nmin N ac (g)

1 10.4 2.3 SP, SM, ML 9 13 0.16 2 NA 1.5 NA NA NA NA 3 38.7 3.9 SP, SM, ML 6 25 0.12 4 9.6 2.4 SP 14 31 0.20 5 14.2 0.8 SM, ML 5 14 0.12 6 12.2 1.0 SP, SM, ML 4 11 0.09 7 12.5 0.6 SP, SM, ML 10 10 0.15 8 20.5 NA SP, SM NA NA NA 9 12.2 2.0 SP, SM, ML NA NA NA 10 18.6 0.3 SP, SM NA NA NA 11 8.8 0.8 SM NA NA NA 12 9.6 1.2 SP, SM, ML 58 0.08 13 22.0 0.0 SM, ML 4 10 0.07 14 30.5 0.0 SM, ML 4 12 0.07 15 26.5 0.5 SP, SM, ML 38 0.06 16 4.6 1.1 SM, ML NA NA NA 17 9.5 0.3 SP, SM, ML 2 18 0.05 18 27.5 2.2 SP, SM 10 30 0.10 19 16.9 2.0 SP, SM, ML 36 0.05 20 30.5 0.0 SP, SM, ML 22 0.05 21 NA 0.2 NA 12 30 0.13 22 30.5 1.2 SP, ML 11 23 0.13 23 23.5 2.0 SP, SM 15 19 0.16 24 7.5 4.0 SP, SM, ML 33 0.10

1Sites are re-numbered from Anderson and others (1982); see figure 2 for locations. 2Depth is a maximum depth if more than one hole was drilled at a site. Abbreviations: USCS = Unified Soil Classification System; N min = minimum SPT blow counts; N= average SPT blow counts; ac = critical acceleration; NA = data not available. 14 Utah Geological Survey deep as 20 meters (65 ft) and are underlain by dense sand umented on slopes as low as 2.3 degrees (Keefer, 1984). and gravel. Borehole logs indicate the top of the medium- Based purely on slope, therefore, flow failure could have dense to very dense sand and gravel ranges in elevation occurred locally, especially near the head of the complex between about 1,268 and 1,289 meters (4,158-4,227 ft). where pre-failure slopes were likely steeper. By excavat - These elevations are below the elevation of the Stansbury ing trenches across hummock flanks and adjacent ground shoreline, which formed during the transgressive phase of in the northern part of the complex, Harty and others Lake Bonneville (Currey and others, 1983; Green and (1993) determined the hummocks are relatively intact Currey, 1988; Oviatt and others, 1990). These deposits, “islands” of lacustrine strata surrounded by liquefied sand, therefore, may represent a transgressive lacustrine se- and concluded that flow failure had occurred in this part quence consisting of relatively dense nearshore sand and of the complex. Also, the nature of deposits exposed in gravel deposited during the early part of the Bonneville trench FST3 provides additional evidence for flow failure. paleolake cycle, or possibly pre-Bonneville alluvium, Unit 1 of trench FST3 consists of cyclically bedded sand overlain by loose/soft, offshore, fine-grained sediments and clayey silt that was very similar in appearance to subsequently deposited in deeper water. The contact be- lacustrine strata exposed in a road cut along Swinton Lane tween these different depositional facies of contrasting in the landslide main scarp approximately 1.8 kilometers density and shear strength may correspond at least locally (1.2 mi) north of the trench (figures 3 and 5). In contrast, to a landslide failure surface. unit 1 did not resemble any of the scarp material exposed in trench FST2, approximately 0.5 kilometer (0.3 mi) Geomorphology northwest of trench FST3. Therefore, the hummock on which trench FST3 was excavated may have been dis - The existence in the northern part of the landslide placed southward a considerable distance, possibly as complex of a main scarp with up to 12 meters (40 feet) of much as a kilometer or more, from its original location. relief indicates flow was a significant slope-failure mode. This likely could only have been achieved by flow trans - Such a scarp would not result from lateral spread alone. port during one or more flow-failure events. Other evidence for flow failure includes: the hummocky topography; the overall negative relief in the head region Structure of the complex, indicating evacuation of a large volume of material; and the overall positive relief in the distal region The trench exposures revealed several contrasting of the complex, indicating an accumulation of landslide styles of deformation: ductile or plastic deformation char - material. acterized by strong folding; brittle deformation character - Harty and others (1993) mapped an area of transverse ized by discrete, nondisrupted, low-angle faulting; brittle lineaments near the middle of the landslide complex that deformation characterized by high-angle faulting; and may represent infilled ground cracks associated with lat - flow characterized by disruption or loss of internal struc - eral spread. Although these lineaments could also repre - ture (figure 17). Some deformation likely predates sub - sent shorelines formed during post-Gilbert regression of aerial landsliding within the complex. This deformation Great Salt Lake (Lowe and others, 1995), three features of includes convolute lamination and recumbent, isoclinal the lineaments support a landslide origin. First, the linea - folds exhibited in trenches FST3 and FST4, and low-angle ments are preserved on the northern (younger) part of the faults in trench FST1. The nature of these structures indi - complex, but not on the southern part. Depending on the cates they were formed under subaqueous conditions, or timing of shoreline development relative to landsliding, possibly during subaerial landsliding but at a depth unaf - shorelines should be preserved either across the entire fected by surficial disturbance and under relatively high complex or only on the southern part. Second, several of confining pressures. In the latter case, extensive erosion the lineaments display an anastomosing pattern uncharac - would have been necessary to result in the present shallow teristic of shorelines. Third, lineaments adjacent to the depth of these deposits. This does not seem likely, given southeast boundary of the northern landslide are consis - the low regional topographic gradient and the relatively tently convex in the downslope direction and terminate fresh geomorphic appearance of the hummocks within along the boundary. This pattern indicates the lineaments which the structures occur. Also, these structures closely likely formed during movement of the northern landslide resemble folds and faults exposed in a road cut along and were drag-folded by differential movement near the Main Street north of downtown Farmington (figure 18), landslide margin. east of the landslide main scarp and therefore not involved Unfailed slopes along the flanks of the landslide com - in landsliding within the complex. These structures likely plex, which approximate the pre-failure slopes within the represent penecontemporaneous deformation associated complex, are within the typical range of 0.5 to 5 percent with slumping beneath Lake Bonneville that predates sub - (0.3-3 degrees) for documented lateral spreads (Youd, aerial landsliding. 1978, 1984; National Research Council, 1985). The com - High-angle faults involving organic soil units were bination of topographic slope and locally conducive encountered in trenches FST2 and FST3 (figure 19) and stratigraphy indicates that lateral spread would be an ex- are clearly associated with subaerial landsliding. The pected failure mode. However, flow failure has been doc - nature of the faults exposed in trench FST2 indicates that Farmington Siding landslide complex, Davis County, Utah 15

A B

Liquefied Displaced sand fragments

Silt C D Figure 17 . Styles of deformation in landslide deposits in the Farmington Siding landslide complex. A) Ductile deformation characterized by strong folding in thin-bedded sand and silt in trench FST3; hammer for scale. B) Brittle deformation characterized by low-angle faulting in trench FST1; hammer for scale. Fault trace (marked by heavy dashed line with arrows showing relative movement) truncates gentle folds (marked by flagging squares); trenching tool for scale. C) Brittle deformation characterized by high-angle faulting in trench FST3. Shadowed area is adjacent to exhumed surface of fault (marked by heavy dashed line with arrows showing relative movement); trowel for scale. D) Flow of liquefied sand in trench FST2. Disruption of silt interbed (left of trowel) is indicated by detached and displaced fragments (above and right of trowel).

Figure 18. Folded lacustrine strata exposed in a road cut along Main Street north of downtown Farmington (see figure 5 for location). Exposure is east of the Farmington Siding landslide complex, and therefore deposits were not involved in landsliding within the complex. Trowel in center of photo for scale. 16 Utah Geological Survey

appreciably older than that on the northern part of the complex based on the abundance and distinctiveness of geomorphic features, especially hummocks, on the north - ? ern part. Variation in the slope of the landslide main scarp also supports this conclusion, as the southern part of the scarp appears to have eroded to a much gentler slope than Paleosol the northern part.

