(I file UNITED STATES DEPARTMENT OF THE INTERIOR 125-3 GEOLOGICAL SURVEY

PROCEEDINGS OF

WORKSHOP XXVIII

On the Borah Peak, Idaho, Earthquake

Volume A

Convened under Auspices of

NATIONAL EARTHQUAKE PREDICTION AND HAZARDS PROGRAMS

3 - 6 October 1984

Editors and Convenors

Ross S. Stein ------Robert C. Bucknam U.S. Geological Survey U.S. Geological Survey Menlo Park, California 94025 Denver, Colorado 80225

Organizing Committee

Roy M. Breckenridge Idaho Geological Survey Anthony J. Crone U.S. Geological Survey Robert B. Smith University of Utah Spencer H. Wood Boise State University

Administrators

Wanda H. Seiders and Kay E. Johnson U.S. Geological ,Survey

OPEN-FILE REPORT 85-290

Compiled by Muriel Jacobson

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards and stratigraphic nomenclature. Any use of trade names is for descriptive purposes only and does not imply endorsement by the USGS.

MENLO PARK, CALIFORNIA 1985 QUATERNARY TECTONIC SETTING OF-THE 1983 BORAH PEAK EARTHQUAKE, CENTRAL IDAHO

William E. Scott, Kenneth L. Pierce, and M. H. Halt, Jr. U.S. Geological Survey .Denver Federal Center, Denver, CO 80225

Abstract

The 1983 Borah Peak earthquake was accompanied by extensive surface faulting along a part of the Lost River fault that has abundant evidence of latest Quaternary (last 15,000 yr) offset. This fault and two similar range- front normal faults along the Lemhi Range and Beaverhead Mountains lie in an area of basin-and-range structure in central Idaho that is part of a semicircular belt of latest Quaternary surface faulting that extends from the Wasatch fault, through the Yellowstone area, to the Lost River fault. The central segments of the Lost River, Lemhi, and Beaverhead faults have been active more recently, and,probably more active throughout Quaternary time, than the southern and northern segments. The 1983 surface faulting occurred in an area of high structural relief along a part of the Lost River fault that has ruptured in latest Quaternary time, which suggests that comparable areas along other range fronts in the area should be regarded as likely sites of future surface faulting. Other perspectives of fault behavior suggest additional possible sites, and all segments of the range-front faults are regarded as capable of surface faulting.

Introduction

The Borah Peak earthquake (Ms-7.3) of October 28, 1983, occurred in an area of basin-and-range structure (Reynolds, 1979) in central Idaho that contains widespread evidence of latest Quaternary (last 15,000 yr) faulting but has had little historic seismicity (Figs. 1, 2, and 3; Smith and Sbar, 1974). A 34-km-long zone of surface faulting along the north-central part of the western front of the Lost River Range accompanied the earthquake; both field evidence and focal mechanism indicate that the fault movement was normal-sinistral on a southwest-dipping plane (Crone and Machette, 1984). This report briefly describes the neotectonic setting of the Borah Peak area, summarizes our knowledge of the distribution and ages of Quaternary faulting there, and discusses possible sites of future surface faulting.

Regional Neotectonic Setting

Figure 1 shows the location of the surface faulting associated with the 1983 earthquake in relation co other areas of historic surface faulting and to major late Cenozoic normal faults in the northeastern part or the Basin and Range province. This part of the Basin and Range can be subdivided into domains based on (1) amount of latest Quaternary surface faulting, (2) high rates of Quaternary faulting as shown by geologic studies or inferred by high structural relief along imposing range fronts, and (3) structural setting

