Bulletin of the Seismological Society of America, 91, 2, pp. 232–249, April 2001

Paleoseismologic Evidence for a Ͼ8 Ka Age of the Most Recent Surface Rupture on the Eastern Sierra Madre Fault, Northern Los Angeles Metropolitan Region, California by Allan Z. Tucker and James F. Dolan

Abstract Paleoseismologic data from the Sierra Madre fault, a major north-dip- ping reverse fault that extends for 75 km across the northern edge of the Los Angeles metropolitan region, indicate that the most recent surface rupture on the eastern part of the fault occurred more than 8000 years ago. Coupled with evidence for a mini- mum reverse-slip rate of 0.6–0.9 mm/yr on the strand that we trenched, the long elapsed interval since the most recent event suggests that the Sierra Madre fault Ն breaks during very infrequent, large-magnitude (MW 7) earthquakes. Events of such large magnitude are much larger than the largest earthquakes that have occurred on any of the Los Angeles area urban faults during the ϳ200-year-long historic

period (e.g., the 1971 MW 6.7 San Fernando and 1994 MW 6.7 Northridge earth- quakes) and must be considered in future seismic hazard analyses for southern Cali- fornia. Although more paleoseismologic data are needed to determine whether or not the Sierra Madre fault ruptures together with adjacent faults, available data already show that the Raymond fault, a west-southwest-trending left-lateral strike-slip fault that intersects the central Sierra Madre fault, has ruptured to the surface at least once, and possibly several times, since the most recent surface rupture on the eastern Sierra Madre fault. Moreover, if the San Andreas fault (SAF) and the Sierra Madre fault ever rupture together, then such events must be exceedingly rare, with at least ϳ 50–100 SAF MW 8 so-called Big Ones occurring between every possible combined SAF–Sierra Madre fault event.

Introduction Over the past decade, ideas about seismic hazards facing more complicated, resulting in north–south convergence and Los Angeles have undergone significant revision and refine- consequent crustal shortening. This north–south shortening ment. Most models now focus not only on the possibility of has given rise to both the east–west topographic highs of the the recurrence of a great (M ϳ8) 1857-type San Andreas Transverse Ranges and a complicated pattern of west-trend- fault rupture (the so-called Big One), but also on the occur- ing reverse faults and left-lateral strike-slip faults (Fig. 1). rence of smaller, moderate to large earthquakes on faults The largest of these reverse faults in the metropolitan region directly beneath the metropolitan area (e.g., Wesnousky, is the 75-km-long Sierra Madre fault, which together with 1986; Dolan et al., 1995; WGCEP, 1995). Because these ur- the Cucamonga fault to the east and the Santa Susana fault ban faults lie so close to the major population centers of to the west forms the central reach of a 140-km-long system southern California, moderate to moderately large earth- of north-dipping reverse faults that extends along the north- quakes that they may generate could be at least as destructive ern edge of the metropolitan region. In this article we present as a much larger event on the more distant San Andreas fault the first paleoseismologic data from the eastern half of the (WGCEP, 1995; Dolan et al., 1995; Heaton et al., 1995). Sierra Madre fault and discuss the implications of these data The metropolitan Los Angeles area is built atop a major for models of past fault interactions as well as for seismic tectonic transition. To the south, Pacific–North American hazard analysis in the Los Angeles metropolitan region. plate boundary deformation is accommodated primarily by right slip along north-northwest-trending strike-slip faults of Regional Geology and Relationship to Other Faults the San Andreas system. In contrast, in the Transverse Ranges region north of Los Angeles, right-slip interactions The Sierra Madre fault bounds the southern margin of between the Pacific and the North American plates become the San Gabriel Mountains, an 85-km-long, west-trending

232 Paleoseismologic Evidence for a Ͼ8 Ka Age of the Most Recent Surface Rupture on the Eastern Sierra Madre Fault 233

119° 118° 30' 118° 117° 30'

San Mojave

Piru 34° Lake 30' San Andreas Desert Lake Caye tano Casitas Fault San R M F Basin Fault Fault Ridge SSF SIERRA Gabriel Oak V Ventura Mountains San SJcF LC MADRE SF Fernando V C- Cucamonga Valley F A Fault Ox FAULT Az P Flt S Thrust ? Figure 2 Santa Monica Mountains Flt Raymond San Gabriel SJF Hol LA ELATB Malibu M WN Valley Chino Coast Flt Fault SCIF Anacapa- Monica Los Angeles Basin 34° Dume Fault Santa PHT Whittier Newport- Flt Fault Compton EPT Elsinore ? Santa Palos LB Pacific Inglewood Ana Flt Thrust Verdes Mtns

Fault NB 025 Ocean ?

Fault km Fault 33° 30'

Figure 1. Regional neotectonic map for metropolitan southern California showing major active faults. The Sierra Madre fault is a 75-km-long active reverse fault that extends along the northern edge of the metropolitan region. Fault locations are from Ziony and Jones (1989), Vedder et al. (1986), Dolan and Sieh (1992), Sorlien (1994), and Dolan et al. (1997, 2000b). Closed teeth denote reverse fault surface trace; open teeth on dashed lines show upper edge of blind thrust fault ramps. Strike-slip fault surface traces shown by double arrows. Star denotes location of Oak Hill paleoseismologic trench site of Bonilla (1973). C- SI, Clamshell-Sawpit fault; ELATB, East Los Angeles blind thrust system; EPT, Elysian park blind thrust fault; Hol Fl, Hollywood fault; PHT, Puente Hills blind thrust fault; RMF, Red Mountain fault; SCII, Santa Cruz Island fault; SSF, Santa Susana fault; SJcF, San Jacinto fault; SJF, San Jose fault; VF, Verdugo fault; A, Altadena study site of Rubin et al. (1998); LA, Los Angeles; LB, Long Beach; LC, La Crescenta; M, Malibu; NB, Newport Beach; Ox, Oxnard; P, Pasadena; PH, Port Hueneme; S, Horsethief Canyon study site in San Dimas; V, Ventura. Dark shading denotes mountains. range composed predominantly of Mesozoic granitic rocks of the Sierra Madre fault strikes between east–west and and pre-Mesozoic metamorphic rocks (e.g., Ehlig, 1975, N55ЊW (Fig. 2). The left step is marked by a sharply defined, 1981). Reverse slip along the Sierra Madre fault, which has southwest-trending topographic inflection point along the raised the San Gabriel Mountains to peak elevations of 2–3 southern edge of the San Gabriel Mountains that we interpret km, is thought to have begun ϳ7 million years ago (Blythe as the fluvially modified scarp of a throughgoing fault con- et al., 2000). necting the Sierra Madre and Cucamonga faults. The ab- The boundary between the Sierra Madre fault and the sence of good three-dimensional control on the downdip ex- relatively more linear trace of the Cucamonga fault to the tent and geometries of the Sierra Madre and Cucamonga east is marked by a 3-km-wide left step in the frontal reverse faults precludes a detailed analysis of the geometry of this fault system at San Antonio Canyon (Figs. 1 and 2). To the fault system at depth. Two possibilities are that (1) the two east of this step, the Cucamonga fault strikes N82E. In con- faults merge into the same plane at seismogenic depths, in trast, to the west of the step, the most recently active trace which case the fault within the left step is a tear fault; or 234 A. Z. Tucker and J. F. Dolan

Figure 2. Map of the easternmost 20 km of the Sierra Madre fault between Azusa and the San Antonio Canyon lateral ramp (location in Fig. 1). Traces of Sierra Madre fault are based on our analysis of 1928-, 1938-, and 1952-vintage aerial photographs (1:20,000 scale) except as noted. Locations of Upper and Lower Duarte faults (UDF and DF, respectively) are modified from Crook et al. (1987). Cucamonga fault (Cuc Flt) location is from Morton and Matti (1987). Northern branch of Sierra Madre fault and San Dimas Canyon fault (SDCF) are from Pentegoff et al. (1965) and Proctor et al. (1970; 1992). Epicenters of 1988 ML 4.6 and 1990 MW 5.3 earthquakes (shown by crosses) are from Hauksson and Jones (1991). Dotted fault trace shown due east of the two earthquake epicenters marks the surface projection of the faulting planes based on focal mechanisms and hypocenter locations of Hauksson and Jones (1991). Vertical hatchures denote fault-bounded lobes discussed in text (“piedmont forelands” of Bull, 1987). “Possible Older Mountain-Front Fault?” corresponds to mountain front 21A of Bull [1987]. Topographic contour interval is 50 m. SAC, San Antonio Canyon; SJF, San Jose fault; SMDF, Sierra Madre fault.