Great Salt Lake Shorelines Van Horn (1975) noted that landsliding in the northern part of the complex truncated the Gilbert shoreline, which formed between 10,900 and 10,300 14 C yr B.P. (Currey, 1990), indicating major post-Gilbert movement. Anderson and others (1982) and Harty and others (1993) mapped the Figure 19. High-angle fault (marked by heavy dashed line with arrows Gilbert shoreline across the southern part of the complex showing relative movement) involving organic soil unit in trench FST3. (although in different places), indicating pre-Gilbert land - Fault juxtaposes paleosol A horizon (dark material) against lacustrine strata (lighter material). sliding in this part of the complex. Aerial-photograph review during this study confirmed the presence of an apparent shoreline feature at an elevation of approxi - rotational or translational block sliding may have played a mately 1,293 meters (4,240 ft) on the southeastern part of significant role in scarp formation. The dominant style of the complex that could represent the Gilbert highstand. deformation in trench FST3 is extensional, as indicated by The northwestern continuation of this feature in the mid - numerous normal faults and horst-and-graben structures. dle part of the complex is evident but difficult to trace, These features appear to be related to internal deformation possibly due to disruption by relatively minor post-Gilbert of the hummock during landsliding. Although displace - landsliding. ments on most of these faults were on the order of a few Harty and others (1993) mapped shorelines on the centimeters, A-horizon soil blocks were downdropped to a southern part of the landslide complex (figure 2) near an depth of about 2 meters (7 ft) near the middle of the trench. elevation of 1,287 meters (4,221 ft), which is the elevation of the Holocene highstand of Great Salt Lake between Trenches FST2, FST3, and FST5 showed evidence for about 2,500 and 1,400 14 C yr B.P. (Currey and others, flow of liquefied sand. In trench FST2, subhorizontal 1988; Murchison, 1989; Currey, 1990). These shorelines fragments of silt within a structureless sand bed indicate at are not evident on the northern part of Harty and others’ least one formerly continuous silt interbed was attenuated (1993) southern landslide or on their northern landslide, and disrupted as the sand liquefied. Lateral extension of indicating possible landsliding since about 1,400 years the sand would likely have been accompanied by subsi - ago of sufficient magnitude to disrupt the shorelines. dence of the overlying strata, which together with vertical Alternatively, the original geomorphic expression of the displacement along scarp-forming slip surfaces produced shorelines may have been subtle relative to pre-existing the overall topographic expression of the scarp. Small landslide geomorphology, making the shorelines difficult sand dikes were present in trenches FST2 and FST3, indi - to recognize, or the shorelines have been obscured by cating minor lateral spread of silt and clay beds over liq - grading or agricultural activities. uefied sand. In trench FST5, surficial liquefied sand Harty and others (1993) mapped shorelines on both flowed over pre-existing landslide deposits. the southern and northern parts of the landslide complex (figure 2) near an elevation of 1,286 meters (4,215 ft), which is the elevation of the late-prehistoric highstand of LANDSLIDE TIMING Great Salt Lake around A.D. 1,600-1,700 (Murchison, 1989; Currey, 1990). The presence of these shorelines Relative and absolute dating techniques used to deter - indicates that major, ground-disrupting landsliding has not mine the timing of landsliding include: geomorphic affected the distal parts of the complex during the past expression of landslide features; cross-cutting relations of approximately 400 years. landslide boundaries and Great Salt Lake shorelines; soil- profile development on landslide deposits; and radiocar - bon dating of organic soils developed on and incorporated Soil-Profile Development into the landslide deposits. Soil-profile development can provide information on Geomorphic Expression of Landslide Features the relative timing of landsliding. In general, a relatively deep, well-developed soil profile on landslide deposits Harty and others (1993) concluded that major move - may indicate a relatively long period of landscape stabil - ment on the southern part of the landslide complex is ity. However, soil survey information (Erickson and Wil - Farmington Siding landslide complex, Davis County, Utah 17 son, 1968) documents wide variability in morphology and deposits in trench FST5, we did not encounter these kinds degree of development of surficial soils on and in the of stratigraphic relations in the trenches. The organic soils vicinity of the Farmington Siding landslide complex. This that were sampled for radiocarbon dating were generally variability, which is typical of late Quaternary surficial modern A-horizon soils and paleosol fragments incorpo - soils along the Wasatch Front, is due more to microcli - rated into the landslide deposits. Thus, we interpret the matic conditions and the physical characteristics of the ages obtained for these soils as representing minimum and parent material than to the age of the parent material (Shro- maximum limiting ages, respectively, of landsliding. ba, 1980). The oldest ages (8,350 ± 80 [Beta-80453] and 7,480 ± Perhaps the most useful applications of soil-horizon 70 [Beta-80454] 14 C yr B.P.) we obtained were from pale - development, in particular clay enrichment and accumula - osol blocks of unit S1(A) in trench FST3. The incorpora - tion of secondary calcium carbonate, to determining land - tion of these paleosol blocks in landslide deposits to slide timing are in differentiating surfaces that likely depths of about 2 meters (7 ft) indicates significant disrup - existed prior to landsliding from surfaces created during tion of the former ground surface. The soil ages represent landsliding, and providing a context within which to inter - a maximum limiting age for a landslide event of about pret radiocarbon ages. For example, profiles with argillic 7,480 14 C yr B.P., assuming the blocks represent parts of Bt horizons may be associated with surfaces at least as old the same soil profile disturbed during a single landslide as early Holocene (Scott and Shroba, 1985), and Bk hori - event; this interpretation is favored based on the physical zons displaying stage II carbonate morphology may be similarities of the paleosol blocks and their relative posi - associated with similarly old surfaces. Bt and stage II Bk tions in the trench wall. The landslide could have oc- horizons were observed only in trenches FST2, FST3, and curred up to several thousand years later than this limiting FST5. In the case of trench FST2, the Bk horizon is age, depending on the age of the soil at the time it was faulted and buried beneath undisrupted colluvial soil with incorporated into the landslide deposits. Given the rela - stage I carbonate morphology. These relations indicate a tively old ages of the soils and the fact that they are over - relatively long period of soil development followed by printed by a relatively well-developed modern soil profile scarp-forming landsliding, in turn followed by landscape (stage II carbonate morphology), landsliding sometime stability. The presence of Bt and stage II Bk horizons in during the early to middle Holocene seems likely. trench FST3 may indicate either early Holocene soil The base of the colluvial wedge (unit 3) in trench development that postdates hummock deformation, or FST2 yielded an age of 7,310 ± 60 14 C yr B.P. (Beta- preservation of a soil profile that predates landsliding. 80450). This age represents a maximum limiting age for Development of Bt horizons on old landslide deposits in the onset of colluvial-wedge deposition. Therefore, an trench FST5 indicates a lack of significant late Holocene episode of landslide scarp formation likely occurred at ground disturbance. In contrast, the maximum stage I car - this location shortly before this time. At least one subse - bonate morphology in the soils in trenches FST1 and quent episode of scarp modification associated with lands - FST4 indicates a relatively shorter period of pedogenesis, liding is indicated by the offset of the buried stage II and therefore a more recent period of significant ground carbonate horizon consisting of unit S1(Bkb). However, disturbance. the age of 3,650 ± 70 14 C yr B.P. (Beta-80451) from the base of the modern A horizon in trench FST2 indicates rel - Radiocarbon Ages ative ground-surface stability at this location on the scarp during the late Holocene. Any landsliding subsequent to Bulk-sediment samples were obtained from the trench- early Holocene scarp formation appears not to have cre - es during this study for radiocarbon age determinations. ated a new scarp in this area. Table 2 summarizes the radiocarbon age data, and appen - Unit S1(Bt2) in trench FST5 yielded an age of 5,280 ± dix B provides information regarding radiocarbon anal- 60 14 C yr B.P. (Beta-81833). Because this unit is at the yses and calendar calibrations. All radiocarbon ages dis- base of the soil profile developed on older landslide cussed in this section are designated as 14 C yr B.P. and are deposits, we interpret the age as indicating relative stabil - listed with a laboratory identification number. Elsewhere ity of this area during the late Holocene. Although the soil in this report, radiocarbon ages are designated as 14 C yr may have developed on an isolated geomorphic feature B.P. and calendar-calibrated ages are designated as cal yr (for example, a hummock) that has remained stable during B.P. late Holocene landsliding relative to the landslide mass as Radiocarbon dating of organic soils developed on and a whole, the locally flat topography makes this unlikely. incorporated into landslide deposits can provide absolute The base of unit S1(A) in trench FST5 yielded ages of information on the timing of landsliding, although the 3,390 ± 50 (Beta-81832) and 2,340 ± 60 (Beta-81831) results may not closely date specific landslide events. To 14 C yr B.P. This unit represents the A horizon of this pale - closely date a flow-failure or lateral-spread event, a sam - osol and is unconformably overlain by younger landslide ple is needed from the upper part of an organic soil hori - deposits (unit 3). Although the cause of the difference in zon that has been buried by deposits produced during ages is uncertain, the ages indicate the soil was buried by landsliding (such as flow or sand-blow deposits). Except the younger landslide deposits sometime after about 2,340 for flow deposits overlying a paleosol on older landslide 14 C yr B.P. or less, depending on the soil age at the time of 18 Utah Geological Survey

Table 2 . Radiocarbon age estimates on bulk sediment from the Farmington Siding landslide complex.

Sample Sample Source 1 Conventional Calendar-Calibrated Age 4, (Lab ID) 14 C Age 2,3 yr B.P. yr B.P., ± 2σ (2 σ intercept ranges)

FST2-RC1 Trench FST2, base of unit 3 (colluvial paleosol) 7,310 ± 60 8,100 + 250, -200 (Beta-80450) (8,350-7,900)

FST2-RC2 Trench FST2, base of unit S2(A) (modern A horizon) 3,650 ± 70 3,950 + 450, -350 (Beta-80451) (4,400-3,600)

FST3-RC1 Trench FST3, base of unit S2(A) (modern A horizon) 1,200 ± 40 1,100 + 200, -150 (Beta-80452) (1,300-950)

FST3-RC2 Trench FST3, middle of unit S1(A) block (paleosol) 8,350 ± 80 9,400 + 200, - 450 (Beta-80453) (9,600-8,950)

FST3-RC3 Trench FST3, middle of unit S1(A) block (paleosol) 7,480 ± 70 8,300 + 200, -300 (Beta-80454) (8,500-8,000)

FST4-RC1 Trench FST4, base of unit S1(A) (modern A horizon) 2,440 ± 70 2,450 + 350, - 300 (Beta-80455) (2,800-2,150)

FST5a-RC1 Trench FST5 (test pit A), base of unit S2(Ap) 840 ± 50 750 + 200, - 100 (Beta-81828) (modern A horizon, disturbed) (950-650)

FST5b-RC1 Trench FST5 (test pit B), base of unit S2(Ap) 370 ± 50 450 + 100, - 200 (Beta-81829) (modern A horizon, disturbed) (550-250)

FST5b-RC2 Trench FST5 (test pit B), middle of thin organic- 1,050 ± 50 (AMS) 5 950 ± 200 (Beta-81830) rich soil within unit 4 (likely infilled animal burrow) (1,150-750)

FST5b-RC3 Trench FST5 (test pit B), base of unit S1(A) 2,340 ± 60 2,350 + 400, - 300 (Beta-81831) (paleosol A horizon) (2,750-2,050)

FST5c-RC1 Trench FST5 (test pit C), base of unit S1(A) 3,390 ± 50 3,650 ± 250 (Beta-81832) (paleosol A horizon) (3,900-3,400)

FST5c-RC2 Trench FST5 (test pit C), base of unit S1(Bt2) 5,280 ± 60 6,000 + 300, - 250 (Beta-81833) (paleosol B horizon) (6,300-5,750)

FST5e-RC1 Trench FST5 (test pit E), base of unit S2(Ap) 540 ± 60 550 + 150, - 200 (Beta-81834) (modern A horizon, disturbed) (700-350)

1Refer to appendix A for detailed descriptions of sample source material. 2All ages δ 13 C corrected. 3All ages determined by standard radiometric analysis unless otherwise noted; see appendix B. 4Calibrated using methods of Stuiver and Reimer (1993); see appendix B. 5Accelerator mass spectrometry. Farmington Siding landslide complex, Davis County, Utah 19 burial. A dark, organic-rich soil fragment within unit 3 was not sufficiently large to completely destroy the geo - yielded an age of 1,050 ± 50 14 C yr B.P. (Beta-81830), and morphic expression of the Gilbert shoreline. is likely an infilled animal burrow as opposed to a frag - A fourth event occurred sometime between 2,750 and ment of the overridden paleosol A horizon. 2,150 cal yr B.P. This interval is based on the maximum Of the remaining ages obtained from the base of the and minimum two-sigma error limits, respectively, of cal - modern A-horizon soils in trenches FST3, FST4, and endar-calibrated ages associated with a pre-existing soil FST5, only the age of 2,440 ± 70 14 C yr B.P. (Beta-80455) dated at 2,340 ± 60 14 C yr B.P. (Beta-81831) at trench from trench FST4 was likely not affected by modern FST5 that was overridden by landslide deposits (maxi - human activity. This age may indicate surface disruption mum limiting age of 2,350 [+400] cal yr B.P.), and post- just prior to about 2,440 yr B.P. and relative stability since landslide soil development dated at 2,440 ± 70 14 C yr B.P. that time. The other ages, which range from 370 ± 50 (Beta-80455) on the hummock slope at trench FST4 (min - (Beta-81829) to 1,200 ± 40 (Beta-80452) 14 C yr B.P. (see imum limiting age of 2,450 [-300] cal yr B.P.). This event table 2), are from relatively thin A horizons in previously involved the northern and western parts of the complex cultivated areas where the bottom of the horizon may have (figure 20). A new main scarp likely formed as the result been contaminated by mechanical mixing with young car - of headward migration to the northeast, whereas the exist - bon and possible soil additives used in agricul - ture.