3 (Fig. 2). Domains 2-5 represent several structural settings and contain few between 10 and 5 my ago contrast with low rates during the last 5 my and widely have moved distributed faults that in latest Quaternary time. In (Covington, 1983, Fig. 5; H. R. Covington, written commun., 1984) and low contrast, domain 1 contains many faults that have moved in latest Quaternary rates of Quaternary deformation (Williams and others, 1982; K. L. Pierce, time and most of these have evidence of a high rate of Quaternary faulting. unpublished data, 1984). Further northeast along the south margin of the In addition, two of the three historic surface-faulting events in the eastern Snake River Plain, the Blackfoot Mountains were uplifted at high rates northeastern Basin and Range occurred in domain 1. (0.8 m/1000 yr) between 5.9 and 4.7 my ago to attain most of their present Domain 1 includes the Wasatch fault (1a; Fig. 2), which has slip rates relief (Allmendinger, 1982). At about this time, major silicic volcanism was that locally exceed 1 m/I000 yr (Swan and others, 1980) and has accomodated centered along the adjacent part of the Y-SRP axis (Armstrong and others, much of the east-west regional extension between the Crest Basin and stable 1975). In contrast, there is no evidence of significant Ouaternary surface interior in latest Quaternary time, and the following active neotectonic faulting along the main range-front fault on the west side of the Blackfoot elements Chet form a semicircular belt (lb) north of the Wasatch fault. These Mountains. are (a) a sec of right -stepping faults that extend from Cache Valley, Utah, to We do not intend to suggest that the cause of Basin and Range deformation Star Valley, Wyoming; (b) the Teton fault; (c) faults in the Yellowstone area; marginal to the eastern Plain is solely the result of thermal activity (d) the Deep Creek, Madison, Centennial, and Red Rock faults in Montana; and associated with the Y-SRP axis, but rather that a thermal front may have (e) the Beaverhead, Lemhi, and Lost River faults in Idaho. localized faulting in a wake-like belt that propagated outward through the This neotectonic belt of major Quaternary faults has a remarkable spatial northeastern Basin and Range from the Y-SRP axis. relation to the Yellowstone-Snake River Plain axis (li-SRP axis; Fig. 2). The The Lost River, Lemhi, and Beaverhead faults define an area of similar northern part of the most active portion (stippled on Fig. 2) of this belt structural pattern and neotectonic activity at the northwestern end of the lies generally from several tens to 100 km beyond the margin of the eastern latest Quaternary belt of surface faulting that is the focus of the rest of Snake River Plain, whereas the southern part lies from 100 to 200 km from the this discussion. Plain margin in the west and merges to within 100 km of the Plain margin in The present topographic relief of the Lost River, Lemhi, and Beaverhead the Teton arca. Both parts join within the Yellowstone area. ranges is the result of late Cenozoic faulting (Baldwin, 1951; Ruppel, Major silicic volcanism started about 15 my ago on the Y-SRP axis in 1982). Most of this faulting may have occurred during the last 4-7 m.y. based southwest Idaho, migrated northeastward along the axis at an average rate of on (1) the presence of fluvial-gravel cleats that must have been transported about 3.5 cm/yr, and presently is centered in the Yellowstone area (Armstrong from west of this area and that now underlie 6.6-m.y.-old volcanic rocks in and others, 1975; Chriatiansen and McKee, 1978). This volcanism was the Lemhi Range (x on Fig. 3) and (2) uplift-induced east dips of basalts that accompanied by crustal heating and uplift; subsequent cooling is reflected in overlie 4-m.y.-old ash-flow tuffs near the southern ends of the ranges (Fig. the decrease in elevation along the Y-SRP southwestward axis (Brott and 3; M. H. Hait, Jr., unpub. data; dates from McBroome and others, 1981). others, 1978; Smith, and others, 1985). The passage of this thermal activity Ruppel (1982) suggests that the Lemhi Range and Beaverhead Mountains are block along the axis also produced an outwardly migrating thermal front with a uplifts of mostly Miocene age, and that the fault scarps on the southwest geometry analogous to the wake of a moving boat. sides of the ranges reflect eastward tilting in late Pliocene to Holocene The belt encompassing the major Quaternary faults of the northeastern time. In contrast, Hait (1984) finds evidence for considerable extension Basin and Range (Fig. 2) also has a wake-like pattern about the Y-SRP axis and during middle Cenozoic time. We favor an interpretation that the three ranges converges with the axis at Yellowstone suggesting the pattern of neotectonic are parts of normal-fault-bounded blocks that range from structurally flat- activity may relate to the thermal front migrating outward from the Y-SRP topped (Ruppel, 1982) co eastward-tilted. Faulting during Ouaternary time axis. If this association between the thermal front and the active appears largely related to eastward tilting because: (1) fault scarps that neotectonic belt is valid, it provides a testable late Cenozoic tectonic cut Quaternary deposits occur mainly on the west sides of the ranges, (2) some history for areas within the belt. That is, the passage of the thermal front parts of the ranges are assymetric with steep west sides and gentler sloping produces first high rates of faulting as the crust is heated and thinned, east sides, and (3) upper Tertiary tuffs and basalts in and between the followed by decelerating rates of deformation or relative quiescence as southern parts of the ranges have easterly dips (Fig. 3). heating ceases and cooling occurs. For range-front faults between the Snake River Plain and the belt of major Quaternary faulting (Fig. 2), tectonic histories that reflect a high Quaternary Surface Faulting in the Lost River-Beaverhead Area rate of faulting followed by a decelerating rate appear co confirm the above prediction, especially on the south side of the Plain where numerous basins Ouaternary surface faulting has broken most of the Lost River, Lemhi, and and ranges attest to active late Cenozoic faulting, but where there is little Beaverhead range-front faults. Similar geometric end temporal patterns of evidence of faulting. For example, Quaternary the Cotterel and Jim Sage breakage and structural relief are repeated along each fault (Fig. 3). Mountains are western part adjacent to the of the eastern Snake River Plain The mapped patterns of the faults are strikingly similar. All three and consist of volcanics and sediments emplaced in a topographic low about 10 .generally hug the western range fronts, change trend by as much as 90°, and, my ago, at about the time silicic volcanism was centered along the adjacent in places, diverge from range fronts and strike across basins. Quaternary part of the Y-SRP axis (Armstrong and others, 1975; Williams and others, offset on the faults is dominantly normal slip, but lateral components can be 1982): Subsequently the basin was extended greatly along an east-dipping, recognized locally by en echelon patterns of faults (Fig. 3). low-angle detachment and the ranges were uplifted to produce about 1 km of Structural reliefalong the three faults, as estimated from topographic structural relief on the 10-my-old volcanics. High rates of deformation and geophysical data, is generally greater along central segments, which are