(2) the Sierra Madre and Cucamonga faults are different possibility is supported by the results of recent computer faults through the entire seismogenic part of the crust, in modeling (Magistrale and Day, 1999). In contrast, if the Si- which case the fault within the left step is perhaps best de- erra Madre and Cucamonga faults are the same fault at seis- scribed as a lateral ramp between sections of a reverse fault mogenic depths separated by a relatively minor, near-surface system of relatively constant dip. Future resolution of this tear fault, then the left step in the frontal reverse fault system question has important implications for seismic hazard as- at San Antonio Canyon may not represent a significant bar- sessment in the region because the termination of some re- rier to rupture propagation. verse fault earthquakes at major lateral ramps—for example, The northeast-trending fault within the left step between the 1971 MW 6.7 San Fernando event (USGS Staff, 1971; the Cucamonga and Sierra Madre faults is subparallel with Whitcomb et al., 1973; Allen et al., 1975; Barrows et al., and several kilometers to the northwest of a left-lateral fault

1975; Oakeshott, 1975; Heaton, 1982; Tsutsumi and Yeats, defined by the 1988 ML 4.6 and 1990 MW 5.3 Upland earth- 1999) and the 1999 MW 7.6 Chi-Chi, Taiwan event (Bilham quakes, both of which exhibited left-lateral strike-slip focal and Yu, 2000; Shin et al., 2000)—suggests that such features mechanisms and steeply west-northwest-dipping faulting represent fundamental barriers to rupture propagation. This planes (Fig. 2) (Hauksson and Jones, 1991). The fault de- Paleoseismologic Evidence for a Ͼ8 Ka Age of the Most Recent Surface Rupture on the Eastern Sierra Madre Fault 235

fined by these two earthquakes may be part of the left-lateral/ central and western reaches of the Sierra Madre fault suggest reverse (?) San Jose fault, which extends west-southwest- a slip rate on the order of 1 to 2 mm/yr (Lindvall et al., 1996; ward for 15 km from the mountain front (Figs. 1 and 2) Walls et al., 1998), but all of these estimates are from west (Hauksson and Jones, 1991). The parallelism of the fault of the Raymond–Sierra Madre fault intersection. defined by the Upland earthquakes with the northeast-trend- The western end of the Sierra Madre fault is marked by ing fault connecting the surface traces of the Cucamonga another major (2.5-km-wide) left step in the frontal reverse and Sierra Madre faults suggests that the fault in the left step fault system onto the Santa Susana fault to the west (Fig. 1) may also accommodate primarily left-lateral motion. (USGS Staff, 1971; Allen et al., 1975; Barrows et al., 1975; If the left-lateral faults within the left-step region do Oakeshott, 1975; Heaton, 1982; Yeats, 1987; Tsutsumi and accommodate significant left-lateral strike-slip that extends Yeats, 1999). The 1971 MW 6.7 San Fernando earthquake, southward of the frontal fault system along the San Jose fault which ruptured the westernmost 20 km of the Sierra Madre system, then the kinematic arguments above suggest that the fault, terminated against this left step (Allen et al., 1975; reverse-slip rate for the Cucamonga fault should be faster Heaton, 1982; Yeats, 1987). During this oblique, left-lateral- than that of the eastern Sierra Madre fault (Dolan et al., reverse event, two parallel, 45Њ to 50Њ north-dipping strands 1995, 1996; Walls et al., 1998). Although the reverse-slip of the Sierra Madre fault ruptured (Heaton, 1982), but along rate of the Cucamonga fault has been determined to be ϳ2– the eastern half of the rupture plane, surface rupture occurred 5 mm/yr (Matti et al., 1982, 1992; Morton and Matti, 1987, only along the southern strand (USGS Staff, 1971; Barrows 1993; Matti and Morton, 1993; Dolan et al., 1995, 1996; et al., 1975; Oakeshott, 1975). The area between the two Walls et al., 1998), no geological slip rate for the eastern strands is marked by the Merrick Syncline, a major fold in Sierra Madre fault has been published. Tertiary strata (Barrows et al., 1975; Tsutsumi and Yeats, Farther west, the kinematics of possible fault interac- 1999). tions between the Sierra Madre, the Raymond, and the Ver- dugo faults are suggested by the geometric and spatial re- Tectonic Geomorphology of the Eastern lations between these faults. The east-northeast-trending Sierra Madre Fault Zone Raymond fault intersects the Sierra Madre fault 50 km west of the left step in the frontal reverse fault system at San The active trace of the Sierra Madre fault zone is char- acterized by several 7- to 15-km-wide, convex-to-the-south Antonio Canyon (Fig. 1). Similar to the San Jose fault, the lobes, in contrast to the relatively linear, east-trending sur- Raymond fault is a left-lateral strike-slip fault, as expressed face trace of the Cucamonga fault to the east (Ehlig, 1975; by the tectonic geomorphology of the fault zone and the Bull, 1987; Crook et al., 1987; Morton and Matti, 1987; focal mechanism of the 1998 M 4.9 Pasadena earthquake W Ziony and Jones, 1989; Dolan et al., 1996) (Fig. 1). In many (Jones et al., 1990; Weaver and Dolan, 2000). The Raymond locations, the front of the San Gabriel Mountains exhibits fault may act to transfer slip from the Sierra Madre fault several strands of the fault, many of which juxtapose bed- southwestward onto the Verdugo fault, a major northeast- rock against older alluvium or older alluvium against dipping, reverse or oblique right-lateral-reverse fault system younger alluvium (e.g., Pentegoff et al., 1965; Proctor et al., that parallels the Sierra Madre fault for 30 km (Ziony and 1970, 1992; Bull, 1987; Crook et al., 1987). Many of these et al., et al., Jones, 1989; Wright, 1991; Dolan 1995; Walls bedrock strands, however, do not exhibit topographic ex- 1998; Tsutsumi and Yeats, 1999). If the Raymond fault does pression or much evidence of recent uplift, and geomorpho- serve as a transfer zone between the Sierra Madre–Cuca- logical observations by us and others indicate that the most monga fault system and the Verdugo fault, then the Verdugo recently active fault trace generally lies at, or to the south fault may accommodate some of the north–south shortening of, the main topographic mountain front (Bull, 1987; Crook accommodated by the Sierra Madre fault to the east. Dolan et al., 1987). These observations suggest that in many lo- et al. (1995) assumed a combined western Sierra Madre– cations the active strand of the fault has propagated south- Verdugo fault slip rate of 4 mm/yr, but no geologically con- ward from the mountain front. This inference is consistent strained slip-rate estimate for the Verdugo fault is currently with several earlier geological and geomorphological studies available. Alternatively, the Raymond fault may be the east- of the location of the Sierra Madre and Cucamonga fault ward continuation of the reverse-left-lateral Santa Monica– zones to the west, and east, respectively, of our study area Hollywood fault system (Weaver and Dolan, 2000). In either that have revealed older, inactive fault strands to the north case, the location, geometry, and kinematics of the Raymond of the most recently active strand (e.g., Bull, 1987; Crook fault indicate that some of the reverse component of slip et al., 1987; Morton and Matti, 1987; Leighton and Asso- accommodated by the Sierra Madre fault to the east of its ciates, 1992). intersection with the Raymond fault is accommodated to the The large-scale geomorphology and geology of the south of the Sierra Madre fault farther west. This observation mountain front also suggest that the most recently active in turn implies that the reverse-slip rate of the Sierra Madre strand of the fault has propagated southward. This phenom- fault may be somewhat faster to the east of the Raymond enon is illustrated in at least three locations along the Sierra fault intersection. Several reverse-slip-rate estimates for the Madre fault: (1) at San Dimas, along the eastern part of the 236 A. Z. Tucker and J. F. Dolan fault; (2) at Bradbury, 14 km to the west; and (3) near the Fernando rupture at Oak Hill revealed evidence for a pre- west end of the fault along the Merrick Syncline (“Little 1971 surface rupture in the form of a colluvial wedge inter- Tujunga Syncline” of Ehlig, 1975). In these locations, the preted to have developed by collapse of a scarp (Fig. 1) most recently active fault trace appears to have stepped 1 to (Bonilla, 1973). This pre-1971 colluvial wedge contained a 4 km southward, generating lobe-shaped hanging-wall single wood fragment that yielded a radiocarbon date of 200 years (Bonilla, 1973), suggesting the possibility that 100 ע -blocks that typically expose Tertiary–Quaternary sedimen tary and volcanic rocks (Fig. 2) (Eaton, 1957; Pentegoff et the penultimate event on the western Sierra Madre fault may al., 1965; Proctor et al., 1970, 1992; Ehlig, 1975; Barrows have occurred during the past several hundred years. et al., 1975; Bull, 1987; Crook et al., 1987; Dibblee, 1991a, Trenches excavated at two sites along the 1971 rupture trace b). Bull (1987) referred to these uplifted lobes as “piedmont by Fumal et al. (1996) (T. Fumal, personal comm., 2000) forelands.” yielded tentative evidence in support of this interpretation, These lobes are bounded on the south by the most re- but further age control is required to confirm this result. cently active fault trace and on the north by steep, south- As part of a comprehensive seismic hazard analysis of facing topographic fronts of the crystalline Mesozoic igne- the fault zone, Crook et al. (1987) excavated more than a ous and metamorphic rocks of the main San Gabriel dozen paleoseismologic trenches across the central and west- Mountains. The fact that these lobes are mantled by Terti- ern parts of the Sierra Madre fault. They found evidence for ary–Quaternary sedimentary and volcanic rocks indicates the fault at numerous sites, but because of the absence of that they have experienced much less exhumation than the datable material in their trenches, they were unable to de- crystalline block of the high-standing San Gabriel Moun- termine the precise ages of any past surface ruptures. The tains to the north. This inference is supported by relatively absence of datable material is probably at least partially a old apatite fission-track ages from two localities near the function of the fact that the accelerator mass spectrometer topographic mountain front (Blythe et al., 2000). (AMS) 14C dating technique, which facilitates age determi- We interpret the pronounced topographic inflection nations of very small carbon samples was not available to point at the base of the crystalline rocks at the northern edges these researchers. Although they were not able to directly of these lobes as the site of older Sierra Madre fault strands date any past events, Crook et al. (1987) suggested, on the that bound the crystalline basement (e.g., “Sierra Madre basis of soil development and geomorphologic observations, fault” of Pentegoff et al., [1965] and Proctor et al., [1970, that the central reach of the fault (east of La Crescenta; Fig. 1992] in the San Dimas area, and the fault exposed along 1) has not ruptured for at least several thousand years, and the northern edge of the Little Tujunga/Merrick Syncline perhaps as long as 11,000 years (the oldest soil age estimate [Ehlig, 1975; mountain front 7C of Bull, 1987]). Coupled that they tentatively assigned to their oldest unfaulted fan with geomorphologic evidence for active reverse faults deposit [their unit 2]). along the southern edge of these lobes, we agree with Bull The conclusion of Crook et al. (1987) was at least par- (1987) in interpreting these relationships as evidence for tially supported by paleoseismologic observations from a relatively recent southward propagation of the Sierra Madre trench in Altadena, along the central reach of the fault, where fault outward from the trace of the mountain-front fault. A Rubin et al. (1998) saw evidence for only two surface rup- similar situation may occur along the easternmost 8 km of tures during the past 15,000 years. Together, these two the Sierra Madre fault, just west of the major left step be- events produced ϳ10.5 m of reverse slip, yielding an aver- tween the Sierra Madre and Cucamonga faults (shown as age slip/event of ϳ5 m at this site; the most recent event “possible older mountain-front fault” on Fig. 2). There, a produced at least 4.2 m of reverse slip. However, the pres- prominent topographic front (mountain front 21A of Bull ence of abundant reworked detrital charcoal fragments of [1987]) occurs approximately along the strike of the Cuca- widely different ages in their trench prevented Rubin et al. monga fault trace to the east. To the south of this topographic (1998) from precisely dating the most recent surface rupture. front, topographically much lower foothills occur between the crystalline mountain front and the most recently active This Study trace of the Sierra Madre fault, resulting in a lobe-shaped, As part of a more regional study of the history of inter- fault-bounded zone of recent uplift similar to those described actions between major faults of the northern Los Angeles above (Fig. 2). metropolitan region (Dolan et al., 1995, 1996, 1997, 2000a, b; Fumal et al., 1995; Lindvall et al., 1995; Rubin et al., 1998; Walls et al., 1998; Weaver and Dolan, 2000) we con- Paleoseismology of the Sierra Madre Fault ducted a detailed air-photo and field-mapping analysis of the tectonic geomorphology of the eastern part of the Sierra Ma- Investigations dre fault zone in order to identify suitable paleoseismologic The first paleoseismologic studies of the Sierra Madre trench sites. Our ultimate goal is to compare paleoseismo- fault were initiated in the aftermath of the 1971 San Fer- logic results from the eastern Sierra Madre fault with similar nando earthquake. A trench excavated across a hanging-wall data from sites on adjacent faults (e.g., the central reach of strand of the western Sierra Madre fault along the 1971 San the Sierra Madre fault, and the Cucamonga, Raymond, and Paleoseismologic Evidence for a Ͼ8 Ka Age of the Most Recent Surface Rupture on the Eastern Sierra Madre Fault 237