Episodes of Landsliding In summary, relative and absolute timing information indicates at least three, and possibly four, landslide events in different parts of the Farmington Siding landslide complex (figure 20). The earliest event occurred after deposition of Provo-aged Lake Bonneville sediments but before formation of the Gilbert shoreline (Harty and others, 1993), and therefore between about 14,500 and 10,900 14 C yr B.P. (Currey, 1990). This event involved at least the extreme south - ern part of the complex, but its northern extent is unknown because it has been obscured by sub - sequent landslides. A second event occurred just prior to 8,100 (+250, -200) cal yr B.P., based on the calendar- calibrated maximum limiting age of the onset of colluvial-wedge deposition on the landslide scarp. This event involved at least the north - western part of the complex (figure 20), and probably incorporated A-horizon soils into the landslide deposits encountered in trench FST3. A possible third event may have occurred some - time before 6,000 (+300, -250) cal yr B.P., based on the calendar-calibrated maximum lim - iting age associated with the onset of soil-profile development on landslide deposits near the mid - dle of the complex. Depending on surface and climate conditions, however, the inception of pedogenesis may not provide a close limiting age for this landslide event. The southeastern margin of this landslide, which involved at least the middle part of the complex (figure 20), cor - Figure 20. Generalized areas of landsliding (shaded areas) within the Farmington responds to the boundary between the two land - Siding landslide complex during four events indicated by geomorphic expression of slides mapped by Nelson and Personius (1993) landslide features, cross-cutting relations of lake shorelines, and soil radiocarbon in the southern part of the complex. ages. Solid line indicates relatively well-constrained boundary formed during land - slide event, dashed line indicates present main-scarp position, queried boundary indi - The landsliding in this area could alternatively cates uncertain extent. Arrows show speculated direction of landslide movement. have been distal movement associated with the Landslide-complex boundary and Gilbert shoreline (lineament with “G” designation) pre-8,100 cal yr B.P. event, rather than a sepa - from Harty and others (1993). Present shoreline of Farmington Bay shown for refer - rate event. In either case, movement in this area ence. 20 Utah Geological Survey ing scarp at the location of trench FST2 apparently acted (Murchison, 1989), Great Salt Lake again transgressed to as a lateral scarp during this event. The timing of this an elevation of about 1,284 meters (4,213 ft) between event determined in this study corresponds well with about 7,300 and 7,100 14 C yr B.P. (Murchison, 1989). Harty and others’ (1993) bracketing ages of 4,530 ± 300 This time period coincides with the second Farmington and 2,730 ± 370 cal yr B.P. for movement on their north - Siding landslide event that occurred prior to 7,310 ± 60 ern landslide, and reasonably well with the radiocarbon 14 C yr B.P. (or 8,100 cal yr B.P.). The topographic expres - age of 2,930 ± 70 14 C yr B.P. from organic mud on the toe sion of the preserved Gilbert shoreline northwest of the of the landslide beneath Farmington Bay (Everitt, 1991). landslide complex indicates that the shoreline may have contributed to landsliding by acting as a free face along which a failure surface could have initiated. GEOLOGIC/HYDROLOGIC CONDITIONS Palynologic data indicate a relatively warm and dry climate between about 8,000 and 6,000 14 C yr B.P. (Mad - DURING LANDSLIDING sen and Currey, 1979), and Great Salt Lake regressed to Inferences regarding geologic and hydrologic condi - near desiccation levels during this time (Currey, 1980; Murchison, 1989). The climate then became wetter tions during landsliding at the Farmington Siding land - 14 slide complex can be made by combining topographic, between about 6,000 and 3,500 C yr B.P. (Madsen and Currey, 1979). Murchison (1989) reports a lake-level rise stratigraphic, and landslide-timing data with postulated 14 ground-water conditions associated with paleoclimatic to 1,283 meters (4,211 ft) around 5,900 C yr B.P., which is near the age of the possible third landslide event that changes and lacustral fluctuations of Lake Bonneville and 14 Great Salt Lake. The Bonneville basin was characterized may have occurred prior to about 5,280 ± 60 C yr B.P. (or 6,000 cal yr B.P.). by closed-basin hydrology throughout most of the Quater - 14 nary. Following the Provo stage of Lake Bonneville, dur - After about 3,500 C yr B.P., precipitation declined ing which the high lake level allowed extra-basinal to near the Holocene average and temperatures declined to drainage via the Snake and Columbia Rivers, climatic below the Holocene average (Madsen and Currey, 1979). Great Salt Lake began a relatively major transgression conditions forced a reversion to closed-basin hydrology 14 after about 14,000 14 C yr B.P. (Currey and Oviatt, 1985). around 3,440 C yr B.P. (Murchison, 1989), possibly the Changes in ground-water levels and associated soil pore- result of reduced evapotranspiration. This transgression water pressures, and therefore changes in susceptibility to culminated in the Holocene highstand at an elevation of 1,287 meters (4,221 ft) between about 2,500 and 1,400 landsliding in lake-margin areas, can be deduced from 14 periods of climate-controlled lacustrine contractions and C yr B.P. (Currey and others, 1988; Murchison, 1989). expansions. Therefore, summaries of late Quaternary cli - This lacustral highstand coincides with the fourth Farm - ington Siding landslide event that occurred sometime matic and lacustral conditions (for example, Madsen and 14 Currey, 1979; Currey and James, 1982; Murchison, 1989; around 2,340 ± 60 to 2,440 ± 70 C yr B.P. (or between Rhode and Madsen, 1995) provide a useful context within limiting ages of about 2,750 and 2,150 cal yr B.P.). The which to reconstruct conditions during landslide events. apparent correspondence throughout the late Quaternary Lake Bonneville, of which Great Salt Lake is a rem - between landslide events and lacustral highstands sup - nant, occupied the Bonneville basin between about 30,000 ports the hypothesis that landsliding occurred under con - and 12,000 14 C yr B.P. (Oviatt and others, 1992). The ditions of relatively elevated ground-water levels and soil Bonneville paleolake cycle was the last of several Pleis - pore-water pressures associated with high lake levels. tocene deep-lake cycles in the Bonneville basin (McCoy, 1987; Oviatt and Currey, 1987) and ended when the lake regressed to a lowstand, marked locally by mirabilite and SEISMIC CONSIDERATIONS red beds (Eardley, 1962; Currey, 1990), after about 12,000 14 C yr B.P. (Oviatt and others, 1992; Rhode and Madsen, Obermeier (1987) and Obermeier and others (1990) 1995). During the subsequent Gilbert transgression, Great have developed criteria for establishing an earthquake ori - Salt Lake reached its highest stage between 10,900 and gin for liquefaction-induced features. The criteria include 10,300 14 C yr B.P. (Currey, 1990; Oviatt and others, 1992). various characteristics of sand blows, the presence of lat - The timing of pre-Gilbert landsliding within the Farming - eral spreads, possible mechanisms of sedimentary dike ton Siding landslide complex is poorly constrained, and and sill formation, mechanical properties of near-surface may have occurred as subaqueous slumping beneath Lake sediment, and the potential for strong ground shaking at Bonneville or subaerial landsliding near the end of the the site. Many of the features associated with the Farm - Lake Bonneville regression when the gently sloping, satu - ington Siding landslide complex (for example, evidence rated lacustrine sediments were draining. Alternatively, of lateral spread, sand dikes, deposits susceptible to lique - landsliding could have occurred when lake and ground- faction) indicate landsliding could have been triggered by water levels (and associated pore-water pressures) were strong ground shaking. rising during the Gilbert transgression. Many earthquake source zones exist in northern Utah, After regression from the Gilbert highstand to near the and we conclude that certain of these source zones could historic mean elevation of 1,281 meters (4,200 ft) produce earthquakes that generate ground shaking of suf - Farmington Siding landslide complex, Davis County, Utah 21 ficient strength and duration to trigger large-scale lique - 7.1 for the central segment of the East Cache fault zone faction-induced landsliding at the Farmington Siding land- (McCalpin, 1994), moment magnitude (M w) 7.0 for the slide complex. Our conclusion is based on five indepen - East Great Salt Lake and Oquirrh fault zones (Hecker, dent means of evaluation: (1) empirical earthquake mag - 1993; Olig and others, 1996), M w 6.7 for the West Valley nitude-distance relations, (2) comparison of expected peak fault zone (Keaton and others, 1987), and M w 6.9 - 7.4 for horizontal ground accelerations with calculated critical the six active central segments of the Wasatch fault zone accelerations, (3) liquefaction severity index, (4) esti - (Youngs and others, 1987; Machette and others, 1991; mated Newmark landslide displacements, and (5) com- Black and others, 1995, 1996). For consistency in the fol - parison of landslide timing with fault-zone paleoseismicity. lowing analyses, the maximum earthquake on the East Cache fault zone is taken to be M w 7.1; M w approximately Earthquake Magnitude-Distance Relations equals M s at this magnitude (Kanamori, 1983). Keefer (1984) compiled worldwide data from a vari - Proximity to earthquake source zones and earthquake ety of geologic and seismic settings on the maximum dis - magnitudes associated with those source zones are two tance of lateral spreads and flows from earthquake factors important in evaluating earthquakes as possible epicenters and fault-rupture zones relative to earthquake triggering mechanisms for liquefaction-induced land - magnitude. Figure 22 shows Keefer’s (1984) empirically slides. Figure 21 shows mapped fault zones nearest to the derived curve indicating maximum distance of lateral Farmington Siding landslide complex with evidence for spreads and flows from fault-rupture zones, as well as Holocene activity; these include the East Cache, East possible maximum distances to lateral spreads and flows Great Salt Lake, West Valley, Oquirrh, and Wasatch fault triggered by maximum earthquakes on the East Cache, zones (Hecker, 1993). Maximum earthquake magnitudes, East Great Salt Lake, West Valley, Oquirrh, and Wasatch based on rupture-length, surface-displacement, and slip- fault zones. We used this curve rather than Keefer’s (1984) rate parameters, are about surface-wave magnitude (M s) distance-from-epicenter curve because of the reduced uncer tainty associated with defining the extent of surface 113 112 111 42 rupture as compared to predicting epicenter locations. Assuming that earthquakes occur on the end of the fault

EAST CACHE zone or segment nearest the Farmington Siding land slide Logan FAULT ZONE Central Segment complex, the relations shown in figure 22 and summarized in

Brigham City table 3 indicate that large earthquakes on any of these fault

Brigham CityW Segment W N

A Y U

O zones, except for the Levan and possibly Nephi segments of S T M Great A A H I T N the Wasatch fault zone, could potentially induce lateral C G

Salt H Ogden spread or flow failure at the landslide complex. Lake Weber Segment

EAST GREAT FARMINGTON 41 SALT LAKE SIDING FAULT ZONE LANDSLIDE Peak Ground Accelerations and Critical COMPLEX Accelerations Salt Lake City The ability of an earthquake to trigger liquefaction-

Salt Lake CityF Segment A induced landsliding at a given locality is controlled to a WEST U OQUIRRH VALLEY L T large degree by attenuation of the seismic energy. Al- FAULT FAULT ZONE ZONE though consideration of attenuation is implicit in the Provo Segment Utah empirically derived curve in figure 22, a more rigorous Provo Lake analysis of the opportunity for liquefaction-induced lands liding at the Farmington Siding landslide complex 40 relative to various earthquake source zones can be accom - Nephi Segment plished by determining the peak ground acceleration Z

Nephi O (PGA) using attenuation curves developed for north-cen - N

E tral Utah by Campbell (1987). The PGA can also be determined for earthquakes of magnitude (M) 5, which are at Levan Segment the lower threshold of liquefaction-induced ground failure (Kuribayashi and Tatsuoka, 1975; Youd, 1977; Keefer,

0 50 MILES 1984) and are not constrained in location to mapped faults

0 50 KILOMETERS (Arabasz and others, 1992). The PGAs can then be com -

39 pared to critical accelerations, the lowest values of peak ground acceleration required to induce liquefaction, at the Figure 21 . Fault zones in the vicinity of the Farmington Siding land - Farmington Siding landslide complex. If the PGA assoc- slide complex with evidence for Holocene movement (modified from Arabasz and others, 1992; Machette and others, 1992; Hecker, 1993). iated with a given earthquake is less than the critical ac- Wasatch-fault-zone segments designated with lower-case script; celeration, the earthquake would be unlikely to trigger arrows show segment boundaries. liquefaction-induced landsliding. 22 Utah Geological Survey

Anderson and others (1982) calculated critical 1000 accelerations at borehole sites across Davis County; table 1 summarizes their critical accelerations at sites 500 within 1.6 kilometers (1 mi) of the Farmington Siding landslide complex. Anderson and others (1982) de- 200 termined critical acceleration using the simplified s

w procedure for calculating cyclic stress ratio (Seed and o

l 100 s f r

r e

t Idriss, 1971) in conjunction with an empirical method o

e s m d 50 developed by Seed and others (1977) that relates o a l i e k r standard penetration resistance of the soil with the s n EAST GREAT i l

, a SALT LAKE, r e 20 cyclic stress ratio required to cause liquefaction.

e OQUIRRH n t o a l

z Critical accelerations were calculated for layers at

f e o E r 10 u H

e least one foot thick consisting of sand, silty sand, or t c C p n A u

a sandy silt with less than 15 percent clay and a plastic - r C y t

t - i t s T

l 5 i C Y

S u d o ity index less than 5 (Anderson and others, 1982). e E

A a v k L f E o

m L r , i L u A

m Anderson and others (1982) assigned a representative h

t l V ,

o p r m a i r e T e

f 2 S x b

S N critical acceleration for each site considering soil ,

a e , E n y t a W M i W v type, limitations of the SPT data, and consistencies C e

1 L m

a within and between individual boreholes. As shown h g i r

0.5 B in table 1, critical accelerations range from 0.05 to 0.2 g at sites within and near the Farmington Siding landslide complex. 0.2 Table 4 summarizes PGA determinations for M 5

0.1 and maximum earthquakes on the fault zones and 4.0 5.0 6.0 7.0 8.0 9.0 segments that could possibly trigger liquefaction Magnitude (M) induced landsliding at the Farmington Siding land - slide complex as indicated in table 3. Again, we as- Figure 22. Empirically derived curve of maximum distance from fault-rupture sume that the earthquakes occur on the end of the zone to lateral spreads or flows (from Keefer, 1984), showing distances for maxi - mum earthquakes on the East Cache, East Great Salt Lake, West Valley, and fault zone or segment nearest the landslide complex. Oquirrh fault zones (upper case) and the six central Wasatch-fault-zone seg - As required by Campbell’s (1987) attenuation curves, ments (lower case). Earthquake magnitudes from Keaton and others (1987), the distance parameter used in this analysis is dis - Youngs and others (1987), Machette and others (1991), Hecker (1993), Black and others (1995, 1996), and Olig and others (1996). Curve is based on tance from the center of the landslide complex to the reported surface-wave and moment magnitudes from 20 earthquakes worldwide. nearest part of the seismogenic-rupture zone, as opposed to the surface-rupture zone in our analysis

Table 3. Opportunity for lateral spread or flow at the Farmington Siding landslide complex, based on empirical earthquake magnitude-distance relations.