2 3

+.0 pertly coincident with en arch north of and parallel to the Snake River Plain clast from the soil in the faulted gravel has uranium-series daces of (see discussion in Rappel, 1982), than along distal segments (Fig. 3). These 23,000*4,000 and 30,000±5,000 yr (John Rosholt, Written commun., 1979). relations suggest that the central segments have higher long-term rates of Carbonate coats from the faulted gravel and slightly younger unfaulted gravel faulting, which is consistent with estimates of short-term slip rates for are about twice as thick as coats from a nearby surface estimated to be about various segments. In the Borah Peak area, which has about 2.7 km of 15,000 yr old (Pierce and Scott, 1982). Based on these 0-series dates, structural relief, the 15,000-yr-old surface of the willow Creek fan (Pierce carbonate-coat thicknesses, and the morphology of the scarp, the youngest and Scott, 1982) is offset 3.5-4.5 m, including the 1.5-2-m offset of 1983 ' faulting on the Arco segment occurred about 30,000 yr ago. (Crone and Machette, /984). The mean slip rate there is about 0.3 m/1000 South of the Arco segment, a zone of discontinuous scarps as high as 10 m yr. Similar relations along the central segments of the Lemhi and Beaverhead that displace surficial deposits and lavas of late and middle Pleistocene age faults suggest they have similar rates. In contrast, the Arco segment of the extends 12 km onto the Snake River Plain (Kuntz, 1978). The exact age Lost River fault, which has about one-half the structural relief of the Borah relationship of these scarps to the scarps along the Arco segment is not Peak area, has a maximum mean slip rate of 0.1 m/1000 yr (see discussion of known, but both sets of scarps displace deposits of similar age. Arco segment; Pierce, 1985); the southern segment of the Lemhi fault probably North of the Arco segment, the Pass Creek segment, which forms a marked has a similar rate. dogleg in the range front, extends 30 km to just south of Lower Cedar Creek. . In addition to having greeter structural relief, the central segments of Fault scarps are preserved only locally along this segment. The range front the faults have moved more recently than the distal segments. The ages of maintains a steep faceted profile as in adjacent segments, has high structural latest faulting shown on Figure 3 are based on the stratigraphic relationship relief, and is doubtless bounded by a fault. Surficial deposits older than of fault scarps to surficial deposits whose ages are estimated by about 30,000 yr are not well exposed along most of this segment, so the stratigraphic, geomorphic, and soil-development evidence (Malde, 1971; Hait general lack of scarps is difficult to interpret, but faulting may not have and Scott, 1978; Pierce and Scott, 1982; Scott, 1982). The latest Quaternary occurred in the last 30,000-50,000 yr. scarps are typically 2-5 m high and displace alluvial and glacial deposits of The 22-km-long Mackay segment extends from Lower Cedar Creek, east of Pinedale age (as young as 12,000-15,000 yr). The late Pleistocene and older, Mackay, to the prominent bend in the range front at Elkhorn Creek. Fault scarps are as high as 20 m. These do not displace latest Quaternary deposits, scarps of latest Quaternary age occur throughout the entire segment, and but do offset surficial deposits that are locally as young as 30,000 yr, but generally are preserved even on steep (30°) slopes at the base of the range that are commonly more like deposits dated about 160,000 yr (see discussion of front. A trench across the fault acarp at the mouth of Lower Cedar Creek in a Arco segment; Pierce, 1985). pre-Pinedale fan deposit records several surface-faulting events, the last of A consistent pattern of ages of fault scarps on the western fronts of the Which occurred after the deposition of a pod of Mazama ash (6800 14C yr old; ranges is evident on Figure 3. Late Pleistocene or older scarps occur along }Oat and Scott, 1978). A date of 4,320*130 14C yr B.P. (W-4427) from organic the southern sections of the range fronts within about 25 km of the Snake matter buried by colluvium derived from the free face formed during the last River Plain; scarps along the central and north-central sections of the ranges .surface-faulting event indicates that this event probably occurred about 4,000 are latest Quaternary; and the absence of scarps along the northernmost range yr ago. The Mackay segment has less structural relief than the Thousand fronts suggests that in these areas the last surface-faulting event is older Springs segment to the north, suggesting that the Mackay segment has a lower than late Pleistocene. long-tern slip rate. However, the lack of evidence of pedimentation of the front, the position of the latest Quaternary scarp on steep faceted spurs, and the burial of the head of extensively exposed middle Pleistocene and older Lost River Fault and 1983 Surafce Faulting alluvial-fan deposits by younger fan deposits suggest that this segment has been very active in late Quaternary time. Reconnaissance and detailed mapping of the Lost River fault defines 6 or The Thousand Springs segment, which extends from Elkhorn Creek north to 7 fault segments that are characterized by different geomorphic expression, the Willow Creek hills, was the site of the greatest amount surface rupturing structural relief, and ages of last movement that follow the pattern discussed during the 1983 Borah Peek earthouake (Crone and Machette, 1984). This above (Fig. 3). segment has the greatest structural relief (2.7 km; Fig. 3) measured along the Fault scarps along the 20-km-long Arco segment do not displace latest Lost River fault, and therefore probably has the highest long-term slip Quaternary deposits, but do offset extensively preserved older fan surfaces by rate. Pre-1983 fault scarps of latest Quaternary age are best preserved at as much as 20 m. In the area of a trench acroas the fault (Malde, 1971, sites where the scarp lies a short distance out from the steep mountain front, 1985), several stratigraphic datums allow estimates of the slip rate of this such as at Cedar. Rock, and Willow Creeks. Trenches across the scarp at the segment (Pierce, 1985). A surface estimated to be about 160,000 yr old based mouth of Willow Creek (Halt and Scott, 1978; D. P. Schwartz and A. J. Crone, on uranium-series ages of layered carbonate coats on cleats in soils is offset personal communication, 1984) where the scarp offsets an alluvial surface about 19 m or possibly more and a volcanic ash estimated to be about 70,000- estimated to be about 15,000 yr old (Pierce and Scott, 1982) and a 110,000 yr old is offset about 8 m. Because of burial and limited exposure, reconstruction of geomorphic surfaces across the fault (Vincent, 1985) the existence of back rotation or graben formation can not be evaluated, so indicate that one pre-1983 surface-faulting event occurred along the central the slip race of 0.07-0.1 m/1000 yr estimated from these datums is regarded as part of this segment in latest Quaternary time. That event occurred in late a maximum. Holocene time based on the following soil-development evidence (Bait and Near the northern end of the Arco segment, the youngest faulting offsets Scott, 1978). The soil formed in the alluvium of the hanging wall where it is fan gravels about 3 m (Pierce, 1985). The inner part of a carbonate coat on a buried by fault-scarp colluvium is almost as well developed as soils formed in