Verdugo faults; Fig. 1) in order to develop a space-time his- borehole (D-12G) along the tunnel alignment (Fig. 4). The tory of earthquake occurrence in the Los Angeles region. main fault strand projects to the surface at the topographic Our geomorphologic analysis reveals that, east of base of slope at our trench site. Projection of two boreholes Azusa, the most recently active trace of the Sierra Madre (D-15G and D-18G) that were drilled along Sycamore Can- fault generally extends along the topographic break-in-slope yon westward onto the cross section suggests that the fault along the southern edge of the San Gabriel Mountains (Fig. steepens below 135 m depth to a dip of ϳ25Њ north (Fig. 4). 2). The active trace of the fault is defined by intermittent The absence of deep borehole data from directly along the south-facing scarps, faceted spurs, vegetation lineaments, main borehole transect, however, makes this conclusion groundwater anomalies, and changes in stream gradients and somewhat speculative. sinuosity (Crook et al., 1987). These features have allowed us to construct the detailed map of the active trace of the Paleoseismologic Investigations at the Horsethief easternmost 20 km of the Sierra Madre fault shown in Fig- Canyon Site, San Dimas: Trench and Large-Diameter ure 2. Borehole Results We concentrated our search for paleoseismologic trench We excavated a 62-m-long, 5-m-deep trench across the sites in the San Dimas area, along one of several geomorph- geomorphically defined main strand at Horsethief Canyon ically defined lobes where the Sierra Madre fault zone ap- Park, a San Dimas city park that is still largely undeveloped pears to have propagated southward from the crystalline (Figs. 3 and 5). The main active strand in this reach of the mountain front. In this region, the hanging wall of the main fault extends along the topographic base of the southern edge active trace consists predominantly of pale yellow-gray of the foothills of the San Gabriel Mountains, where it is mudstone of the Miocene Puente Formation and purple to locally obscured by small landslides (Fig. 5). Our N20Њ– dark gray basalt and associated rocks of the Glendora Vol- 35ЊW-trending trench was excavated along the eastern mar- canics, also of Miocene age (Eaton, 1957; Pentegoff et al., gin of a small canyon that has incised into the mountain 1965; Proctor et al., 1970, 1992). The active strand of the front, thereby creating a re-entrant that allowed us to cross fault separates these rocks from Pleistocene–Holocene al- the entire width of the main active fault zone in an area of luvium to the south. Our air-photo and field analysis revealed active sediment accumulation (Fig. 5). After the trench walls the presence of several closely spaced strands that generally were mapped, we backfilled the trench and then excavated extend along, or just to the south of, the topographic moun- a transect of eight large-diameter (70 cm) boreholes (com- tain front. This narrow zone of faults is traceable continu- monly known as bucket-auger holes) along the length of the ously both to the east and west of the study area (Fig. 3). trench directly through the back-filled trench in order to de- We sited our trench along what our geomorphologic fine the geometry of the fault and the deformed strata below analysis suggests is the main active strand of the Sierra Ma- the trench. The walls of boreholes H1, H3, and H7 were dre fault in this reach (Fig. 3). This trace is marked by south- examined directly by lowering a geologist downhole, facing scarps and a generally linear mountain front that ex- whereas the remaining boreholes were described by exami- hibits triangular facets at the southern terminations of many nation of cuttings taken every 25–50 cm of borehole depth. ridges. In addition, the fault crossings of several canyons, including the small canyon along which we excavated our Stratigraphy. We encountered 13 distinct stratigraphic trench, are marked by vegetation lineaments, suggestive of units in the trench and large-diameter boreholes (Fig. 6). We either springs or ponding of groundwater along the fault. refer to these as units 1 through 13, from youngest to oldest. Vegetation lineaments and a linear alignment of several Six of these units were exposed in the trench. The upper four springs suggest the presence of at least two fault strands to units extend most of the length of the trench, but the deeper the north of the main, active strand at our trench site. Neither units are truncated by a prominent fault (discussed below). of these apparent fault strands, however, has any topographic The fault separates the lower part of the trench and the bore- expression, suggesting that they are either no longer active, hole transect into two distinct lithologic sections, with al- or are much less active than the topographically well- luvial units exposed below and to the south of the fault and expressed main strand that we trenched. Several small, sur- bedrock overlain by alluvium exposed above the fault. All the ficial landslides have locally obscured the active fault trace colors mentioned in this text are from the Munsell Soil Color (Fig. 5). Chart (1994), and all the locations are listed in meters to the Our trench was located 150 m southwest of a major north of the south end of the trench (e.g., m 34 describes a water-diversion tunnel (Fig. 3). Geotechnical investigations location that is 34 m north of the southend of the trench). for this tunnel project included a series of deep boreholes, The southern half of the trench, south of the projected as well as examination of the tunnel bore itself (Pentegoff topographic base of slope along the scarp to the east, is cap- et al., 1965; Proctor et al., 1970, 1992). Several of the bore- ped by a 0- to 2-m-thick section of historic fill that contains holes and the tunnel penetrated the main strand of the Sierra numerous man-made objects (e.g., barbed wire), as well as Madre fault (their “Cucamonga fault”), which dips north- several Ͼ70-cm-diameter, 3-m-long buried tree trunks. The ward at ϳ11Њ north to a depth of at least 135 m, the maxi- basal contact of the fill sequence with unit 1 is sharp. mum depth in which it was encountered in the northernmost Unit 1 is the A horizon of the active surface soil. In the 238 A. Z. Tucker and J. F. Dolan

wind gaps

1800

1400

1600

scarps 1200

WC

920 Fig. 4

1400 1200 D-18G

ShC D-15G springs 960 VL D-12G

SC D-13G 1200

AVE wind gaps VL VL A-4G SAN faceted D-17G Fig. 5 A-1G

AMELIA AVE AMELIA spurs VL scarp at edge FOOTHILL V.L. ? DIMAS VL of drainage 1080