1 2 FAULT ZONE/ Mw Rmax (km) RSR (km) Earthquake Could Induce Segment Landsliding At FSLC? 3

EAST CACHE (central segment) 7.1 110 75 yes EAST GREAT SALT LAKE 7.0 100 25 yes WEST VALLEY 6.7 70 20 yes OQUIRRH 7.0 100 45 yes WASATCH: Brigham City 7.1 110 40 yes Weber 7.4 130 2 yes Salt Lake City 6.9 90 15 yes Provo 7.4 130 55 yes Nephi 7.1 110 110 maybe Levan 6.9 90 190 no

1Earthquake magnitudes from Keaton and others (1987), Youngs and others (1987), Machette and others (1991), Hecker (1993), McCalpin (1994), Black and others (1995, 1996), and Olig and others (1996). 2See figure 22. 3 Earthquake could potentially trigger liquefaction-induced landsliding at the Farmington Siding landslide complex if R max > RSR . Abbreviations: R max = maximum distance from surface rupture to induced lateral spread or flow; R SR = distance from middle of Farmington Siding landslide complex to nearest surface rupture on fault. Farmington Siding landslide complex, Davis County, Utah 23 using Keefer’s (1984) curve. For the East Cache, West ing, appears restricted to M >5 earthquakes on the Weber Valley, and most of the Wasatch fault zone, where the segment and maximum earthquakes on the Salt Lake City measured distances are approximately along the strike of segment. the fault zones, the difference between distance to seismo - Based on the attenuation curves of Campbell (1987), a genic rupture versus distance to surface rupture is negligi - M 5 earthquake would theoretically need to be within a ble for the distances being considered. This is based on 20-kilometer (12-mi) radius of the Farmington Siding the assumptions that the fault zones are steeply dipping landslide complex to trigger local liquefaction-induced (Cook and Berg, 1961; Smith and Bruhn, 1984; Zoback, ground failure there. Empirical data, however, indicate 1992) and that low-density (non-seismogenic) Cenozoic the actual radius within which significant landslide move - basin-fill sediments are less than 4 kilometers (2.5 mi) ment is triggered by M 5 earthquakes may be much thick (Peterson and Oriel, 1970; Zoback, 1983; Mabey, smaller, on the order of 5 kilometers (3 mi) or less (Kee- 1992). Therefore, the distance values used to determine fer, 1984). Duration of shaking is a primary factor influ - PGA for these fault zones are the same as the distance-to- encing liquefaction severity (Youd and Perkins, 1987), surface-rupture (R SR ) values listed in table 3. For the and lateral spreads and flows are more commonly trig - west-dipping Weber segment of the Wasatch fault zone, gered by the long-duration, low-frequency shaking char - which extends directly beneath the Farmington Siding acteristic of large earthquakes (Keefer, 1984). Given the landslide complex, we use a distance of 4 kilometers to apparent large ground displacements within the Farming - determine PGA. For the west-dipping East Great Salt ton Siding landslide complex, such movement triggered Lake and Oquirrh fault zones (Viveiros, 1986; Olig and by an earthquake as small as M 5 seems unlikely, unless others, 1996), which are southwest of the landslide com - perhaps the earthquake occurred directly beneath the land - plex, the distance values used to determine PGA are about slide complex. 4 kilometers greater than the R SR values listed for these fault zones in table 3, based on an assumed 45-degree dip. Liquefaction Severity Index As summarized in table 4, earthquakes of M >5 (including maximum earthquakes) on the West Valley fault The relative amounts of liquefaction-induced ground zone and Weber and Salt Lake City segments of the displacement triggered by earthquakes on various source Wasatch fault zone, as well as maximum earthquakes on zones can be compared by calculating the liquefaction the East Great Salt Lake and Oquirrh fault zones and severity index (LSI). The LSI was originally developed Brigham City and Provo segments of the Wasatch fault by Youd and Perkins (1987) to represent the maximum zone, could cause ground accelerations at the Farmington horizontal ground-failure displacement for lateral spreads Siding landslide complex sufficient to trigger at least local in gently sloping Holocene fluvial deposits, which are liquefaction-induced ground failure. Widespread liquefac - among the materials that are especially susceptible to tion-induced ground failure, which is possible when PGA earthquake-induced landsliding (Keefer, 1984). The fol - exceeds the highest critical acceleration in the area (0.2 g) lowing LSI equation was subsequently developed (T.L. and would be most likely to result in large-scale landslid - Youd and D.M. Perkins, personal communication, 1988,

Table 4. Opportunity for liquefaction-induced ground failure somewhere within the Farmington Siding landslide complex, based on peak ground acceleration and critical acceleration.

M = 5.0 Maximum Earthquake 1 FAULT ZONE/ Liquefaction Liquefaction Segment PGA 2(g) Opportunity 3 PGA (g) Opportunity

EAST CACHE (central segment) <0.02 none 0.04 none EAST GREAT SALT LAKE 0.03 none 0.12 moderate WEST VALLEY 0.05 sufficient 0.14 moderate OQUIRRH <0.02 none 0.06 sufficient WASATCH: Brigham City 0.02 none 0.09 sufficient Weber 0.23 high 0.54 high Salt Lake City 0.07 sufficient 0.22 high Provo <0.02 none 0.07 sufficient Nephi <0.02 none 0.02 none

1See table 3. 2Approximate peak ground acceleration at the Farmington Siding landslide complex based on attenuation curves of Campbell (1987). 3None (PGA <0.05 g); sufficient (PGA = 0.05 - 0.1 g); moderate (PGA = 0.1 - 0.2 g); high (PGA >0.2 g). 24 Utah Geological Survey in Mabey and Youd [1989]) to account for the regional then a tentative conclusion can be made that flow failures variation of attenuation appropriate for Utah: may have initiated as earthquake-triggered block slides. Thus, this analysis can also be used to compare the rela - log (LSI) = -3.53 - 1.60 log (R) + 0.96 M w (1) tive potential triggering effects of earthquakes from vari - ous source zones. where LSI is the maximum ground displacement (inches), Newmark displacement can be estimated from the fol - R is the distance (kilometers) to the nearest surface fault lowing empirical equation relating displacement, critical rupture, and M w is moment magnitude. LSI values can acceleration, and Arias intensity (Jibson, 1993; Jibson and range from 0 to 100; Youd and Perkins (1987) considered Keefer, 1993): displacements greater than 100 inches (2.5 m) to be so erratic and damaging that the corresponding LSI values log D N = 1.460 log I a - 6.642a c + 1.546 (2) would not be meaningful. Table 5 shows calculated LSIs for earthquakes on nearby source zones that could gener - where D N is Newmark displacement (centimeters), I a is ate PGAs at the Farmington Siding landslide complex suf - Arias intensity (the integral over time of the square of the ficient to trigger liquefaction-induced ground failure (see ground acceleration; Arias, 1970) (meters per second), table 4). The LSI values range from 0.15 for a M 5 earth - and a c is critical acceleration (g). Arias intensity can be quake on the West Valley fault zone to greater than 100 for estimated from the following equation relating Arias a M w 7.4 earthquake on the Weber segment of the Wasatch intensity, earthquake magnitude, and source distance fault zone. These values support the idea that significant (Wilson and Keefer, 1985): lateral displacements at the Farmington Siding landslide complex are most likely associated with large earthquakes log I a = M w - 2 log R - 4.1 (3) on nearby fault segments, primarily the Weber segment of the Wasatch fault zone. where I a is in meters per second, M w is moment magni - tude, and R is earthquake source distance (kilometers). Newmark displacements can therefore be estimated for Table 5. given critical accelerations relative to earthquakes of vari - Liquefaction severity index (LSI) at the Farmington Siding ous size at various distances from the Farmington Siding landslide complex. landslide complex. LSI (in) Table 6 summarizes estimated Newmark displace - ments at sites with critical accelerations of 0.05 and 0.2 g, 1 FAULT ZONE/segment M=5.0 Maximum2 which is the range of critical accelerations in the vicinity Earthquake of the Farmington Siding landslide complex (Anderson EAST GREAT SALT LAKE - 9.0 and others, 1982). Newmark displacements were esti - WEST VALLEY 0.15 6.6 mated for maximum earthquakes on the fault zones and OQUIRRH - 3.5 segments that could possibly trigger liquefaction-induced WASATCH: landsliding at the landslide complex as indicated in table Brigham City - 5.3 3. Values of earthquake source distance are based on the assumptions that the earthquakes occur on the end of the Weber 6.1 >100 fault zone or segment nearest the landslide complex, that Salt Lake City 0.24 16.3 the earthquakes originate at a depth of 10 kilometers (6 Provo - 6.2 mi) (Smith and Bruhn, 1984), and that the faults dip 45 1LSI was not calculated for earthquakes resulting in no liquefaction opportunity as noted degrees. The significance of the estimated Newmark dis - in table 4. 2See table 3. placements can be considered relative to a critical dis - placement that leads to general slope failure. A range of 5 to 10 centimeters (2-4 in) has been identified as represent - Estimated Newmark Landslide Displacements ing critical displacements for various landslides in Cali - fornia and the Mississippi Valley (Wieczorek and others, The presence of scattered, relatively intact blocks of 1985; Keefer and Wilson, 1989; Jibson and Keefer, 1993). lacustrine sediment suggests that at least some flow fail - Comparison of estimated Newmark displacements with ures within the Farmington Siding landslide complex may this range of critical displacement indicates that coherent have initiated as coherent block slides that transformed block slides which could evolve into flow failures could into flows after some threshold amount of coseismic iner - be triggered in areas of low critical acceleration (0.05 g) tial displacement. The extensional style of deformation by maximum earthquakes on the East Great Salt Lake and evident from the structural relations in trench FST2 also West Valley fault zones, as well as the Brigham City, Weber, indicates that block sliding occurred at least locally. Ini - Salt Lake City, and Provo segments of the Wasatch fault tial movement of these landslides can be modeled under zone. However, widespread earthquake-induced block various earthquake scenarios using the sliding-block tech - sliding, which would be characterized by slope failure in nique of Newmark (1965). If the model generates suffi - areas of high (up to 0.2 g) as well as low critical accelera - cient coseismic displacement of the intact landslide mass, tions, would likely only occur during a maximum earth - Farmington Siding landslide complex, Davis County, Utah 25

Table 6. Estimated Newmark landslide displacements at the Farmington Siding landslide complex associated with maximum earthquakes.