4 5

15 the same alluvial surface above the rates, and latest Quaternary offset. Consequently, similar segments can scarp. This indicates that most of the slip 15,000-yr-interval since the future surface faulting. The part of the Lost stabilization of the fan surface and be viewed as likely sites for of soil development had initiation central segments of the Lemhi and passed prior to the pre-1983 event. River fault that broke in 1983 and the Near Willow Creek, many these characteristics (Fig. 4A) and therefore are features of the 1983 break (Crone and Beaverhead faults have 1984, fig. 4) closely mimic Machette, features of the pre-1983 scarp. likely sites. Willow Creek fan The offset of the accompanying the 1983 earthquake was pre-1983 1.5-2 m, compared to the offset of 2-2.5 m. Other features Movement triggered along faults linked at depth with the 1983 break: if of include a right-stepping pattern short faults, a broad complex graben the Lost River, Lemhi, and Beaverhead faults are linked to a single west- with a conspicuous horat, and minor thrust features west of the graben. dipping detachment at depth, then the 1983 slip along the central segment of The segment of the 1983 break the Lost River fault might trigger movements along one or several of the that diverges from the Lost River strikes west into the fault and faults co the northeast (Fig. 4A) that would lie Willow Creek hills in part occurs on central segments of the scarps marked by remnants of older This is analogous to the displacement conspicuous benches and changes in higher on the assumed detachment plane. surfaces slope, but also displaces having no apparent evidence of late of blocks in a landslide, in which the movement of a block that provides a Pleistocene offset. The higher structural position of the Willow Creek buttress for blocks higher on the failure plane would trigger movement of some hills compared to the basins to the north and south suggests recurrent of those blocks. uplift of the hills during the late Cenozoic. Elsewhere this similar pattern of a fault splay crossing a occurs at Middle Ridge (Fig. 3). basin Segments adjacent to the ends of the main 1983 break: The Mackay and Warm The Warm the Thousand Springs Springs segment extends for 15 km Springs segments have less structural relief than hills as a north of the Willow Creek conspicuous fault scarp of latest segment, which accounted for the main part of the 1983 surface rupture; The Quaternary age as high as 5 m. scarp ends at Devils Canyon, which however, the similar height and morphology of the range along these segments is also the northern end of cracking associated with the ground Quaternary fault scarps on the lower slopes of the 1983 earthquake. Along this and the position of latest ground breakage segment the 1983 three segments have generally lies on the mid-slope of the steep faceted spurs of the- range front suggest that all ranges from 1-m older fault scarp, and scarps to small, discontinuous had comparable activity in late Quaternary time. Therefore, future surface- Mackay cracks. As is the case for the segment, the structural relief faulting events along the adjacent segments seem likely if these sections of along the Warm Springs segment great as that of the is not as segment Thousand Springs segment; however, range front are to keep pace with slip along the Thousand Springs range front and the character of the the position of the young scarps (Fig. 48). segment has suggest that the Warm Springs had late Quaternary slip rates Unfortunately, we don't know the age relations of the last events on the similar to those of the other two segments. Mackay and Warm Springs segments and the pre-1983 event on the Thousand The northernmost segment of the Lost Springs segment. Based on the 14C date from the Lower Cedar Creek trench, the River fault extends from Devils Canyon to the end of the Lost Willow Creek trench, and scratigraphic and River Range near Challis, has low soil-development evidence at the relief, and shows no evidence structural segment, any sequence of events is of late Quaternary faulting. geomorphic relations along the Warm Springs permissable. Knowledge of this sequence is critical to predictions of sites of future surface faulting based on fault history. For instance, if the pre- Likely Sites of Future Faulting in 1983 event on the Thousand Springs segment were significantly older than the the Lost River-Beaverhead Area last events on the Mackay and Warm Springs segments, then the 1983 event could In view of our -1983 event on understanding of the 1983 Borah be viewed as a gap-filling event. Of more concern, if the pre Quaternary Peak earthquake and its tectonic setting, what are likely the Thousand Springs segment preceded closely the last events on the Warm extensive sites in the area for future surface faulting accompanying large Springs and Mackay segments, then surface faulting along both of these evidence of earthquakes? Widespread lace Quaternary surface faulting segments adjacent to the 1983 rupture would be considered highly likely. fronts and the morphology of the suggest that future ruptures range could occur along any part of River, Lemhi, or Beaverhead the Lost seismic gap is a fault segment that has not faults. Prediction of specific Surface-faulting gap: A surface faulting sites of future historic fault scarps; such a requires an understanding of several ruptured in historic time and that lies between including patterns and conditions at depth Whitney, rates of strain accumulation, gap is thought to be a likely site for future rupture (Wallace and the rocks, and physical properties of linkages between the range-front 1984). By analogy, a fault segment that lies between ones having evidence of know little faults. Unfortunately, we about these. Nevertheless, one can more recent, but prehistoric, breakage can be called a surface-faulting gap, faulting view the history of surface and the geomorphic and structural and be considered a likely site for future faulting. The Pass Creek segment break features of the area and the 1983 from the following no evidence of late Pleistocene or Holocene perspectives to gain insight as to which of of the Lost River fault has segments are more likely to the structural relief and an rupture than others. The site of movement; however, it lies in an area having high is difficult to predict the next event with confidence based on these imposing range front (Fig. 4C), which suggests that it probably has a long- are useful for perspectives, but they identifying potential sites for the term rate of faulting similar to chat of adjacent segments that have ruptured ruptures in next 10 or so surface the area. in late Pleistocene or Holocene time. A few segments of the Lemhi and Beaverhead faults also fall in this category. Sites comparable to 1983 break: The 1983 Other possible gaps are shown in Figure 4C by the dashed bold lines along a segment with surface faulting occurred along both high structural relief, latest Quaternary faults; however, so little Is known about the age relations which indicates high long-term