BLVD CW scarp

HORSETHIEF CANYON

LONE HILL 0 CANYON SITE 100 1000

fluvial AVE

1020

SAN DIMAS SAN BASELINE RD RD

I-210 0 0.5 1 1.5 km N contours in feet

Figure 3. Map of Horsethief Canyon study site. Shaded box shows location of Fig- ure 5. Note location of geotechnical boreholes (gray-filled circles) used to construct cross section shown in Figure 4 along route of water-diversion tunnel (Pentegoff et al., 1965; Proctor et al., 1970, 1992). Irregular patches of pale gray shading denote young landslides. VL, vegetation lineament; ShC, Shuler Canyon; SC, Shay Canyon; SyC, Sycamore canyon; WC, Wildwood Canyon. Topography is digitized from U.S. Geo- logical Survey San Dimas 7.5-minute quadrangle. northern part of the trench, unit 1 extends to the surface, 15 m of the trench, the lower part of unit 2 contains several whereas in the southern half of the trench it underlies, and pockets and lenses of pale grayish yellow-brown pebble to is locally cut out by, the historical fill. Unit 1 consists of a cobble gravels consisting of Puente Formation mudstone medium to dark gray-brown, organic-rich, silty, fine- to clasts. Several of these gravels exhibit oblique channel cross medium-grained sand that locally contains disseminated sections. Unit 2 overlies unit 3 along a sharp, undulating small pebbles of Puente Formation mudstone. The unit contact marked by local small pebble gravel lags along the ranges in thickness from 70 to 130 cm over most of the base of unit 2 (Fig. 6). trench, but locally reaches ϳ300 cm at the north end of the Unit 3 is a pale yellow-brown to slightly reddish yellow- trench. Unit 1 overlies unit 2 along a gradational contact that brown (7.5YR 4/4 to 10YR 5/4) silty clay with ϳ20% is locally marked by discontinuous stringers of pebble gravel pebble-sized Puente Formation mudstone clasts. Most of consisting of Puente Formation mudstone clasts. In the unit 3 is massive, but it locally exhibits horizontal to gently southern part of the trench, where unit 1 is locally cut out south-dipping sandy silt layers. Unit 3 extends from the by the historic fill, the contact between the fill and unit 2 is southern end of the trench northward for 45 m, where it sharp. pinches out to the north. At the southern end of the trench, Unit 2 is a pale yellow-brown (10YR 4/5 to 10YR 5/5), the unit is ϳ4 m thick, gradually thinning northward to ϳ2 clayey, silty sand that contains sparse, disseminated Puente m at 34 m. At 34 m, unit 3 abruptly thins to ϳ1 m thickness Formation mudstone pebbles and granules. In the northern along an irregular, interfingering contact. We examined this Paleoseismologic Evidence for a Ͼ8 Ka Age of the Most Recent Surface Rupture on the Eastern Sierra Madre Fault 239

Fault in deep boreholes projects to Proposed Landslide surface at scarp/trench exposure SD-1 Glendora Pentegoff et al. (1965) Volcanics Figure 6 (projected) D-12G D-17G & Puente Formation A-4G Projection of Trench SD-1 400 D-13G

Elevation (meters) D-18G D-15G FAULTS? A-1G #16 Water Well GLENDORA TUNNEL THRUST Qt 300 Topanga Formation FAULT Break in Section Tunnel bearing N39W Sycamore Canyon Topographic Profile 200 NOTE: Boreholes D-15G and D-18G 0 100 are projected onto section westward 200 from Sycamore Canyon. 50 150 250 m

Figure 4. N40W cross section constructed along route of water-diversion tunnel across the major active strand of Sierra Madre fault at Horsethief Canyon in San Dimas (Cucamonga fault strand of Pentegoff et al., 1965; Proctor et al., 1970, 1992). Note location of Figure 6. See text for discussion.

contact in some detail in order to determine whether or not and appears to pinch out to the south of borehole H1. Unit it is a fault. Our investigations, however, showed that it was 5 overlies unit 6 along a gradational contact. a purely sedimentary feature. Unit 6 consists of medium brown to medium purplish Unit 3B is a cohesive, pale yellow-brown (10YR 5/4) brown (10YR 4/3) silty clay to silty sand that is generally sandy clay with 5–10% pebbles. It is distinguished from similar to overlying unit 5 except for a much lower gravel overlying unit 3 by having fewer clasts and a higher clay content (Ͻ10%–40% clasts) and a less purplish color. Unit content. The base of unit 3B is marked by a pebble-cobble 6 can be traced laterally for ϳ20 m in boreholes H6, H7, gravel between m 10 and m 15. South of ϳm 6, unit 3B H2, and H1. Unit 6 overlies unit 7 along a relatively sharp cannot be distinguished from overlying unit 3. contact (gradational over Ͻ5 cm). The part of unit 4 that is exposed in the trench is a Unit 7 is a medium to dark brown to yellow-brown cohesive, red-brown (8.5YR 5/6) silty clay with 5%–15% (7.5YR 3/4 to 10YR 4/3 to 10YR 5/4) clay to clayey sand pebble-sized Puente Formation mudstone clasts. In the that contains Ͻ5% clasts larger than sand size. The dark northern part of the trench, unit 4 contains several pebble- color and abundant clay in this unit lead us to interpret it as to cobble-gravel channels consisting of Puente Formation a buried soil horizon. This unit, which can be correlated mudstone clasts. Farther south, in boreholes H1 and H3, unit across all five southern boreholes, dips southward at 10Њ, 4 contains more gravel (15%–30%) and is a pale yellow- subparallel with the ground surface. brown. Unit 4 overlies unit 5 along a sharp to locally gra- Unit 8 consists of pale brown to locally dark brown dational (Ͻ15 cm) contact. (10YR 3/3 to 10YR 5/6) clay, clayey sand, and fine-grained Unit 5 is a moderately well-indurated, pale purplish sand with a southward decreasing amount of gravel. Pebbles brown (10YR 4.5/3 to 10YR 5/4, with variable purplish and cobbles within unit 8 were derived from both the Puente hue), clast-supported pebble to cobble conglomerate. More Formation mudstone and Glendora Volcanics. In boreholes than 95% of the clasts consist of purple-gray volcanic rocks H6 and H7, the unit contains ϳ60% pebbles and cobbles, derived from outcrops of the Miocene Glendora Volcanics whereas in more southerly borehole H1 and H3, the unit to the north and northeast of the trench. The matrix consists contains Ͻ20% clasts. of a pale purplish gray-brown silty sand. This unit can be Unit 9 is a southward-thickening wedge composed of traced southward as a southward-thinning wedge from its clast-poor, generally dark brown (7.5YR 3/2 to 10YR 4/4) exposure at the base of the trench through boreholes H6, H7, clay with 5%–20% pebbles and cobbles derived from the H2, and H1 (Fig. 5). Unit 5 is not exposed in borehole H3 Puente Formation, Glendora Volcanics, and granitic rocks. 240 A. Z. Tucker and J. F. Dolan

Trail

orse H SD-H4

SD-H5 TRACE FAULT SD-H6 Qls SD-H7 TRAIL SD-H2 Qls SD-H1 QlsTRENCH SD1 SD-H3 Qls VEGETATION LINEAMENT

? Dirt Road ?

contours in feet 0 25 50 75 meters N

Figure 5. Detailed topographic map of our Horsethief Canyon study site showing location of trench SD-1 and transect of large-diameter boreholes. Note pronounced change in slope at fault trace. Also note approximate borehole locations from earlier studies of Pentegoff et al. (1965) and Proctor et al. (1970; 1992); “D” borehole prefix is for small-diameter boreholes drilled with a diamond bit, whereas “A” prefix denotes large-diameter bucket-auger boreholes. Topographic contour interval is 5 feet (1.5 m) in steep terrain and 1 foot (0.30 m) in flatter areas. Topography digitized from detailed topographic map of Horse Thief Canyon Park, kindly provided by the City of San Dimas.

Units 10 is made up of highly deformed pale yellow- 2- to 5-m-thick, orangish yellow-brown (7.5YR 4/6) to brown (2.5 Y 6/6 to 5Y 6/4) Puente Formation mudstone yellow-brown (10YR 5/8) sand with abundant angular frag- that is exposed only in the hanging wall of the prominent ments of granitic rock; it is unclear if these are individual fault described below. Unit 10 comprises several 25- to 50- clasts in a gravel or whether they are more intact pieces of cm-thick blocks of intact, well-bedded mudstone, as well as a pod or lens of granitic rock interleaved with the Puente more brecciated pods of altered, iron oxide–stained mud- Formation mudstone of unit 10 to the south. There is a no- stone (7.5YR 5/8). Bedding in the intact blocks dips 35Њ table absence of Puente Formation mudstone clasts in unit north, parallel with the underlying fault plane. Several 11. We suspect that the sand observed in the cuttings is ac- prominent shear zones, some of them marked by 5- to 35- tually the remains of an intact granitic body that was ground cm-thick white to purple-gray clay gouge zones, cut through to sand by the bucket-auger cutting teeth during drilling. The unit 10. presence of intact granitic rock in the borehole could not Units 11 and 12 were encountered only in boreholes H4 be confirmed because, as noted above, neither borehole in and H5 (Fig. 5). These two boreholes were not logged di- which unit 11 was encountered was logged downhole. rectly due to unsafe borehole conditions (water in bore- Unit 12 is a friable, blue-gray (Gley 2 colors) sand with holes). Thus, these units, unlike the other stratigraphic units angular granodiorite clasts (ground-up bedrock?) and sparse we encountered, are known only from cuttings obtained Glendora Volcanics pebbles and cobbles. every 25 to 50 cm. These cuttings indicate that unit 11 is a Directly below unit 12, beneath the fault gouge dis- 0 10 -15 S Outline (SD H3) of Trench 20910 +/- 70 BP UNIT 7 FILL

Ground Surface SD-H3 ? UNIT 3b 79 (SD-41) ± UNIT 9 UNIT 4 UNIT 8 Dimas, CA UNIT 3 SD-H1 BC 6252 Sierra Madre Fault SD-H2 East Wall of Trench SD-1 Trench of Wall East 29 ± (SD-26) Horse Thief Canyon Park, San Park, Thief Canyon Horse SD-H7 UNIT 6 UNIT 5 10 BC 5467 SD-H6 92 ± UNIT 4 5 sheared clay meters UNIT 1 East of Trench UNIT 10 (SD-14)

Ground Surface 25 m 0 BC 3980 Cross section showing east wall of trench SD-1 and large-diameter borehole results from the Horsethief Canyon site. UNIT 2 SD-H5 6. 60 50 40 30 20 10 0 ? fault zone fault ? ? UNIT 13

Figure Large-diameter boreholes were drilled directlyoutlines through are back-filled not material shown along original where alignment they of cross trench. the For clarity, original the trench. borehole No vertical exaggeration. Water Table Water in sheared clay