1 DN (cm)

FAULT ZONE/ Mw R (km) Ia (m/s) ac = 0.05 g ac = 0.2 g Segment

EAST CACHE (central segment) 7.1 76 0.17 1.23 0.12 EAST GREAT SALT LAKE 7.0 37 0.58 7.39 0.75 WEST VALLEY 6.7 24 0.69 9.52 0.96 OQUIRRH 7.0 56 0.25 2.16 0.22 WASATCH: Brigham City 7.1 42 0.57 7.20 0.73 Weber 7.4 13 11.81 600 60.7 Salt Lake City 6.9 21 1.43 27.6 2.78 Provo 7.4 57 0.61 7.95 0.80 Nephi 7.1 111 0.081 0.42 0.042

1Newmark displacement; displacements are given for the range of critical accelerations (0.05 - 0.2 g) at selected sites in the vicinity of the Farmington Siding landslide complex as calculated by Anderson and others (1982). Abbreviations: M w = moment magnitude; R = distance from earthquake source to center of Farmington Siding landslide complex; Ia = Arias intensity; a c = critical acceleration. quake on the Weber segment. 4 kilometers, hence the earthquake would have to origi - Newmark displacement can also be used to evaluate nate in non-seismogenic material. A M w 6.0 earthquake, the relative effects of smaller earthquakes with regard to which is believed to be about the threshold for surface block sliding. Of interest is the maximum source distance fault rupture in Utah (Arabasz, 1984; Smith and Arabasz, of an earthquake that could result in a critical displace - 1991; Arabasz and others, 1992), would need to be on or ment leading to general slope failure. Rearranging the in the immediate vicinity of the Weber segment of the terms in equation 2 allows calculation of Arias intensity Wasatch fault zone; a shallow earthquake (less than about from Newmark displacement and critical acceleration: 5 kilometers deep) would be necessary to produce 5 cen - log I a = 0.685 (log D N + 6.642 a c - 1.546) (4) timeters of coherent-block displacement in areas of high critical acceleration. A M w 6.5 earthquake would need to Likewise, rearranging the terms in equation 3 allows cal - be on or near the Weber segment to produce 5 centimeters culation of earthquake source distance from Arias inten - of coherent-block displacement in areas of high critical sity and moment magnitude: acceleration, but could be as far away as the near end of

log R = 0.5 (M w - log I a - 4.1) (5) the Salt Lake City segment, and possibly the West Valley fault zone, to produce 5 centimeters of coherent-block dis - To generate 5 centimeters (2 in) of Newmark dis - placement in areas of low critical acceleration. placement, equation 4 indicates that an Arias intensity of 0.44 meters per second would be required in areas with a Table 7. critical acceleration of 0.05 g, and an Arias intensity of Maximum distance at which earthquake can generate 5 cm of 3.03 meters per second would be required in areas with a Newmark displacement. critical acceleration of 0.2 g. Using these values and ac (g) Mw R (km) equation 5, we calculated earthquake source distances for earthquakes of M w 5.0, 6.0, and 6.5. As summarized in 0.05 5.0 4.2 6.0 13.4 table 7, a M w 5.0 earthquake would need to originate 6.5 23.9 directly beneath the Farmington Siding landslide com - plex, at or very near the top of the seismogenic crust, to 0.2 5.0 1.6 6.0 5.1 produce 5 centimeters of coherent-block displacement in 6.5 9.1 areas of low critical acceleration (0.05 g). A M w 5.0 earth - quake could not produce 5 centimeters of coherent-block Abbreviations: ac = critical acceleration; displacement in areas of high critical acceleration (0.2 g) Mw = moment magnitude; because the earthquake source distance would be less than R = distance from earthquake source. 26 Utah Geological Survey

Fault-Zone Paleoseismicity not extend beyond 6,000 years ago, so the possibility exists that unrecognized and/or undated older earthquakes To further evaluate earthquakes as possible triggering may also correspond to the earlier landslide events. Of mechanisms for landsliding at the Farmington Siding particular significance is evidence provided by geometric landslide complex, landslide timing can be compared to reconstructions and slip-rate estimates for two or three the paleoseismic histories of nearby earthquake source early Holocene to latest Pleistocene earthquakes on the zones. Unfortunately, the timing of paleoseismic events Weber segment (McCalpin and others, 1994). Given the on the East Great Salt Lake, Oquirrh, and West Valley high potential for large Weber-segment earthquakes to fault zones remains unclear. The paleoseismic history of trigger liquefaction-induced landsliding at the Farmington the East Great Salt Lake fault zone has not been studied, Siding landslide complex, an association between these limiting radiocarbon ages of the Holocene-aged most earthquakes and landslide events seems likely. Further - recent event on the Oquirrh fault zone poorly constrain the more, the landslide timing therefore appears to provide timing of this event to a period spanning 3,100 years (Olig indirect evidence for the approximate timing of these and others, 1996), and the six events on the West Valley Weber-segment earthquakes that predate the late Holo- fault zone during the past 13,000 years (Keaton and oth - cene events documented in paleoseismic trenching studies ers, 1987) have not been dated. Also, paleoseismic histo - (Hylland, 1996). ries in Utah are based on fault-trenching studies, which Figure 23 shows a likely correspondence between are subject to two inherent limitations. First, surface fault apparent earthquake-triggered landslides and lacustral rupture in the Utah region only occurs in earthquakes with highstands approximately at or above the historical high magnitudes greater than about 6.0-6.5 (Arabasz, 1984; of 1,284 meters (4,212 ft). At the same time, numerous Smith and Arabasz, 1991; Arabasz and others, 1992), large earthquakes have occurred on the central segments whereas liquefaction-induced ground failure can be trig - of the Wasatch fault zone that apparently do not coincide gered by smaller earthquakes. Second, the oldest earth - with Farmington Siding landslide events, including the quake that can be documented in a fault study is de- most recent earthquake on the nearby Weber segment termined by the depth to which a backhoe can dig. Reli - which occurred after the most recent major landslide. able earthquake chronologies on the Wasatch fault zone Either geologic and hydrologic conditions at the time of typically are limited to about the last 6,000 years. How - these earthquakes were such that little or no slope move - ever, despite the lack of a clear record of the timing of some possible earthquakes that AGE (103 cal yr B.P.) might have generated liquefaction-induced 0 1 2345 6 7 8 9 10 11 12 13 14 15 Brigham City ground failure, the timing of the largest segment Weber earthquakes (and therefore the most likely segment WASATCH FAULT ZONE Salt Lake City earthquakes to trigger landsliding) on the segment PALEOSEISMICITY well-studied Wasatch fault zone can still pro - Provo vide insight into its possible role in lands lid - segment ing at the Farmington Siding landslide complex . FARMINGTON SIDING LANDSLIDE EVENTS 4 3 (?) 2 1 Numerous fault studies have been com - 4250 pleted that constrain the timing of past sur - 1295 face-faulting earthquakes on the Brigham City, GREAT SALT LAKE 4240 )

s FLUCTUATION HISTORY

Weber, Salt Lake City, and Provo segments ) r t e e t e e of the Wasatch fault zone (Machette and oth - 1290 f 4230 (

m ( ers, 1987; Schwartz and others, 1988; Perso - N N O I O T nius, 1990; Forman and others, 1991; Lund I A T 4220 V A ? V and others, 1991; McCalpin and Forman, E L E 1285 L E