6 between individual segments of these latest Quaternary faults that surface- Mines and Geology Bulletin 26, p. 505-516. faulting gaps can not be defined with certainty. Arabasz, W. J., and Smith, R. B., 1979, Introduction: What you've always wanted to know about earthquakes in Utah, in, Smith, R. B., Arabass, W. Constant strain accumulation and characteristic offset: Schwartz and J., and Richins, W. D., eds., Earthquake studies in Utah 1850-1978: Coppersmith (1984) propose that a given fault segment ruptures when a certain Publication of the University of Utah Seismograph Stations, Salt Lake strain threshold is reached and results in a characteristic surface offset. City, p. 1-31. If we assume that slip rates estimated from geologic relations reflect mean Armstrong, R. L., Leeman, W. P., and Malde, H. E., 1975, K-Ar dating, strain rates on a segment, then an offset is likely when the product of the Quaternary and Neogene volcanic rocks of the Snake River Plain, Idaho: slip rate and the time since the lest surface-faulting event approaches the American Journal of Science, v. 275, no. 3, p. 225-251. characteristic offset. The Arco segment has an estimated maximum long-term Baldwin, E. H., 1951, Faulting in the Lost River Range area of Idaho: slip rate of about 0.1 m/1000 yr and has not ruptured in the past 30,000 yr, American Journal of Science, v. 249, p. 884-902. ' which suggests a potential strain accumulation of as much as 3 m (Pierce, Brott, C. A., Blackwell, D. D., and Kitchell, J. C., 1978, Tectonic 1985). As the surface offset accompanying the 1983 earthquake and prehistoric ' implications of the heat flow of the western Snake River Plain, Idaho: offsets estimated from trench studies are less than this, movement on the Arco Geological Society of America Bulletin, v. 89, p. 1697-1707. segment can be considered overdue (Fig. 4D). The potential strain Christiansen, R. L., and McKee, E. R., 1978, Late Cenozoic volcanic and accumulation on other fault segments is not as well known; however, limited tectonic evolution of the Great Basin and Columbia intermontane regions, geomorphic information on the southern segment of the Lemhi fault suggests in, Smith, R. 8., and Eaton, G. P., eds., Cenozoic tectonics and that surface-faulting events may have a similarly long (104 yr) recurrence geophysics of the western Cordillera: Geological Society of America interval. The time since the last surface-faulting event on this segment is Memoir 152, p. 283-312. >15,000 yr, which implies a potential strain accumulation of >1.5 m by using Covington, H. R., 1983, Structural evolution of the Raft River basin, Idaho, the slip rate for the Arco segment. in Miller, D. M., Todd, V. R., and Howard, K. A., eds., Tectonic and stratigraphic studies in the eastern Great Basin: Geological Society of Grouping of events: Surface faulting along segments may not occur at Memoii 157, p. 229-237. uniform rates, but rather as America intervals of activity along one segment Surface breakage caused by the Borah of several segments or a belt Crone, A. J., and Machette, M. N., 1984, separated by periods of diminished which or no activity during Peak.earchquake, central Idaho: Geology, v. 12, no. 11, p. 664-667. surface faulting is concentrated in another area (e.g., Wallace and Crosthwaite, E. G., Thomas, C. A., and Dyer, K. L., 1970, Water resources in Whitney, 1984; Wallace, 1985). if such grouping in space and time occurs in the Big Lost River basin, south-central Idaho: U.S. Geological Survey the Lost River-Beaverhead area, some of the preceding perspectives may be manifestations of it. simply ' Open-File Report, 109 p. For instance, grouping provides an River Plain, surface-faulting explanation for Hait, M. H., Jr., 1984, Detachment tectonics north of the Snake gaps; the gaps are segments that are in a interval. relatively inactive east-central Idaho and aouthwest Montana: Geological Society of America Likewise, the occurrence of latest Quaternary central offsets on the Abstracts with Programs, v. 16, no. 6, p. 527. segments of the faults may be a function of grouping. In addition, Halt, M. H., Jr., and Scott. W. E., 1978, Holocene faulring, Lost River