Ground Surface ? UNIT 2 UNIT 11 UNIT 12 70 SD-H4 N 0 -10 10

241 242 A. Z. Tucker and J. F. Dolan cussed below, is alluvial unit 13. This unit is a purple to Evidence for Faulting. A prominent fault zone was ex- brown sand and clay that locally contains pebbles derived posed in the base of the trench and in the northernmost two from the Puente Formation mudstone facies and Glendora boreholes (Fig. 6). The fault, where it is exposed in the Volcanics. In general, this unit is quite similar to several trench, consists of a 10- to 30-cm-thick, intensely sheared shallower units exposed in the footwall of the fault. The pale-gray clay gouge. The clay gouge thickens down-dip, 4-m thickness of unit 13 that was exposed in borehole H5 and it is 75–120 cm thick in boreholes H4 and H5. The fault consists of a medium- to dark-brown (2.5Y 3/2 to 10YR separates completely different stratigraphic sections, with 3/6) silty sand matrix with 30%–50% petroleum-stained sheared and brecciated, pale yellow-brown Puente Forma- Puente Formation mudstone clasts. Below the oil-stained tion mudstone of Miocene age exposed in the hanging wall gravel is a pebble to cobble gravel containing 30%–50% and late Pleistocene to early Holocene(?) alluvium of units clasts in a purple-brown sand matrix (2.5Y 3/2 to 10YR 5–10 exposed in the footwall. The fault strikes N40ЊE and 5/4 with variable pale purple hue). In borehole H4 unit 13 dips northward at 22Њ in the uppermost 18 m. The near- is a pale yellow-brown to dark brown (10YR 3/2 to 10YR surface part of the fault exposed in the trench and boreholes 5/6) clay with 5%–10% pebble gravel and local sand string- projects down-dip directly to the gently north-dipping main ers and thin beds with Ͻ10% pebble-sized clasts. Below unit strand of the fault observed in the water-diversion tunnel 9 in boreholes H6, and H7, unit 13 consists of a pale to dark boreholes adjacent to our trench site (Fig. 3) (Pentegoff et yellow-brown (10YR 3/2 to 10YR 5/8) sand with 10%–20% al., 1965; Proctor et al., 1970; 1992). Puente Formation mudstone and Glendora Volcanics peb- In the base of the trench, the clay gouge is truncated bles. Further south, in borehole H1, unit 13 is a brown along a sharply defined, erosional unconformity at the base (10YR 4/4) silt to fine-grained sand matrix with 25%–60% of unit 4 that is not faulted. The shallowest part of the fault Glendora Volcanics, Puente Formation, and granitic pebbles juxtaposes sheared, pale yellow-brown Puente Formation and small cobbles. mudstone in the hanging wall against purple-gray gravel of unit 5 (Figs. 6, 7). Several of the faulted alluvial units in the Age Control. We recovered four detrital charcoal samples footwall, including the dark brown clay of unit 7, can be from our excavations, three from the trench and one from traced northward through several boreholes to within 10 m borehole H3, for accelerator mass spectrometer (AMS) ra- of the fault (Fig. 6). Any hanging-wall strata correlative with diocarbon dating (Figs. 6 and 7; Table 1). Samples SD-14, these footwall units must have been deposited above the SD-26, and SD-41 were collected from unfaulted strata di- Miocene bedrock currently exposed in the hanging wall at rectly above the main fault zone exposed in the trench (dis- the base of the trench. Thus, projection of the base of unit 7 cussed below). These samples yielded calibrated, calendric to its prefaulting position requires at least 14 m of reverse 6272 ,123 ע .yr, 3920 B.C 71 ע .AMS ages of 5367 B.C respectively (Table 1). The ages of the samples slip. This reverse-slip estimate is a minimum because any ,98 ע .B.C indicate that they are in correct stratigraphic order, suggest- strata that were correlative with unit 7 have been eroded off ing that they have not been reworked. All three of these the hanging wall prior to deposition of unit 4. samples were small and consequently received only a single The fault locally controls the depth of near-surface acid wash in pretreatment. Samples SD-14, SD-26, and SD- groundwater. Groundwater was encountered immediately 41 were recovered from 3 to 4 m depth, however, and we above the fault in hanging-wall boreholes H4 and H5. We therefore do not think they have been significantly contam- interpret this as evidence for groundwater flowing along and/ inated by young illuviated carbon. or ponded against the impermeable clay of the fault gouge. We also recovered a detrital charcoal fragment (SD-H3) In contrast, the only groundwater encountered in the foot- from the drill rig cutting teeth when borehole H3 had reached wall of the fault was in borehole H6, where the gravels at a depth of 12.4 m. We are confident that the detrital charcoal the base of unit 5 were wet. We interpret this as evidence fragment was from unit 7, as we were careful to clean the for groundwater flowing southward from the fault zone cutting teeth after each drill run, and the sample was taken through the permeable unit 5 gravels along the top of the from a freshly broken face of a coherent mass of distinctive impermeable clay of underlying unit 6. unit 7 clay. We emphasize that the color and grain size of Relationship of Horsethief Canyon unit 7 are very distinctive, as demonstrated by direct down- Trench/Borehole Observations to Water-Diversion hole observations. This charcoal sample, which received a Tunnel/Borehole Observations full acid-alkali-acid wash in pretreatment, yielded an uncal- years B.C. Although this Possible landsliding in the vicinity of our trench and 70 ע ibrated AMS date of 20,930 sample is beyond the maximum calendric calibration age of along the youngest trace of the Sierra Madre fault nearby CALIB 4.1.2 (Stuiver and Reimer, 1993), we can use the 14C may introduce confusion between shear surfaces related to production rate curves of Voelker et al. (1998) to estimate faulting and those related to slope failure. For example, on the approximate calendric age of this sample at ϳ22,000 the basis of their borehole results from between our trench B.C. Error bars are difficult to determine for this crude ca- site and the water-diversion tunnel 125 m to the east, Pen- lendric age estimate, but they are probably on the order of tegoff et al. (1965) and Proctor et al. (1970, 1992) suggested years. the presence of large, deep-seated landslides beneath the 1,000עϳ Paleoseismologic Evidence for a Ͼ8 Ka Age of the Most Recent Surface Rupture on the Eastern Sierra Madre Fault 243

Figure 7. Detail of northern part of the east wall of trench SD-1 showing the lo- cations of the three detrital charcoal samples recovered from unfaulted alluvium in trench SD-1.