1994; McCalpin and others, 1994; Black and E others, 1995, 1996). As shown in figure 23, 4210 comparison of the results of these studies Historical Mean ? 4200 with the timing of Farmington Siding land - 1280 slide events indicates a close correspondence ? 4190 between landsliding and certain earth quakes. 01 2 3 456 7 8 9 10 11 12 13 Within uncertainty limits, earthquakes on the AGE (103 14C yr B.P.) Brigham City segment coincide with all four possible landslide events, and earth quakes on Figure 23. Comparison of the timing of landslide events (shaded areas) at the Farming - ton Siding landslide complex with Wasatch-fault-zone paleoseismicity (top) and Great Salt the Weber segment occurred around the times Lake fluctuations (bottom). Earthquake chronologies from McCalpin and Nishenko of the third(?) and fourth (most recent) land - (1996). Reliable pre-6,000 yr B.P. earthquake chronologies for the Weber, Salt Lake City, slide events. Earthquakes also occurred on and Provo segments are not available. Great Salt Lake hydrograph after Murchison (1989). Shaded area for landslide event 1 represents period between the Provo Stage of the Salt Lake City and Provo segments the Bonneville paleolake cycle and the Gilbert highstand of Great Salt Lake. Shaded around the time of the fourth landslide event. areas for landslide events 2 through 4 represent 2 σ limits of radiocarbon ages discussed in The earthquake chronologies generally do text. Dashed lines represent limiting ages. Farmington Siding landslide complex, Davis County, Utah 27 ment occurred, or evidence for landsliding has not yet uefaction-induced ground failure in these areas would be been observed. Some ground disturbance associated with loss of bearing capacity and lateral spread. As indicated minor landsliding seems likely to have occurred during at by the LSI values in table 5, ground displacements associ - least some of these earthquakes, especially those on the ated with future lateral spread at the landslide complex nearby Weber segment. could vary considerably depending on earthquake magni - tude and proximity. The LSI values can be compared to the ranges given in table 8 to determine relative levels of expected damage to structures as summarized by Youd HAZARD POTENTIAL (1980). The results of this study indicate a correspondence between large earthquakes on the Wasatch fault zone, high Table 8 . lacustral and associated ground-water levels, and liquefac - Relationships between ground displacement and damage tion-induced landsliding. Comparison of the average to structures (from Youd, 1980). recurrence intervals of large earthquakes on the Brigham City, Weber, and Salt Lake City fault segments with the Ground Displacement Level Of Expected Damage elapsed time since the most recent events (see figure 23) indicates that the chance of a large earthquake on one of Less than 4 inches Little damage, repairable these segments in the near future cannot be discounted 4 to 12 inches Severe damage, repairable (McCalpin and Nishenko, 1996). Under certain circum - 12 to 24 inches Severe damage, non-repairable stances, large earthquakes on other nearby fault zones as More than 24 inches Collapse, non-repairable well as moderate “random” earthquakes may also have the potential to trigger liquefaction-induced landsliding at the Farmington Siding landslide complex. Anderson and others (1982) mapped the crown area The susceptibility to liquefaction-induced landsliding east and northeast of the Farmington Siding landslide in the vicinity of the Farmington Siding landslide complex complex as a moderate-liquefaction-potential zone, based may presently be less than at other times during the on a 10 to 50 percent probability of critical accelerations Holocene, given the lower average lake and associated being exceeded sometime within a 100-year period, with ground-water levels during historical time as compared to ground water at depths of less than 9 meters (30 ft). those that characterized much of the Holocene. Under Anderson and others (1982) concluded that, based on modern long-term climatic conditions, however, an ground slope, the most likely mode of liquefaction- episode of three to five years of above-average precipita - induced ground failure in this area would be flow failure. tion that could cause an abrupt lake-level rise can be The amount of ground displacement associated with flow expected on average about once every 100-110 years failure is difficult to quantify. However, flow failure is the (Karl and Young, 1986). In historical time, Great Salt most catastrophic mode of liquefaction-induced ground Lake has risen as much as about 3 meters (12 ft) above its failure (Tinsley and others, 1985), and therefore can cause mean level to reach an elevation of 1,284 meters (4,212 the most severe damage to structures. Flow failures com - ft), once in 1873 and again in 1986-1987 (Currey, 1987, monly displace soil masses tens of meters, and under 1990); the lake could rise to an elevation of 1,285 meters favorable circumstances can displace material tens of (4,215 ft) during a more prolonged episode of increased kilometers at velocities of tens of kilometers an hour precipitation (Atwood and Mabey, 1995). Also, the strati - (Tinsley and others, 1985). Rapid flows are among the fied nature of the sediments in the vicinity of the landslide predominant landslide-related threats to life, and along complex can result in shallow, locally “perched” ground with lateral spreads are among the leading causes of prop - water, especially during and following periods of intense erty damage from earthquake-induced landslides (Keefer, or prolonged precipitation or snowmelt. Therefore, a rela - 1984). tively high potential for liquefaction-induced landsliding Harty and others (1993) concluded that the relative would exist if the area experienced strong ground shaking potential for future liquefaction-induced landsliding at the during a time when lake levels were near or above the his - Farmington Siding landslide complex is higher in the torical high, or when local ground-water levels were northern part of the complex than in the southern part. increased as the result of abnormally high seasonal precip - The results of this study support their conclusion, as the itation or snowmelt. sequence of major landslide events appears to have gener - Anderson and others (1982) mapped the Farmington ally progressed from south to north. Various features indi - Siding landslide complex and adjacent flank areas to the cate late Holocene stability of the southern part of the northwest and southeast as a high-liquefaction-potential complex, but not of the northern part. Thus, the relative zone, based on a greater-than-50 percent probability of hazard associated with future liquefaction-induced land- critical accelerations being exceeded sometime within a sliding appears to be higher in the northern part of the 100-year period, with ground water at depths of less than complex than in the southern part. Likewise, the relative 3 meters (10 ft). Anderson and others (1982) concluded hazard associated with future liquefaction-induced land- that, based on ground slope, the most likely modes of liq - sliding in the unfailed crown area adjacent to the north 28 Utah Geological Survey and northeast margins of the complex appears to be higher the Brigham City, Weber, Salt Lake City, Provo, and pos - than in the unfailed flank and crown areas adjacent to the sibly Nephi segments of the Wasatch fault zone. How - northwest, east, and southeast margins of the complex. ever, comparison of expected PGAs with calculated This is because the existing landslide scarp appears to critical accelerations, as well as quantitative estimates of have resulted from headward (northeastward) migration in LSI and Newmark landslide displacements, confirm that the former area, but not in the latter areas, during the most the most likely earthquakes to trigger significant liquefac - recent landslide event. tion-induced landsliding at the Farmington Siding land - slide complex are large earthquakes on the nearby Weber segment. The high liquefaction opportunity and potential CONCLUSIONS AND RECOMMENDATIONS for long-duration strong ground shaking associated with large Weber-segment earthquakes results in a relatively The Farmington Siding landslide complex is a large higher potential for significant liquefaction-induced land- area of liquefaction-induced landslides showing evidence sliding. of recurrent movement during latest Pleistocene and The timing of the three earliest Farmington Siding Holocene time. Based on geotechnical borehole data, liq - landslide events corresponds well with documented earth - uefaction appears to have occurred in loose offshore- quakes on the Brigham City segment of the Wasatch fault lacustrine deposits that overlie relatively dense transgres- zone, and may correspond with earthquakes on other sional-lacustrine or alluvial deposits. Geologic evidence nearby segments for which chronologies do not extend for liquefaction within the complex includes injected back far enough to include those events. These landslide sand, attenuation and disruption of silt and clay interbeds events likely were associated with suspected, but as yet within sand beds, and failure of very gentle slopes not oth - undated, earthquakes on the Weber segment. The fourth erwise susceptible to landsliding. Lateral spread and flow landslide event occurred around the time of large earth - have both been important slope-failure modes, but flow quakes on the Brigham City, Weber, Salt Lake City, and has had a dominant influence on the morphology of the complex. Rotational or translational block sliding may Provo segments. Other large earthquakes on these fault have played a significant role at least locally in scarp for - segments apparently have occurred that did not trigger mation. Hummocks within the complex may have been landsliding at the Farmington Siding landslide complex, at displaced laterally as much as a kilometer or more during least not of sufficient magnitude to cause major ground one or more landsliding events. Landslide deformation disturbance and to have been recognized in this and previ - within the hummocks is characterized by extensional ous studies. In addition to an association with earth - high-angle faults with apparent displacements ranging quakes, landslide timing also corresponds well with Great from a few centimeters to 2 meters (7 ft). Strong folding Salt Lake highstands. Therefore, relatively major land - and low-angle faulting within the hummocks probably sliding appears to be associated with large earthquakes represent penecontemporaneous deformation of sediments coincident with high lake and ground-water levels. beneath Lake Bonneville that predates subaerial landsliding. The susceptibility to liquefaction-induced landsliding Relative and absolute timing information indicates at in the vicinity of the Farmington Siding landslide complex least three, and possibly four, landslide events: the first may presently be less than at other times during the sometime between 14,500 and 10,900 14 C yr B.P.; the Holocene, given the lower average lake and associated second just prior to 7,310 ± 60 14 C yr B.P. (8,100 [+250, ground-water levels during historical time as compared to -200] cal yr B.P.); the third(?) sometime prior to 5,280 ± those that characterized much of the Holocene. However, 60 14 C yr B.P. (6,000 [+300, -250] cal yr B.P.); and the a higher potential for liquefaction-induced landsliding fourth between 2,340 ± 60 and 2,440 ± 70 14 C yr B.P. would exist if the area experienced strong ground shaking (2,750 and 2,150 cal yr B.P.). The landslide events gener - during a time of increased soil pore-water pressures asso - ally progressed from south to north, and the southern part ciated with abnormally high lake and/or ground-water lev - of the complex has remained relatively stable during the els. Based on geologic conditions and the pattern of late Holocene. The present main scarp in the northeastern previous landsliding, the relative hazard associated with part of the complex formed as the result of probable head - liquefaction-induced landsliding is higher in the northern ward migration during the most recent landslide event, part of the landslide complex and in the crown area adja - whereas the pre-existing scarp in the northwestern part of cent to the north and northeast margins of the complex, the complex acted as a lateral scarp during this event and and is lower in the southern part of the complex and in the was only slightly modified. The timing of these landslide flank and crown areas adjacent to the northwest, east, and events corresponds well with the timing of Great Salt southeast margins of the complex. Given the relative like - Lake highstands near or above the historical high, and lihood of a large earthquake in this part of Utah in the near associated high ground-water levels. future and the possible consequences of large-displace - Empirical earthquake magnitude-distance relations in- ment slope failure involving lateral spread or flow, special dicate that liquefaction-induced landsliding at the Farm - consideration of the potential for liquefaction-induced ington Siding landslide complex could have been trig - landsliding in the northern part of the complex and in the gered by large earthquakes on the East Cache, East Great crown area north and northeast of the complex is war - Salt Lake, West Valley, and Oquirrh fault zones, as well as ranted in land-use planning. Farmington Siding landslide complex, Davis County, Utah 29

To ensure safe and responsible development in the be relatively ineffective in reducing the ground-displace - vicinity of the Farmington Siding landslide complex, par - ment hazard if applied only locally within a large area of ticularly in the northern part of the complex and in the potential landsliding. Therefore, the hazard associated crown area north and northeast of the complex, we recom - with the Farmington Siding landslide complex can proba - mend that site-specific geotechnical-engineering and bly best be reduced by addressing the issue on an area- engi neering-geologic studies be completed within a frame- wide scale. The hazard can be reduced administratively work of area-wide land-use planning. Although this study by simply avoiding development in areas with a relatively addresses various geologic aspects of the relative hazard higher potential for liquefaction-induced landsliding. associated with liquefaction-induced landsliding, it is not This option, however, is often impractical in rapidly grow - a substitute for site-specific hazard evaluations. ing areas with limited vacant land, and is not a considera - Lowe (1990, 1993) provides recommendations for tion in previously developed areas. Kockelman (1986) land-use planning in liquefaction-hazard areas and scope lists other administrative alternatives that can be consid - of site-specific liquefaction evaluations in Davis County, ered to reduce the hazards associated with landsliding. and Keaton and Jalbert (1991) present a useful method for The final decisions regarding development on and in the analyzing liquefaction-induced ground-failure hazard. vicinity of the Farmington Siding landslide complex Site-specific studies must go beyond evaluating liquefac - should be based on careful consideration of the nature of tion susceptibility; they need to determine the potential for past landsliding, liquefaction potential, seismic risk, and liquefaction-induced ground failure relative to an appro - sound land-use planning practices involving conscious priate level of earthquake ground acceleration (for exam - decisions to define levels of acceptable risk. ple, a probabilistic acceleration associated with a given exposure time as shown on maps by Youngs and others [1987], Algermissen and others [1990], or Frankel and others [1996]). If the analysis indicates ground failure can ACKNOWLEDGMENTS be expected, then the amount of ground displacement should be estimated. Bartlett and Youd (1992) have This research was sponsored jointly by the Utah Geo - developed a comprehensive empirical model that can be logical Survey and U.S. Geological Survey, National used to determine ground displacement based on site-spe - Earthquake Hazards Reduction Program (award no. 1434- cific factors, including: earthquake magnitude; distance to 94-G-2498). We appreciate the cooperation and assistance seismic source; ground slope or free-face ratio; and thick - of Davis County and the city of Farmington. Frank Ash - ness, fines content, and average grain size of liquefiable land, Charlie Bishop, Bill Black, Dar Day, Janine Jarva, soils. This and other techniques for estimating liquefac - Bea Mayes, Noah Snyder, and Brad and Rob Squires pro - tion-induced ground displacement are summarized in vided assistance in the field. Backhoe services were pro - Glaser (1994). Finally, the site-specific study should eval - vided by Ken Hardy’s Backhoe Work of Farmington, uate the consequences of ground failure and recommend Utah, and the city of Farmington. Les Youd (Brigham appropriate hazard-reduction measures. As stated in Young University) and Jeff Keaton (AGRA Earth & Envi - Keaton and Jalbert (1991), this involves consideration of ronmental, Inc.) visited the trench sites, as did Gary Chris - the site conditions, type of development, probability of tenson (UGS), who also provided insight and guidance earthquake ground accelerations large enough to induce during the course of this project and critically reviewed liquefaction, and likely mode of ground failure. the manuscript. Figures were drafted by Bea Mayes, Jim In general, the liquefaction hazard can be reduced on a Parker, Vicky Clarke, and Sharon Hamre (UGS). We site-specific scale through a variety of engineering tech - appreciate helpful reviews of the manuscript by Kimm niques, such as special foundation design, removal and Harty (UGS) and David Keefer, Randy Jibson, and Steve replacement of material susceptible to liquefaction, soil Obermeier (USGS). Finally, we thank the people of densification, grouting or chemical stabilization, loading Farmington who granted access for trenching on their or buttressing, and dewatering through the use of wells, property, including Rodney Hess, Merlin Morrison, Cand - drains, or other ground-water controls (National Research land Olsen, Aaron Richards, Milton Sessions, and the Council, 1985). Any of these techniques, however, could Oakridge Country Club. 30 Utah Geological Survey

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Miller, R.D., Olsen, H.W., Erickson, G.S., Miller, C.H., and Odum, Scott, W.E., and Shroba, R.R., 1985, Surficial geologic map of an area J.K., 1981, Basic data report of selected samples collected from six along the Wasatch fault zone in the Salt Lake Valley, Utah: U.S. test holes at five sites in the Great Salt Lake and Utah Lake valleys, Geological Survey Open-File Report 85-448, 18 p., scale 1:24,000. Utah: U.S. Geological Survey Open-File Report 81-179, 49 p. Seed, H.B., and Idriss, I.M., 1971, Simplified procedure for evaluating Murchison, S.B., 1989, Fluctuation history of Great Salt Lake, Utah, soil liquefaction potential: Proceedings of the American Society of during the last 13,000 years: Salt Lake City, University of Utah, Civil Engineers, Journal of the Soil Mechanics and Foundations Ph.D. thesis, 137 p. Division, v. 97, no. SM9, p. 1249-1273. National Research Council, 1985, Liquefaction of soils during earth - Seed, H.B., Mari, K., and Chan, C.K., 1977, Influence of seismic his - quakes: Washington, D.C., National Academy Press, 240 p. tory on liquefaction of sands: Proceedings of the American Society Nelson, A.R., and Personius, S.F., 1993, Surficial geologic map of the of Civil Engineers, Journal of the Geotechnical Engineering Divi - Weber segment, Wasatch fault zone, Weber and Davis Counties, sion, v. 103, no. GT4, p. 257-270. Utah: U.S. Geological Survey Miscellaneous Investigations Series Shroba, R.R., 1980, Influence of parent material, climate, and time on Map I-2199, 22 p. pamphlet, scale 1:50,000. soils formed in Bonneville-shoreline and younger deposits near Newmark, N.M., 1965, Effects of earthquakes on dams and embank - Salt Lake City and Ogden, Utah [abs.]: Geological Society of ments: Geotechnique, v. 15, p. 139-160. America Abstracts with Programs, v. 12, no. 6, p. 304. Obermeier, Stephen, 1987, Identification and geologic characteristics Smith, R. B., and Arabasz, W.J., 1991, Seismicity of the Intermountain of earthquake-induced liquefaction features, in Crone, A.J., and seismic belt, in Slemmons, D.B., Engdahl, E.R., Zoback, M.D., and Omdahl, E.M., editors, Directions in paleoseismology: Denver, Blackwell, D.D., editors, Neotectonics of North America: Boulder, Colorado, Proceedings of Conference XXXIX, U.S. Geological Colorado, Geological Society of America, Decade Map Volume 1, Survey Open-File Report 87-673, p. 173-177. p. 185-228. Obermeier, S.F., Jacobson, R.B., Smoot, J.P., Weems, R.E., Gohn, G.S., Smith, R.B., and Bruhn, R.L., 1984, Intraplate extensional tectonics of Monroe, J.E., and Powars, D.S., 1990, Earthquake-induced lique - the eastern Basin-Range -Inferences on structural style from seis - faction features in the coastal setting of South Carolina and in the mic reflection data, regional tectonics, and thermal-mechanical fluvial setting of the New Madrid seismic zone: U.S. Geological models of brittle-ductile deformation: Journal of Geophysical Survey Professional Paper 1504, 44 p. Research, v. 89, no. B7, p. 5733-5762. Olig, S.S., Lund, W.R., Black, B.D., and Mayes, B.H., 1996, Paleoseis - Soil Survey Staff, 1981, Soil survey manual - Examination and descrip - mic investigation of the Oquirrh fault zone, Tooele County, Utah, tion of soils in the field (Chapter 4): U.S. Department of Agricul - in Lund, W.R., editor, Paleoseismology of Utah, Volume 6, The ture, unpublished Soil Conservation Service manuscript, p. 4-1 to Oquirrh fault zone, Tooele County, Utah - Surficial geology and 4-107. paleoseismicity: Utah Geological Survey Special Study 88, p. 22-64. Stuiver, M., and Reimer, P.J., 1993, Extended 14 C database and revised Oviatt, C.G., and Currey, D.R., 1987, Pre-Bonneville Quaternary lakes CALIB radiocarbon calibration program: Radiocarbon, v. 35, p. in the Bonneville basin, in Kopp, R.S., and Cohenour, R.E., editors, 215-230. Cenozoic geology of western Utah - Sites for precious metal and Tinsley, J.C., Youd, T.L., Perkins, D.M., and Chen, A.T.F., 1985, Evalu - Farmington Siding landslide complex, Davis County, Utah 33