Range, non-uniform rates of surface-faulting for a given segment would negate the Geological Society of America Abstracts with Program, v. assumption of uniform strain Idaho (abs.): accumulation for the characteristic perspective. The offset 10, no. 5, p. 217. demonstration of grouping is a poorly component in understood but key Howard, K. A., and others, 1978, Preliminary map of young faults in the United understanding the kinematics of late Cenozoic Lost River deformation in the States as a guide to possible fault activity: U.S. Geological Survey -Beaverhead area (as well as other sites and in the Basin and Range), Miscellaneous Field Studies Hap HF-916, scale 1:5,1100,000. thereby predicting the most likely sites of future faulting. Kuntz, M. A., 1978, Geologic map of the Arco-Big Southern Butte area, Butte, Blaine, and Bingham Counties, Idaho: U.S. Geological Survey Open-File Report 78-302, scale 1:58,000. Acknowledgments Mabey, D. R., Peterson, D. L., and Wilson, C. W., 1974, Preliminary gravity -File We map of southern Idaho: U.S. Geological Survey Open Report 74-78, thank S. S. Oriel for encouraging and the supporting our research during scale 1:500,000. late 1970's on Quaternary faulting in the Borah Peak region, as part of Malde, H. E., 1971, Geologic investigation of faulting near the National the U.S. Geological Survey's Snake River Plain Project. We appreciate the Reactor Testing Station, Idaho, with a section on microearthquake studies constructive reviews of the manuscript provided by A. J. Crone, S. S. Oriel, by Pitt, A. M., and Eaton, J. P.: U.S. Geological Survey Open-File M. W. Reynolds, and E. T. Ruppel Report, 167 p. Malde, H. E., 1985, Geologic investigation of faulting near Arco, Idaho, in ' Workshop XXVIII on the Borah Peak Earthquake: U.S. Geological Survey References Open-File Report, in press. McBroome, L. A., Doherty, D. J., and Embree, G. F., 1981, Correlation of major Allmendinger, R. W., 1982, Sequence of late Cenozoic deformation in the Pliocene and Miocene ash-flow sheets, eastern Snake River Plain, Idaho, in Blackfoot Mountains, southeastern Idaho, in, Donnichaen, Bill, and Tucker, T. E., ed., Guidebook to southwestern Montana: Montana Geological Breckenridge, R. H., eds., Cenozoic Geology of Idaho: Idaho Bureau of Society 1981 Field Conference and Symposium, p. 323-330. Figure Captions Nakata, J. K., Wentworth, C. M., and Machette, M. N., 1982, Quaternary fault map of the Basin and Range and Rio Grande Rift Provinces, western United States: U.S. Geological Survey Open-File Report 82-579, scale Figure 1. Neorectonic setting 'of the 1983 Borah Peak earthquake in the 1:2,500,000. northeastern Basin-and-Range Province shoving major, late Cenozoic O'Neill, J. H., and Lopez, D. A., 1985, Character and regional significance of normal-slip faults (modified from Howard and others, 1978; Nakata and Great Falls tectonic zone of east-central Idaho and west-central • others, 1982). Those with latest Quaternary movement that lie at the Montana: American Association of Petroleum Geologists Bulletin, in press. base of high, steep range fronts are shown by a bold line; those with Pierce, K. L., 1985, Neotectonic history of the Arco segment of the Lost River historic rupture are hachured. Light-shaded areas include zones of fault, Idaho, in Workshop XXVIII on the Borah Peak Earthquake: earthquake epicenters between 1850 and 1974 (from Arabasz and Smith, U.S.Geological Survey Open-File Report, in press. 1979) that define the Intermountain Seismic Belt (ISB) and Idaho Seismic Pierce, K. L., and Scott, W. E., 1982, Pleistocene episodes of alluvial-gravel Zone (ISZ) of Smith and Sbar (1974). .indicates epicenter of 1983 deposition, southeastern Idaho, in Bonnichsen, Bill and Breckenridge, R. Borah Peak earthquake. H., eds., Cenozoic Geology of Idaho: Idaho Bureau of Mines and Geology Bulletin 26, p. 685-702. Reynolds, M. W., 1979, Character and extent of basin-range faulting, western Figure 2. Neotectonic domains (see text) of the same area as Figure 1, Montana and east-central Idaho: Rocky Mountain Association of Geologists , including latest Quaternary faults from Figure 1. Large arrows show and Utah Geological Association 