Table 1 toe of a major, southward-directed landslide (Pentegoff et Radiocarbon Samples and Ages (in Years) for Sierra Madre al., 1965; Proctor et al., 1970, 1992; R. Proctor, personal Fault Trench SD-1 comm., 2000). Along the tunnel route, this slide surface would project to the ground surface Ͼ50 m south of the (sigma) Calendric Age (2 sigma 1 ע .Sample No. Beta Lab No. 14C Age (B.P topographic mountain front (Fig. 4 is constructed along the tunnel alignment). Although the shear surface noted in 71 ע B.C. 5367 40 ע SD-14 Beta-123849 6380 123 ע B.C. 3920 50 ע SD-26 Beta-125076 5140 -the water-diversion tunnel boreholes may be the basal de 98 ע B.C. 6272 40 ע SD-41 Beta-123851 7330 -Beyond calibration; tachment of a large, old landslide, the lack of any geomor 70 ע SD-H3 Beta-123852 20,910 see text for discussion phic expression of this feature to the south of the mountain front suggests that it is no longer active. Moreover, in our Calendric ages were calculated using CALIB 3.0 (Stuiver and Reimer, study area just to the west of the earlier water-tunnel bore- 1993; method B). holes, the presence of an undeformed, well-defined alluvial section Ͼ20 m thick in the footwall of the fault rules out the southern mountain front (Fig. 4). In their large-diameter presence of large, deep-seated landslides that extend south boreholes located along, and just south of the mountain front of the topographic mountain front. (e.g., A-4G in Fig. 3), geologists lowered down into the The cross section constructed along the water-diversion boreholes noted the presence of a gently north-dipping shear tunnel route shows that a thin sheet of alluvium extends plane, which they interpreted as the basal slip plane near the beneath the Sierra Madre fault for at least 250 m to the north 244 A. Z. Tucker and J. F. Dolan of the surface trace of the fault (Fig. 4). The presence of this period between the maximum 24 ka and 8 ka limits provided alluvium beneath the fault far to the north of the surface trace by the trench and borehole observations; and (3) the single, of the fault indicates major thrust motion along the fault ϳ24 ka detrital charcoal sample recovered from borehole strand identified in the water tunnel. The alluvium beneath H3 was reworked from an older deposit, and had a signifi- the fault overlies rocks that are interpreted by Pentegoff et cant age when it was incorporated into unit 7. In this last al. (1965) and Proctor et al. (1970, 1992) to be of Miocene case, unit 7 would be younger than the age of the charcoal age (“Topanga Formation” in the footwall of fault in Fig. sample, and the Ն14 m of reverse slip could have occurred 4). The presence in our trench at the southern tip of the during a much shorter period than 24–8 ka. Whatever the hanging-wall block of Miocene-aged bedrock, which may exact reverse-slip rate on the fault, the long elapsed time correlate with the Miocene bedrock exposed beneath the un- since the most recent event on the main active strand of the derthrust alluvium in the footwall of the fault, provides a Sierra Madre fault suggests that at least the eastern part of means of estimating the thrust displacement on this strand. the fault ruptures in infrequent, but therefore probably large, A simple measurement of the distance along the dip of the events. fault from the southernmost hanging-wall exposure of Mio- Any colluvial material that may have accumulated as a cene bedrock at the north end of our trench to the deepest result of scarp collapse during the most recent surface rup- and northernmost location of the alluvium-over-Topanga ture at the trench site have been eroded, and we therefore Formation contact in the footwall of the fault indicates a cannot directly measure slip per event. Nevertheless, we can minimum of ϳ260 m of reverse slip on this fault (Fig. 4). use the minimum reverse-slip rate and the Ն8000-year-long We note, however, that it is possible that the rocks that Pen- interval since the most recent surface rupture to speculate tegoff et al. (1965) and Proctor et al. (1970, 1992) interpret about the magnitude of potential earthquakes on the eastern as Miocene bedrock (“basal conglomerate of [Miocene] Pu- Sierra Madre fault. The minimum 0.6–0.9 mm/yr reverse- ente Formation or Topanga Formation”) might be easily con- slip rate implies that at least 4.6 to 7.0 m of elastic strain fused in a drill core with much younger alluvium. In this energy has been stored on the strand we trenched during the case, rocks of Miocene age in the footwall of the fault would past ϳ8000 years. If we assume that all of this strain were lie deeper than interpreted by Pentegoff et al. (1965) and to be accommodated during a single rupture, we can use Proctor et al. (1970, 1992), and total post-Miocene slip on regressions of average and maximum slip against moment this strand of the Sierra Madre fault at the study site may be magnitude to estimate the sizes of potential Sierra Madre much greater than the 260-m minimum value we derive. fault events. If 4.6 m is the average slip across the rupture plane in ע ϳ Discussion a future event, this is equivalent to a MW 7.5 0.2 earth- quake; if 4.6 m is the maximum slip, this would be a MW event (Wells and Coppersmith, 1994). If 7.0 0.15 ע Our paleoseismologic observations indicate that it has ϳ7.2 ϳ been at least 8000 years since the most recent surface rupture m is average slip in a future event, this would be a MW 7.6 earthquake (Wells and Coppersmith, 1994), yielding 0.2 ע .on the strand of the Sierra Madre fault that we trenched ϳ Although several strands of the fault exist in the study area, a range of potential values of MW 7.0 to 7.8. We re- the geomorphologic data indicate that the strand that we emphasize, however, that the slip rate upon which these trenched is the only one with discernible topographic ex- magnitude estimates are based is a minimum value, and that pression, suggesting that it is the main, and probably the the higher end of this magnitude range is therefore probably only, active strand of the Sierra Madre fault zone in the more likely to be correct. The regressions of Dolan et al. vicinity of our trench site. This inference is supported by our (1995), specific to southern California earthquakes, yield a large-diameter borehole data, which indicate that this strand range of somewhat larger magnitudes from MW 7.5–7.9 for accommodated at least 14 m of reverse slip between average displacements of 4.6 m and 7.0 m; Dolan et al. ϳ 24,000 and 8000 years ago. This displacement is a mini- (1995) did not regress maximum slip against MW. mum, because strata correlative with unit 7 in the footwall The long elapsed time interval since the most recent have been eroded off the hanging wall at the trench site. event on the eastern Sierra Madre fault is consistent with These relationships yield an average, minimum reverse-slip results from elsewhere along the Sierra Madre fault zone. rate for this strand of at least ϳ0.6 mm/yr (measured over Only two events apparently have occurred along the central the interval 24 ka–present) to ϳ0.9 mm/yr (measured over reach of the fault since 15 ka in Altadena, 35 km to the west the interval 24 ka to 8 ka only). We prefer the latter estimate of San Dimas (Fig. 1) (Rubin et al., 1998). Moreover, our because the absence of deformation in the unfaulted early- estimate of a minimum 4.6 to 7.0 m of stored slip on the late Holocene strata exposed in the trench and boreholes eastern part of the fault is consistent with the suggestion by suggests that the fault has been locked and storing elastic Rubin et al. (1998) that the Sierra Madre fault produces an strain energy since at least 8 ka. average of ϳ5.5 m of reverse slip per event along its central The actual reverse-slip rate could be somewhat faster if reach. At both sites the fault has thus experienced large sur- (1) total slip has been much greater than the 14-m minimum; face displacements during latest Pleistocene–Holocene time: (2) the 14-m measured minimum slip occurred during a brief Ն14 m of reverse slip between 24 ka and 8 ka at San Dimas Paleoseismologic Evidence for a Ͼ8 Ka Age of the Most Recent Surface Rupture on the Eastern Sierra Madre Fault 245 and ϳ10.5 m of reverse slip in Altadena during the past 15 year-long current quiescent period on the eastern Sierra Ma- ka (Rubin et al., 1998). dre fault, however, shows that the fault ruptures much less While these similarities may be merely coincidence, the frequently than the San Andreas fault, which along the Mo- large displacements suggested by the colluvial wedges at the jave Desert reach exhibits a recurrence interval of ϳ100– Altadena site and the long elapsed time since the most recent 130 years (Sieh, 1978; Sieh et al., 1989; Fumal et al., 1993). event at the San Dimas site argue that the events recorded Thus, something on the order of 60 to 80 SAF Big Ones (MW at these sites are large enough to rupture long stretches of ϳ7.5–8 earthquakes) have occurred since the most recent the Sierra Madre fault. Such large displacements are typical surface rupture on the eastern Sierra Madre fault. While of ruptures that involve large fault-plane areas (Wells and these observations do not rule out the possibility that the Coppersmith, 1994; Dolan et al., 1995). The most likely Sierra Madre fault could rupture together with the San An- faulting scenario involves simultaneous rupture of large dreas fault in a scenario similar to the 1957 Gobi–Altay stretches of the Sierra Madre fault itself. If this is the case, earthquake, they indicate that such combined southern Cali- and if the long elapsed time since the most recent event on fornia events, if they ever occur, must be extremely rare. the eastern part of the fault is typical, then we speculate that This is not to say that a major event involving rupture the Altadena and San Dimas trench sites may record the of a large section of the Sierra Madre fault by itself would same earthquakes. not have serious repercussions for the Los Angeles metro- Alternatively, it is possible that the Sierra Madre fault politan region. The largest events that have occurred on could rupture together with contiguous faults, for example faults directly beneath the metropolitan region during the the Cucamonga or Raymond faults. Published paleoseis- 150- to 200-year-long historic period are the 1971 MW 6.7 mologic data, however, indicate that the Raymond fault has San Fernando and 1994 MW 6.7 Northridge events. The MW ruptured to the surface at least once, and possibly several Ͼ7 events suggested by the long elapsed time interval on times, since the most recent surface rupture on the eastern the eastern Sierra Madre fault and the estimates of large slip Sierra Madre fault (Fig. 8) (Crook et al., 1987; Weaver and per event along the central part of the fault (Rubin et al., Dolan, 2000). Bayarsayhan et al. (1996) have even sug- 1998) would be much larger than either the 1971 or 1994 gested that the Sierra Madre fault could rupture together with events, and would differ from them in several important re- the Mojave section of the San Andreas fault in southern Cali- spects. Specifically, strong ground motions generated by a Ͼ fornia’s “most devastating earthquake” scenario. Their sug- MW 7 Sierra Madre fault event would be experienced over gestion is based on the geometric similarity between the Si- a much larger region, and the duration of these strong ground erra Madre–San Andreas fault pair and faults that ruptured motions would be longer than for more moderate-sized ϳ during the 1957 MW 8 Gobi–Altay event in southern Mon- events. golia. That event involved simultaneous rupture of Ͼ300 km Source directivity could also play a critical role in con- of the Bogd fault, a major left-lateral strike-slip fault similar trolling damage patterns during a future Sierra Madre fault to the San Andreas fault, and two long stretches of a north- event. If a large earthquake were to nucleate near the base dipping reverse-fault system to the south that is similar to of the seismogenic zone, as occurred during the 1971 and the Sierra Madre fault (Florensov and Solonenko, 1963; Bal- 1994 events (Heaton, 1982; Scientists of the USGS and jinnyam et al., 1993; Bayarsayhan et al., 1996). The Ͼ8000- SCEC, 1994; Wald et al., 1995), up-dip directivity along the

Weaver and Dolan, 2000 RAYMOND FAULT 2-3 Events ? 2-3 Events Crook et al., 1987 31.5ka

Most Recent Event This study SIERRA MADRE ? FAULT 2 events, most recent <15ka ? Rubin et al., 1998