ating liquefaction potential, in Ziony, J.I., editor, Evaluating earth - — 1980, Ground failure displacement and earthquake damage to build - quake hazards in the Los Angeles region - An earth-science per - ings: Knoxville, Tennessee, American Society of Civil Engineers, spective: U.S. Geological Survey Professional Paper 1360, p. Proceedings, Conference on Civil Engineering and Nuclear Power, 263-315. v. 2, p. 7-6-1 - 7-6-26. Van Horn, Richard, 1975, Largest known landslide of its type in the — 1984, Geologic effects - Liquefaction and associated ground failure, United States - A failure by lateral spreading in Davis County, in Kitzmiller, Carla, compiler, Proceedings of the geologic and Utah: Utah Geology, v. 2, no. 1, p. 83-87. hydrologic hazards training program: U.S. Geological Survey Open-File Report 84-760, p. 210-232. Viveiros, J.J., 1986, Cenozoic tectonics of the Great Salt Lake from seismic reflection data: Salt Lake City, University of Utah, M.S. Youd, T.L., and Perkins, D.M., 1987, Mapping of liquefaction severity thesis, 81 p. index: Journal of Geotechnical Engineering, v. 113, no. 11, p. 1374-1392. Wieczorek, G.F., Wilson, R.C., and Harp, E.L., 1985, Map showing slope stability during earthquakes in San Mateo County, Califor - Youngs, R.R., Swan, F.H., Power, M.S., Schwartz, D.P., and Green, nia: U.S. Geological Survey Miscellaneous Geologic Investiga - R.K., 1987, Probabilistic analysis of earthquake ground shaking in tions Map I-1257E, scale 1:62,500. hazard along the Wasatch Front, Utah, Gori, P.L., and Hays, W.W., editors, Assessment of regional earthquake hazards and risk Wilson, R.C., and Keefer, D.K., 1985, Predicting areal limits of earth - along the Wasatch Front, Utah, Volume II: U.S. Geological Survey quake-induced landsliding, in Ziony, J.I., editor, Evaluating earth - Open-File Report 87-585, p. M1-M110. quake hazards in the Los Angeles region - An earth- science Zoback, M.L., 1983, Structure and Cenozoic tectonism along the perspective: U.S. Geological Survey Professional Paper 1360, p. Wasatch fault zone, Utah, in Miller, D.M., Todd, V.R., and Howard, 316-345. K.A., editors, Tectonic and stratigraphic studies in the eastern Youd, T.L., 1977, Discussion of “Brief review of liquefaction during Great Basin: Geological Society of America Memoir 157, p. 3-28. earthquakes in Japan,” by E. Kuribayashi and F. Tatsuoka: Soils — 1992, Superimposed late Cenozoic, Mesozoic, and possible Protero - and Foundations, v. 17, no. 1, p. 82-85. zoic deformation along the Wasatch fault zone in central Utah, in — 1978, Major cause of earthquake damage is ground failure: Civil Gori, P.L., and Hays, W.W., editors, Assessment of regional earth - Engineering, American Society of Civil Engineers, v. 48, no. 4, p. quake hazards and risk along the Wasatch Front, Utah: U.S. Geo - 47-51. logical Survey Professional Paper 1500-A-J, p. E1-E20. APPENDIX A DESCRIPTIONS OF TRENCH UNITS

Trench FST1 with unit 2; locally juxtaposed against units 1 and 2 along shear surfaces; grades upward Landslide deposits derived from lacustrine sediment: into unit S1(Bk). Unit 1 Interbedded fine sand, silty fine sand, and Modern soil: clayey silt with very fine sand. Fine sand is olive brown (2.5Y 4/3 1); micaceous; very Unit S1(Bk) Bk horizon developed in units 2 and 3; clay- well sorted; structureless to poorly bedded ey silt with very fine sand; dark yellowish with brown (10YR 5/3) silty fine sand; bed brown (10YR 4/4); moderately plastic; few thickness 2-11 cm. Clayey silt is brown soft CaCO 3 nodules at base of unit, scattered filaments throughout (stage I+ carbonate (10YR 5/3) to olive brown (2.5Y 4/3); mod - 4 erately plastic 2; 5% very fine sand, 95% morphology ); vigorous reaction with HCl; fines 3; bed thickness 0.5-2 cm. Unit has no uncemented; diffuse lower boundary; clear visible CaCO 3; no to slight reaction with upper boundary. HCl; uncemented; gently folded with small Unit S1(A) A horizon; clayey silt with fine sand; dark brittle offsets (microfaults) evident in clay grayish brown (10YR 4/2); granular struc - layers; lower contact not exposed; upper ture; moderately plastic; abundant rootlets; contact conformable/gradational with unit 2; no visible CaCO 3; vigorous reaction with locally juxtaposed against units 2 and 3 along HCl; uncemented; clear lower boundary. shear surface. Unit 2 Interbedded fine sand and clayey silt with very fine sand. Fine sand is yellowish brown Trench FST2 (10YR 5/4); very well sorted; structureless; Disturbed lacustrine sediments: bed thickness 14-18 cm. Clayey silt is brown (10YR 5/3); moderately plastic; laminated; Unit 1 Fine sand with a trace of silt; brown (10YR 1% very fine sand, 99% fines; bed thickness 5/3); well sorted; structureless with thin (1-4 2-3 cm. Unit has no visible CaCO 3; no to cm), poorly laminated, disrupted silt beds; slight reaction with HCl; uncemented; gently 95% sand, 5% fines; scattered Fe-oxide mot - folded with small brittle offsets (microfaults) tling; no visible CaCO 3 or reaction with HCl; evident in clay layers; lower and upper con - uncemented; scattered, minor bioturbation; tacts conformable/gradational with units 1 lower contact not exposed; upper contact and 3; locally juxtaposed against units 1 and conformable/sharp with unit 2; locally over - 3 along shear surfaces; locally grades up ward lain by units S1(Bkb) and S2(Bk). into unit S1(Bk). Unit 2 Clayey silt with fine sand; brown (7.5YR Unit 3 Interbedded clayey silt and fine sand with 5/3); poorly laminated; slightly plastic; 5% silt. Clayey silt is brown (10YR 5/3); moder - sand, 95% fines; no visible CaCO 3; moderate ately plastic; laminated with a distinctive reaction with HCl; uncemented; scattered, green horizon; moderate oxidation along minor bioturbation; locally cross-cut by sand partings; bed thickness 10-24 cm. Fine sand dikes (1-4 cm thick) from unit 1; lower con - is yellowish brown (10YR 6/4); well sorted; tact conformable/sharp with unit 1; grades structureless to thin bedded with green and upward into units S1(Bkb) and S2(Bk). brown silt horizons; 95% sand, 5% fines; bedding thickness 6-10 cm. Unit has no visi - Paleosol: ble CaCO 3; slight reaction with HCl in silt, no reaction in sand; uncemented; gently Unit S1(Bk) Bk horizon developed in unit 2; clayey silt folded; lower contact conformable/gradational with fine sand; yellowish brown (10YR 5/4);

1Munsell colors reported are for moist material unless otherwise noted. 2Plasticity estimated in the field; for coarse-grained units plasticity is reported for the matrix (fine portion) of the deposit. 3Reported percentages of grain-size fractions are field estimates using the American Society for Testing and Materials (ASTM) D 2488-90 (Visual- Manual Procedure) classification system. 4Carbonate morphology from Gile and others (1966), Machette (1985), and Birkeland and others (1991). Farmington Siding landslide complex, Davis County, Utah 35

slightly plastic; few soft and locally abun - plastic; laminated to thin bedded (up to 3 cm dant hard CaCO 3 nodules, pervasive fila - thick). Unit contains thin (1-3 cm), disrupted ments throughout (stage II carbonate lens of coarse sand and gravel at north end of morphology); vigorous reaction with HCl; trench (possible channel deposit); no visible weakly cemented; extensive bioturbation; CaCO 3 at north end of trench, but abundant, abrupt to gradual lower boundary; clear to hard nodules in clay horizons at south end of diffuse upper boundary. trench; slight to moderate reaction with HCl; Unit S1(Bkb) Same as unit S1(Bk), but buried by unit 3. uncemented; deformation characterized by inclined bedding offset by high-angle faults; Colluvial wedge: clay beds locally cross-cut by sand dikes; lower contact not exposed; upper contact Unit 3 Colluvial soil derived from eroded scarp free conformable/gradational with unit 2; locally face; silty fine sand with clay, gravel, and juxtaposed against unit 2 along shear sur - disseminated organic matter; dark grayish faces; locally grades upward into unit brown (10YR 4/2); moderately sorted; struc - S2(Bk2). tureless; pervasive pinhole texture (vesicu - lar); moderately plastic; 5% gravel 5, 60% Unit 2 Clayey silt with minor interbedded fine sand. sand, 35% fines; maximum clast size 2 cm; Clayey silt is yellowish brown (10YR 5/4) to subangular to subround; sand is micaceous; light brownish gray (2.5Y 6/2) with some pervasive CaCO 3 filaments (stage I carbon - olive (5Y 5/6) horizons; moderately plastic; ate morphology); vigorous reaction with laminated to thin bedded (up to 3 cm thick). HCl; uncemented; wedge-shaped unit with Fine sand is yellowish brown (10YR 5/4) to indistinct upper and lower contacts; pinches olive gray (5Y 4/2); very well sorted; struc - out towards the east; western end juxtaposed tureless; bed thickness 0.5-12 cm. Unit has against units 2 and S1(Bk) along sharp, sparse Fe-oxide mottling; no visible CaCO 3; steep, east-dipping contact (buried scarp free slight reaction with HCl; uncemented; gently face). to strongly folded with local convolute lami - nation; clay beds locally cross-cut by sand Modern soil: dikes (1-7 cm thick); lower contact con - Unit S2(Bk) Bk horizon developed in units 2 and 3; formable/gradational with unit 1; locally jux - locally overprints unit S1(Bk); sandy silt taposed against units 1, 3, and S1(A) along with clay and gravel; brown to dark brown shear surfaces and unconformities; locally (10YR 4/3); moderately plastic; 5% gravel, grades upward into unit S2(Bk2). 25% fine sand, 70% fines; maximum clast Unit 3 Clayey silt; pale yellow (2.5Y 7/3) to pale size 2 cm; scattered fine rootlets; scattered to brown (10YR 6/3); slightly plastic; structure - pervasive CaCO 3 filaments (stage I to stage less to brecciated; pervasive pinhole texture II carbonate morphology); moderate to vig - (vesicular); sparse Fe-oxide mottling; sparse orous reaction with HCl; uncemented; CaCO 3 filaments and pore infillings (stage I locally extensive bioturbation; gradual lower carbonate morphology); moderate reaction boundary; clear upper boundary. with HCl; uncemented; scattered distribu - Unit S2(A) A horizon; clayey silt with gravel and fine tion, locally juxtaposed unconformably and sand; very dark grayish brown (10YR 3/2); along shear surfaces against unit 2; contains granular structure; slightly plastic; abundant abundant dark soil blocks (paleosols and/or rootlets; no visible CaCO 3; moderate reac - infilled burrows); locally grades upward into tion with HCl; uncemented; clear lower unit S2(Bk2). boundary. Paleosol:

Unit S1(A) A horizon soil blocks incorporated into land - Trench FST3 slide deposits; silt with clay and fine sand; Landslide deposits derived from lacustrine sediment: color blotchy, but averages very dark grayish brown (10YR 3/2); slightly plastic; structure - Unit 1 Cyclically bedded fine sand and clayey silt. less; pervasive pinhole texture (vesicular); Fine sand is olive brown (2.5Y 4/4) and yel - sand is micaceous; no visible CaCO 3; mod - lowish brown (10YR 5/4); well sorted; strati - erate reaction with HCl; uncemented; sharp fied; bed thickness 1-4 cm. Clayey silt is contacts with units 2 and 3; locally over - yellowish brown (10YR 5/4); moderately printed by units S2(Bk2) and S2(Bk1). 5Clast composition is predominantly metamorphic (gneiss, amphibolite) unless otherwise noted. 36 Utah Geological Survey

Modern soil: isoclinal folds and convolute lamination); lower contact not exposed; locally juxta - Unit S2(Bk2) Bk horizon developed in units 1, 2, 3, and posed against units 2 and 3 along shear sur - S1(A); silt and clayey silt with varying faces; locally grades upward into unit amounts of sand depending on parent mater - S1(Bk). ial; average color very pale brown (10YR 7/3); slightly to moderately plastic; abundant Unit 2 Interbedded clay, silt, and clayey very fine CaCO 3 filaments, soil matrix whitened by sand. Clay is yellowish brown (10YR 5/4); CaCO 3, locally abundant nodules (stage II moderately plastic. Silt is yellowish brown carbonate morphology); vigorous reaction (10YR 5/4); non-plastic; interlaminated with with HCl; weakly cemented; moderate bio - clay. Sand is olive brown (2.5Y 4/3); mica - turbation; gradual lower boundary; clear to ceous; well sorted; moderately plastic. Unit gradual upper boundary. has no visible CaCO 3; no reaction with HCl in sand, slight reaction in clay; uncemented; Unit S2(Bk1) Bk horizon overlying unit S2(Bk2); silt and 30% sand, 70% fines; bed thickness 1-3 cm; clayey silt with sand; brown (10YR 4/3); gently folded and locally faulted; juxtaposed moderately plastic; scattered fine rootlets; against unit 1 along shear surface (fault abundant CaCO 3 filaments (stage I carbonate marked by thin layer of fine to medium sand morphology); vigorous reaction with HCl; and fine gravel); grades upward into unit uncemented; moderate bioturbation; gradual S1(Bk). lower boundary; clear to gradual upper boundary; grades laterally into units S2(Bt) Unit 3 Clayey silt with sand and scattered gravel; and S2(Bw). yellowish brown (10YR 5/4); moderately plastic; blocky structure; laminae locally pre - Unit S2(Bt) Weakly developed Bt horizon; clay; pale served within blocks; scattered lenses and brown (10YR 6/3); weakly developed angu - stringers of olive (5Y 4/4), well-sorted fine lar blocky structure; plastic; abundant root- sand; 10% gravel, 10% sand, 80% fines; lets; no visible CaCO 3; no to slight reaction dominant gravel clast size 1-2 cm, maximum with HCl; uncemented; gradual lower and clast size 6 cm; angular to round; no visible upper boundaries; grades laterally into unit CaCO 3; moderate reaction with HCl; unce - S2(Bk1). mented; unit is enclosed within unit 1 with sharp to obscure, unconformable (fault?) Unit S2(Bw) Weakly developed Bw horizon; clayey silt contact; grades upward into unit S1(Bk). with fine sand; yellowish brown (10YR 5/4) to dark yellowish brown (10YR 4/6); slightly Modern soil: plastic; abundant fine rootlets; scattered CaCO 3 filaments (stage I carbonate mor - Unit S1(Bk) Bk horizon developed in units 1, 2, and 3; phology); vigorous reaction with HCl; unce - clayey silt with scattered sand and gravel; mented; gradual lower boundary; clear upper light yellowish brown (10YR 6/4); plastic; boundary; grades laterally into unit S2(Bk1). 10% gravel, 10% sand, 80% fines; maximum clast size 3 cm; scattered fine rootlets; scat - Unit S2(A) A horizon; clayey silt with fine sand; very tered CaCO 3 filaments and pore fillings dark gray (10YR 3/1); granular structure; (stage I carbonate morphology); moderate to slightly plastic; abundant rootlets; no visible vigorous reaction with HCl; uncemented; CaCO 3; no to slight reaction with HCl; unce - local bioturbation; diffuse lower boundary; mented; clear to gradual lower boundary. gradual upper boundary. Unit S1(Bw) Weakly developed Bw horizon; clayey silt; Trench FST4 yellowish brown (10YR 5/4); homogeneous texture; moderately plastic; sparse fine Landslide deposits derived from lacustrine sediment: rootlets; no visible CaCO 3; vigorous reaction with HCl; uncemented; local bioturbation; Unit 1 Sandy silt and clay with scattered fine gradual lower boundary; clear upper bound - gravel; yellowish brown (10YR 5/6); moder - ary. ately plastic; laminated to bedded with thin (0.2-2 cm) sand horizons; 5% gravel, 40% Unit S1(A) A horizon; clayey silt with gravel; very dark sand, 55% fines; maximum clast size 10 cm; grayish brown (10YR 3/2); granular structure; gravel subangular/tabular to subround; no moderately plastic; abundant rootlets; no visi - visible CaCO 3; slight reaction with HCl; ble CaCO 3; moderate to vigorous reaction with uncemented; strongly folded (recumbent, HCl; uncemented; clear lower boundary. Farmington Siding landslide complex, Davis County, Utah 37

Trench FST5 (Composite) Unit S1(A) A horizon; clay with fine sand; black (10YR 2/1); micaceous; weakly developed granular Older landslide deposits derived from lacustrine sedi - structure; plastic; abundant rootlets; no visi - ble CaCO 3; no reaction with HCl; unce - ment: mented; gradual lower boundary; diffuse upper boundary except where locally over - Unit 1 Silty fine sand with a trace of clay, and fine lain by unit 3 with unconformable/sharp con - sand with silt. Silty sand is gray (10YR 5/1); tact. micaceous; moderately plastic; structureless; 75% sand, 25% fines; sparse Fe-oxide mot - Younger landslide deposit derived from lacustrine sed - tling. Sand with silt is very dark grayish iment: brown (10YR 3/2); micaceous; non-plastic; structureless; 90% sand, 10% fines. Unit has Unit 3 Silty fine sand with scattered pockets and no visible CaCO 3; no reaction with HCl; discontinuous beds of fine to medium sand uncemented; sand with silt occurs as pockets with a trace of silt; grayish brown (10YR within the silty sand; lower contact not 5/2) to dark grayish brown (10YR 4/2); mod - exposed; grades upward into unit S1(Bt2) in erately to very well sorted; micaceous; test pits B, C, and D; upper contact uncon - slightly plastic; unstratified in test pit A, formable/sharp with unit 2 in test pit E. poorly developed horizontal bedding in test pit B; bed thickness 1-4 cm; contains numer - Unit 2 Clay with fine sand and a trace of silt, and ous, variously oriented fragments of dark, silty fine sand. Clay is brown (10YR 5/3) to organic-rich material (possible infilled bur - very dark gray (10YR 3/1); micaceous; plas - rows); 80-98% sand, 2-20% fines; pervasive tic; thin, discontinuous, inclined bedding; Fe-oxide mottling; no visible CaCO 3; no bed thickness 3-6 cm; 5% sand, 95% fines. reaction with HCl; uncemented; wedge- Silty sand is gray (10YR 5/1); micaceous; shaped unit, pinches out towards the east (as non-plastic; structureless; 80% sand, 20% exposed in test pit B); lower contact uncon - fines; pervasive Fe-oxide mottling; forms formable/sharp with units S1(Bt1) and prominent lens. Unit has no visible CaCO 3; S1(A); generally grades upward into unit no reaction with HCl; uncemented; lower S2(A), but locally overlain by unit S2(Ap) contact unconformable/sharp with unit 1; with sharp contact. grades upward into Unit S1(Bt2). Modern soil: Paleosol: Unit S2(A) A horizon; clayey silt with fine sand; very Unit S1(Bt2) Weak Bt horizon developed in units 1 and 2; dark brown (10YR 2/2); weakly developed clay with fine sand and disseminated organic granular structure; moderately plastic; scat - matter; very dark grayish brown (10YR 3/2); tered rootlets; pervasive Fe-oxide staining plastic; scattered fine rootlets; no visible (strong brown; 7.5YR 4/6); no visible CaCO 3; no reaction with HCl; uncemented; CaCO 3; no reaction with HCl; uncemented; gradual lower boundary; clear upper bound - gradual lower boundary; diffuse upper ary. boundary. Unit S1(Bt1) Weakly developed Bt horizon overlying unit Unit S2(Ap) Cultivated A horizon; clayey silt; black S1(Bt2); similar to unit S1(Bt2), but slightly (10YR 2/1); granular structure; slightly plas - gleyed; dark grayish brown (10YR 4/2); tic; abundant rootlets; no visible CaCO 3; no gradual upper boundary except where locally reaction with HCl; uncemented; diffuse to overlain by unit 3 with sharp contact. clear lower boundary. APPENDIX B

RADIOCARBON ANALYSES AND CALIBRATIONS

Radiocarbon dating of bulk-sediment samples from plex by Harty and others (1993). All of the samples were the trenches provided limiting estimates of landslide tim - analyzed by conventional radiometric techniques except ing. We obtained low-carbon-content samples ranging in FST5b-RC2 (Beta-81830), which was analyzed by accel - size from approximately 1 to 2.5 kilograms (2-5 lb) from erator mass spectrometry (AMS). Sample pretreatment the base of soil A horizons and middle of paleosol blocks consisted of acid (HCl) washes for conventional analysis, incorporated into the landslide deposits. We did not apply and acid and alkali (NaOH) washes for AMS analysis. All carbon mean-residence-time (MRT) corrections because of the radiocarbon ages were δ13 C corrected. the ages of samples obtained from the base of A horizons We converted radiocarbon ages to calendric ages (0 yr are believed to approximate the time when the soil began BP = AD 1950) using the methods of Stuiver and Reimer forming, and because of uncertainty in applying an appro - (1993). We used a bidecadal calibration data set, assigned priate correction to ages of samples obtained from the a laboratory error multiplier of 2 to each age estimate, and paleosol blocks. smoothed the calibration curve using a 100-year moving The samples were analyzed by Beta Analytic, Inc. of average. The calibrated ages were rounded to the nearest Miami, Florida, the same laboratory that analyzed sam - 50 years and are reported with 2 σ intercepts. ples obtained from the Farmington Siding landslide com -