1979 Basin and Range Symposium, p. 185- trajectory of thermal activity along the Yellowstone-Snake River Plain 193. (Y-SRP) axis. JC • Jim Sage andlCoterell Mountains; BFM - Blackfoot Ruppel, E. T., 1982, Cenozoic block uplifts in east-central Idaho and Mountains. southwest Montana: U.S. Geological Survey Professional Paper 1224, 24 p. (1) Wasatch (1a)-Yellowstone-Lost River (16) belt of latest Quaternary Schwartz, D. P., and Coppersmith, K. J., 1984, Fault behavior and surface faulting. Area of greatest activity is stippled. characteristic earthquakes: Examples from the Wasatch and San Andreas (2) Southwestern Montana. Many late Cenozoic faults, but only a few, fault zones: Journal of Geophysical Research, v. 89, p. 5681-5698. widely distributed range-bounding faults with evidence of.latest Scott, V. E., 1982, Surficial geologic map of the eastern Snake River Plain Quaternary surface faulting. Recent studies (e.g., O'Neill and Lopez, and adjacent areas, 111° to 115° vest, Idaho and Wyoming: U.S. Geological 1985) indicate that some additional latest Quaternary surface faults Survey Miscellaneous Investigations Series Map 1-1372, scale 1:250,000, occur in this area, but the widely distributed pattern is maintained. Smith, R. B., and Sbar, M. L., 1974, Contemporary tectonics and seismicity of (3) Idaho batholith. Relatively rigid block broken locally by the western United States with emphasis on the Intermountain seismic Quaternary faults. belt: Geological Society of America Bulletin, v. 85, p. 1205-1218. (4) Eastern Snake River Plain. Little evidence of late Quaternary Smith, R. B., Richina, V. 0., Doser, D. I., Eddington, P. K., Leu, L. L., and faulting except for rifts associated with basaltic volcanism. Chen, G, 1985, The Borah Peak earthquake: Seismicity, faulting kinematics, (5) Northeastern Basin and Range west of (1). Many late Cenozoic and tectonic mechanism, in Workshop XXVIII on the Borah Peak Earthquake: faults, but only widely distributed evidence of latest Quaternary U.S. Geological Survey Open-File Report, in press. surface faulting. Swan, F. H., III, Schwartz, D. P., and Cluff, Lloyd, 1980, Recurrence of moderate to large magnitude earthquakes produced by surface faulting on the Wasatch fault zone, Utah: Bulletin Seismological Society of America, Figure 3. Major frontal faults in the Lost River-Beaverhead area. 1983 break v. 70, no. 5, p. 1431-1462. from Crone and Machette (1984). Estimates of minimum structural relief Vincent, K. R., 1985, Measurement of vertical tectonic offset using in kilometers 2.7 from sum of thickness of basin fill (Crosthwaite and longitudinal profiles of faulted geomorphic surfaces near Borah Peak, others, 197D; Ruppel, 1982) and altitude of range, and, at north end of Idaho: A preliminary report, in, Workshop XXVIII on the Borah Peak Lost River fault, from minimum offset of Challis Volcanics. Thick basin Earthquake: U.S. Geological Survey Open-File Report, in press. fills (stippled) interpreted from gravity lows with >15 mgal of closure Wallace, R. E., 1985, Variations in slip rates, migration, and grouping of (Mabey and others, 1974). Direction of dip of upper Tertiary volcanics slip events on faults in the Great Basin Province, in, Workshop XXVIII on are shown by strike-end-dip symbol. X - gravel transported from west of the Borah Peak Earthquake: U.S.Geological Survey Open-File that underlies 6.6-my-old basalt (see text). Segments that have press. Report, in area evidence of a left- or right-lateral component of slip in pre-1983 Wallace, R. E., and Whitney, A. A., 1984, Late Quaternary history of the events are shown by L and R, respectively. DC Devils Canyon; WSV - Stillwater seismic gap, Nevada: Bulletin Seismological Society of Warm Springs valley; WCH Willow Creek hills; TSV Thousand Springs America, v. 74, no. 1, p. 301-314. Valley; VC Willow Creek; BP - Borah Peak; EC - Elkhorn Creek; LCC Williams, P. L., Covington, H. R., and Pierce, K. L., 1962, Cenozoic Laver Cedar Creek. stratigraphy and tectonic evolution of the Raft River basin, Idaho, in, Bonnichsen, Bill, and Breckenridge, R. H., eds, Cenozoic Geology of Idaho: Idaho Bureau of Mines and Geology Bulletin 26, p. 491-504.