0105 15 20 Age in Calender Years (x 1000) Figure 8. Comparison of paleoseismologic data from the Sierra Madre and Ray- mond fault. Compiled from Crook et al. (1987); Rubin et al. (1998); Weaver and Dolan (in press); and this study. 246 A. Z. Tucker and J. F. Dolan north-dipping Sierra Madre fault would focus energy south- by a large reverse-fault earthquake on the Sierra Madre fault. wards, directly toward the metropolitan region, including the There is nothing “special” about the faults that have gener- Los Angeles Basin which is the location of many older, ated large reverse-fault events in southern California and multistory structures and numerous high-rise buildings. The elsewhere around the world, and the evidence presented in stability of some of these high-rises during relatively close, this article adds to a growing body of evidence that indicates large earthquakes has been the subject of intense scientific that such large events could occur on reverse faults directly discussion (e.g., Heaton et al., 1995). beneath or adjacent to the Los Angeles metropolitan region The Ͼ8000-year-long elapsed period since the most re- (e.g., Dolan et al., 1995; Rubin et al., 1998; Shaw and cent surface rupture on the eastern Sierra Madre fault has Shearer, 1999). Future assessments of the seismic hazards important implications for seismic hazard calculations for facing the Los Angeles metropolitan region therefore must the Los Angeles metropolitan region. The standard proba- consider the possibility of very large ruptures along the Si- bilistic seismic hazard assessment model for California (Pe- erra Madre fault. Although such events may be less frequent tersen et al., 1996) assumes an average recurrence interval and somewhat smaller than large-to-great earthquakes on the of 384 years for an assumed MW 7.0 event size on the Sierra San Andreas fault, the proximity of large faults such as the Madre fault. The slip rate on which this model is based, Sierra Madre fault to the metropolitan region indicates that mm/yr (similar to the rate proposed for urban earthquakes on these faults could cause at least as 1 ע however, is 3 the Sierra Madre fault by WCGEP [1995] and for the com- much, if not more, damage than a larger event on the more bined Sierra Madre-Verdugo fault system presented in Dolan distant San Andreas fault. et al., [1995]). This slip rate is much faster than the rate for the main, active strand of the eastern Sierra Madre fault that Conclusions we document in this article (0.6–0.9 mm/yr) and is also faster than the ϳ1–2 mm/yr rates that have been documented on Paleoseismologic data from the eastern Sierra Madre the central and western reaches of the fault (Lindvall et al., fault at San Dimas reveal a minimum reverse-slip rate of 1995; Walls et al., 1998). Moreover, our data and those of 0.6–0.9 mm/yr and an elapsed time interval since the most

Rubin et al. (1998) suggest that the assumed MW 7.0 size recent surface rupture on the main, active strand of the fault for hypothetical earthquakes used in the probabilistic model of Ն8000 years. These observations suggest that the Sierra Ͼ may be smaller than at least some events that occur on the Madre fault ruptures in infrequent, but very large (MW 7), central and eastern parts of the fault. These factors should earthquakes, much larger than any events that have occurred be considered in any future probabilistic seismic hazard as- on any metropolitan Los Angeles faults during the 150- to sessment models for southern California. 200-year-long historic period. Moreover, the long elapsed Ͼ The potential for large earthquakes (MW 7) on the time interval since the most recent event on the eastern Sierra Sierra Madre fault should not come as a surprise. Numerous Madre fault implies that the fault in that region is in the late reverse fault systems elsewhere in the world that are similar stages of a strain accumulation cycle. Large earthquakes on Ͼ to the Sierra Madre fault have generated large (MW 7) the Sierra Madre fault conceivably could involve simulta- earthquakes such as the 1964 MS 7.5 Niigata, Japan (Ka- neous rupture of contiguous faults, such as the Cucamonga wasumi, 1973; Satake and Abe, 1983; Okamura et al., 1994); or Raymond faults. Published paleoseismologic data, how- the 1978 MS 7.4 Tabas-e-Golshan, Iran (Berberian, 1979; ever, indicate that the Raymond fault has ruptured to the Hartzell and Mendoza, 1991; Berberian and Yeats, 1999), surface at least once, and possibly several times, since the the 1980 MW 7.3 El Asnam, Algeria (King and Vita-Finzi, most recent surface rupture on the eastern Sierra Madre fault. 1981; Philip and Meghraoui, 1983; Swan, 1988); the 1992 Previous authors have proposed that the Sierra Madre fault

MW 7.4 Suusamyr, Kyrgyzstan (Mellors et al., 1997; Ghose could sometimes rupture together with the San Andreas fault et al., 1997); and the 1999 MW 7.6 Chi-Chi, Taiwan (Bilham in southern California’s “most devastating earthquake”. The and Yu, 2000; Shin et al., 2000). In fact, several such large long elapsed time interval since the most recent eastern Si- reverse-fault earthquakes have already occurred in southern erra Madre fault event, however, shows that ϳ60–80 San California during the historic period, such as the 21 Decem- Andreas fault Big Ones have occurred since the most recent ber 1812 western Transverse Ranges (Toppozada et al., surface rupture on the eastern Sierra Madre fault. Thus, if ϳ 1981; Ellsworth, 1990); the 1927 MW 7(?) Lompoc (Gaw- the Sierra Madre fault ever does rupture together with the thorp, 1978; Savage and Prescott, 1978; Hanks, 1979; Helm- San Andreas fault, then such events much be extremely rare. berger et al., 1992; Satake and Somerville, 1992); and the 1952 MW 7.3–7.5 Kern County (Hanks et al., 1975; Stein Acknowledgments and Thatcher, 1981; Ellsworth, 1990). Fortunately, the regions affected by strong ground motions during the pre- We would like to thank Kristin Weaver, Jeff Beard, Meredith Rob- vious large southern California events were sparsely popu- ertson, and Ilene Cooper for their assistance with field work. We are in- debted to the City of San Dimas, and especially to John Garcia and Khrishna lated when the earthquakes occurred, and damage and loss Patel, for allowing us permission to conduct this investigation on city prop- of life were minimal. This is certainly not the case for the erty. We would also like to thank Ned Field, Frank Jordan, Scott Lindvall, densely populated areas that would be most directly affected Mark Petersen, and Jerry Treiman for helpful discussions. Jonathan Matti, Paleoseismologic Evidence for a Ͼ8 Ka Age of the Most Recent Surface Rupture on the Eastern Sierra Madre Fault 247