10 11

Ct Figure 4. Likely sites of future surface faulting in the Lost River- Beaverhead area based on several perspectives (see text). 1983 surface fault shown by hachures. Identified sites are shown by bold line; others identified with less certainty are shown by dashed bold line. Areas of thick basin fills (dark stipple) are taken from Figure 3. Upland areas are in light stipple; basins in white.

N \ \ \\ \ , \ ,„ \ ,.. \ \\\L 44, c.e• l i A 140,4TAHA _i... is.\ . —\ I OAMO f if .1;1 k.NN $,.._,.--\( 4 /. :, . .... / \ II 1 7 - • ./::-.'. \ Ates of figure 3 .I 1 2,.::,..:_\____

isz \.-2,----z..---,...... _ •

sy 1„,/ wyomN0 ....,, / .'r-... ci.. ..-1 0 I I

Ar .-- (I4- i\ - )i P 4 i , I /T- 1 /4) NEVADAI I ,1 \ 1/ c i qi 1 \ I/ —.--- — i \ UTAH it— ----- — — t I 1, I/ ‘I 1 \ I I

0 KO 10.0MIFTEAS

FIGURE 1

12 13 8P

110

MONTANA

ar—

DAN°

MOON FAUST 3 OUP CASES FAUST ""81„._ RIO ROCK

I LEPA111 FAULT ,,, ----- .,,• ci.TE.Nim. X.iYELLOWSTONS FIrVL FAULT ISAVERHEAD A FAULT , Aft. ..., / TETON FAULT LOST AMER - , / 11.:Ey.-. FAULT „/". .,,, i , 1 ..?.:2;i1- 1 ‘ )'.."..... WY01•UND I I .i:-‘.:. % ./'4 "' A. 112:;:- FrT•Ft VALLEY FAULT Er ,/ ./. 0." ./.. 1A1 /..1.:4:,.::1 —• '' KAM LAKE F•UST -..1 1 =I

EAST CACNE FAULT IF NEVADA r ' 5

L------VAWAUTCN FAULT

FIGURE 2

14 •

VARIATIONS IN SLIP RATES, MIGRATION, AND GROUPING OF SLIP EVENTS ON FAULTS IN THE GREAT BASIN PROVINCE

Robert E. Wallace U.S. Geological Survey Menlo Park, California 94025

Abstract • Nonuniformity of slip rate on seismogenic faults, both in time and space, probably representS the norm in the Great Basin province. Varia- tions through time of slip rate on faults are exemplified by the change in tilt rates of the East Range and Cortez Mountains, Nevada. Temporal grouping of faulting events is represented on the Lost River fault, Idaho, where several events occurred along the Thousand Springs segment of the fault while segments previously active in late Quaternary time remained Quiescent. Migration and extension of slip along the strike of a fault apparently occurred on the northwest flank of the Humboldt Range, Nevada. Long-term migration or shifting of slip back and forth from one fault to another along subparallel range-front faults is shown by the history of displacement along the faults that generated the 1954 earthquakes in Dixie Valley, Nevada.

Introduction

Slip rates on seismogenic faults in the Great Basin province, western 5.41 1011.1 .... a.TT... lop•renn oipanwa 11. !num. mei F.• wry. IN TIP .w Y.... United States, have not been uniform. 1.kon TOT IS IT 10/04 Variations exist on many time scales ranging from less than a thousand years to several million years. Variations involve both temporal and spatial grouping of large slip events and migration of slip both along individual faults and regionally.

In general, the average recurrence interval for large-scale displace- ments {those measured in meters of displacement along fault traces several kilometers long) on individual faults or segments of faults have been several thousands of years to more than a hundred thousand years. Slip rates are of the order of 0.01-0.5 mm/yr on the average, but for a few hundred or a few thousand years the slip rate may be an order of magnitude greater or smaller depending on some of the factors described below. Only a few examples are presented to illustrate several types of variations in slip rate.

Long-term chances in slip rate

Two ranges in north central Nevada, the East Range and Cortez Moun- tains, are members of a set of southeast or east-tilted ranges that are bounded on their western flanks by active faults. Both range blocks con- tain a core of rocks of Paleozoic and Mesozoic age and are cappeo by basaltic or andesitic lava flows of ages between 10 and 14 my. The flows on each range dip between 8 and 10 degrees southeastward forming cruoe dip FIGURE 4 slopes on their east or south-east flanks. The west or northwest flanks of both ranges are fault generated ano an active fault lies at the base of each.

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