Jerry Treiman, and Lisa Grant provided very helpful reviews of the manu- Dolan, J. F., F. Jordan, G. Rasmussen, D. Stevens, W. Reeder, and L. M. script. In addition to his review of the manuscript, Jerry Treiman kindly McFadden (1996). Evidence for moderate-sized (MW 6.5–7.0) pa- allowed us access to his aerial photograph collection for our geomorpho- leoearthquakes on the Cucamonga fault, northwestern Los Angeles logic studies. This research was supported by the Southern California Earth- metropolitan region, California, EOS 77, 461 (abstract). quake Center (SCEC). SCEC is supported by NSF Cooperative Agreement Dolan, J. F., K. Sieh, T. K. Rockwell, P. Guptill, and G. Miller (1997). EAR-8920136 and USGS Cooperative Agreements 14-08-0001-A0899 and Active tectonics, paleoseismology, and seismic hazards of the Hol- 1434-HQ-97AG01718. The SCEC Contribution Number for this article is lywood fault, northern Los Angeles basin, California, Bull. Geol. Soc. 549. Am. 109, 1595–1616. Dolan, J. F., D. Stevens, and T. K. Rockwell (2000a). Paleoseismologic References evidence for an early to mid-Holocene age of the most recent surface rupture on the Hollywood fault, Los Angeles, California, Bull. Seism. Allen, C. R., T. C. Hanks, and J. H. Whitcomb (1975). Seismological stud- Soc. Am. 90, 334–344. ies of the San Fernando earthquake and their tectonic implications, Dolan, J. F., K. Sieh, and T. K. Rockwell (2000b). Late Quaternary activity in San Fernando, California, earthquake of 9 February 1971, G. B. and seismic potential of the Santa Monica fault system, Los Angeles, Oakeshott, (Editor), Calif. Div. Mines Geol. Bull. 196, 257–262. California, Geol. Soc. Am. Bull. 112, 1559–1581. Barrows, A. G., J. E. Kahle, R. B. Saul, and F. H. Weber Jr. (1975). Geo- Eaton, G. P. (1957). Miocene volcanic activity in the Los Angeles Basin logic map of the San Fernando earthquake area, in San Fernando, and vicinity, Ph.D. Dissertation, California Institute of Technology, California, Earthquake of 9 February 1971, G. B. Oakeshott (Editor), Pasadena, California, 388 pp. Calif. Div. Mines Geol. Bull. 196, Plate 2. Ehlig, P. L. (1981). Origin and Tectonic History of the Basement Terrane Baljinnyam, I., A. Bayasgalan, B. A. Borisov, A. Cisternas, M. G. of the San Gabriel Mountains, Central Transverse Ranges, in The Dem’yanovich, L. Ganbaatar, V. M. Kochetkov, R. A. Kurushin, P. Geotectonic Development of California, W. G. Ernst (Editor), Pren- Molnar, H. Philip, and Yu. Ya. Vashchilov (1993). Ruptures of major tice Hall, Englewood Cliffs, New Jersey, 253–283. earthquakes and active deformation in Mongolia and its surroundings, Ehlig, P. L. (1975). Geologic framework of the San Gabriel Mountains, in in Geol. Soc. Am. Memoir, vol. 181, 62 pp. San Fernando Earthquake of 9 February, 1971. Oakeshott, G. B., (Ed- Bayarsayhan, C., A. Bayasgalan, B. Enhtuvshin, K. Hudnut, R. Kurushin, itor), Calif. Div. Mines Geol. Bull. 196, 7–18. P. Molnar, and M. Olziybat (1996). 1957 Gobi-Altay, Mongolia, Ellsworth, W. L. (1990). Earthquake history, 1769–1989, in The San An- earthquake as a prototype for southern California’s most devastating dreas fault system, R. E. Wallace (Editor), U.S. Geol. Surv. Profess. earthquake, Geology, 24, 579–582. Pap. 1515, 153–187. Berberian, M. (1979). Earthquake faulting and bedding thrust associated Florensov, N. A. and V. P. Solonenko (Editors) (1963). The Gobi-Altai with the Tabas-e-Golshan (Iran) earthquake of September 16, 1978, earthquake, Moscow, Akademiya Nauk, USSR (in Russian; English Bull. Seism. Soc. Am. 69, 1861–1887. translation, 1965, U.S. Department of Commerce, Washington, D.C.), Berberian, M., and R. S. Yeats (1999). Patterns of historical earthquake 424 pp. rupture in the Iranian Plateau, Bull. Seism. Soc. Am. 89, 120–139. Fumal, T., S. Pezzopane, R. Weldon, II, and D. Schwartz (1993). A 100- Bilham, R., and T-T Yu (2000). The morphology of thrust faulting in the year average recurrence interval for the San Andreas fault at Wright- 21 September 1999, Chichi, Taiwan earthquake, J. Asian Earth Sci. wood, California, Science, 259, 199–203. 18, 351–367. Fumal, T. E., A. B. Davis, W. T. Frost, J. O’Donnell, G. Sega, and D. P. Bonilla, M. G. (1973). Trench exposures across surface fault ruptures as- Schwartz (1995). Recurrence studies of Tujunga segment of the 1971 sociated with San Fernando earthquake, in San Fernando, California, San Fernando, California, earthquake, EOS Trans. Am. Geophys. Earthquake of February 9, 1971, L. M. Murphy (Editor), Vol. 3, Union, Supplement, (76) 46, 364. NOAA, Washington, D.C., 173–182. Gawthorp, W. (1978). The 1927 Lompoc, California earthquake, Bull. Blythe, A. E., D. W. Burbank, K. A. Farley, and E. Fielding (2000). Struc- Seism. Soc. Am. 68, 1705–1716. tural and topographic evolution of the central Transverse ranges, Cali- Ghose, S., R. J. Mellors, A. M. Korjenkov, M. W. Hamburger, T. L. Pavlis, fornia, from apatite fission track, (U-Th)/He and digital elevation G. L. Pavlis, M. Omuraliev, E. Mamyrov, and A. R. Muraliev (1997). Suusamyr, Kyrgyzsatn, earthquake in the Tien 1992 7.3 ס model analyses, Basin Research 12, 97–114. The Ms Bull, W. B. (1987). Relative rates of long-term uplift of mountain fronts, Shan. II. Aftershock focal mechanisms and surface deformation, Bull. in Directions in Paleoseismology, A. Crone and E. Omdahl (Editors), Seism. Soc. Am. 87, 23–38. U.S.G.S. Open-File Rept. 87-673, 192–202. Hanks, T. C., J. A. Hileman, and W. Thatcher (1975). Seismic moments of Crook Jr., R., C. R. Allen, B. Kamb, C. M. Payne, and R. J. Proctor (1987). the larger earthquakes of the southern California region, Geol. Soc. Quaternary geology and seismic hazard of the Sierra Madre and as- Am. Bull. 86, 1131–1139. sociated faults, western San Gabriel Mountains, U.S. Geol. Surv. Pro- Hanks, T. C. (1979). The Lompoc, California earthquake (November 4, –and its aftershocks, Bull. Seism. Soc. Am. 69, 451 (7.3 ס fess. Pap. 1339, 27–63. 1927; M Dibblee, T. W. (1991a). Geologic map of the Sunland and Burbank (north 462. 1/2) quadrangles. Dibblee Geologic Foundation Map DF-32, P. O. Hartzell, S., and C. Mendoza (1991). Application of an iterative least- Box 60560, Santa Barbara, California, scale 1:24,000. squares waveform inversion of strong-motion and teleseismic records Dibblee, T. W. (1991b). Geologic map of the San Fernando and Van Nuys to the 1978 Tabas, Iran, earthquake, Bull. Seism. Soc. Am. 81, 301– (north 1/2) quadrangles. Dibblee Geologic Foundation Map DF-31, 335. P.O. Box 60560, Santa Barbara, California, scale 1:24,000. Hauksson, E., and L. M. Jones (1991). The 1988 and 1990 Upland earth- Dolan, J. F., and K. Sieh (1992). Tectonic geomorphology of the northern quakes: Left-lateral faulting adjacent to the central transverse Ranges, Los Angeles basin: Seismic hazards and kinematics of young fault J. Geophys. Res. 96, 8143–8165. movement, in Engineering Geology Field Trips: Orange County, Heaton, T. H. (1982). The 1971 San Fernando earthquake: A double event?, Santa Monica Mountains, and Malibu, Guidebook and Volume, P. L. Bull. Seism. Soc. Am. 72, 2037–2062. Ehlig and E. A. Steiner (Editors), Association of Engineering Geol- Heaton, T. H., J. F. Hall, D. J. Wald, and M. W. Halling (1995). Response ogists, Los Angeles, B-20–B-26. of high-rise and base-isolated buildings to a hypothetical Mw 7.0 Dolan, J. F., K. Sieh, T. K. Rockwell, R. S. Yeats, J. Shaw, J. Suppe, G. blind thrust earthquake, Science 267, 206–211. Huftile, and E. Gath (1995). Prospects for larger or more frequent Helmberger, D. V., P. G. Somerville, and E. Garnero (1992). The location earthquakes in greater metropolitan Los Angeles, California, Science and source parameters of the Lompoc, California earthquake of 4 267, 199–205. November 1927, Bull. Seism. Soc. Am. 82, 1678–1709. 248 A. Z. Tucker and J. F. Dolan

Jones, L. M., K. E. Sieh, E. Hauksson, and L. K. Hutton (1990). The 3 of California, Calif. Div. Mines Geol. Open-File 96–08 and U.S. Geol. December 1988 Pasadena earthquake: Evidence for strike-slip motion Surv. Open-File Report 96–706. on the Raymond Fault, Bull. Seism. Soc. Am. 80, 474–482. Philip, H., and M. Meghraoui (1983). Structural analysis and interpretation Kawasumi, H. (Editor) (1973). General Report on the Niigata Earthquake of the surface deformation of the El Asnam earthquake of October of 1964, Tokyo Electrical Engineering College Press, Tokyo, Japan. 10, 1980, Tectonics 2, 17–49. King, G. C. P., and C. Vita-Finzi (1981). Active folding in the Algerian Proctor, R. J., C. M. Payne, D. C. Kalin (1970). Crossing the Sierra Madre earthquake of 10 October 1980, Nature 292, 22–26. fault zone in the Glendora tunnel, San Gabriel Mountains, California, Leighton and Associates (1992). Report on supplemental fault trench in- Eng. Geol. (4) 1, 5–63. vestigations, proposed La Vina residential development, vesting ten- Proctor, R. J., C. M. Payne, D. C. Kalin (1992). Glendora Tunnel crossing tative tract 45546, Altadena, County of Los Angeles, California, Con- of the Sierra Madre Fault, in Engineering Geology Practice in sulting Report for Leighton and Associates Project 286-11-8-23, Southern California, B. W. Pipkin, and R. J. Proctor (Editors), As- prepared for Southwest Diversified Corp., Leighton and Associates, sociation of Engineering Geologists, Los Angeles, 756–762. Irvine, California, 18 pp. and 11 plates. Rubin, C., S. Lindvall, and T. K. Rockwell (1998). Evidence for large Lindvall, S. C., T. K. Rockwell, C. Walls, and Bornyasz (1995). Late Qua- earthquakes in metropolitan Los Angeles, Science 281, 398–402. ternary deformation of Pacoima Wash terraces in the vicinity of the Satake, K., and K. Abe (1983). A fault model for the Niigata, Japan, earth- 1971 San Fernando earthquake rupture, northern San Fernando Val- quake of June 16, 1964, J. Phys. Earth 31, 217–223. ley, California, (abstract), EOS 76, F363. Satake, K., and P. G. Somerville (1992). Location and size of the 1927 Magistrale, H., and S. Day (1999). 3-D simulations of multi-segment thrust Lompoc, California earthquake from tsunami data, Bull. Seism. Soc. fault rupture, Geophys. Res. Lett. 26, 2093–2096. Am. 82, 1710–1725. Matti, J. C., J. C. Tinsley, L. D. McFadden, and D. M. Morton (1982). Savage, J. C., and W. H. Prescott (1978). Geodetic control and the 1927 Holocene faulting history as recorded by alluvial history within the Lompoc, California earthquake: Bull. Seism. Soc. Amer., v. 68, p. Cucamonga fault zone: A preliminary view, in Late Quaternary Ped- 1699–1703. ogenesis and Alluvial Chronologies of the Los Angeles and San Ga- Scientists of USGS/SCEC (1994). The magnitude 6.7 Northridge, California, briel Mountains Areas, Southern California, Guidebook for Field Trip earthquake of 17 January 1994, Science 266, 389–397. 12, J. C. Tinsley, L. D. McFadden, and J. C. Matti (Editors), Geol. Shaw, J. H., and P. Shearer (1999). An elusive blind-thrust fault beneath Science 283, Soc. Amer. Cordilleran section, 78th Annual meeting, Anaheim, Cali- metropolitan Los Angeles, 1516–1518. Shin, T. C., K. W. Kuo, W. H. K. Lee, T. L. Teng, and Y. B. Tsai (2000). fornia, 21–44. A preliminary report on the 1999 Chi-Chi (Taiwan) earthquake, Matti, J. C., D. M. Morton, and B. F. Cox (1992). The San Andreas fault Seism. Res. Lett. 71, 24–30. system in the vicinity of the central Transverse Ranges province, Sieh, K. E. (1978). Prehistoric large earthquakes produced by slip on the southern California, U.S. Geol. Surv. Open-File Rept. 92–354, 49 pp., San Andreas Fault at Pallett Creek, California, J. Geophys. Res. 83, 2 plates. 3907–3939. Matti, J. C., and D. M. Morton (1993). Paleogeographic evolution of the Sieh, K. E., M. Stuiver, and D. Brillenger (1989). A more precise chro- San Andreas fault in southern California: a reconstruction based on a nology of earthquakes produced by the San Andreas fault in southern new cross-fault correlation, in The San Andreas Fault System: Dis- California, J. Geophys. Res. 94, 603–623. placement, Palinspastic Reconstruction, and Geologic Evolution, Sorlien, C. C. (1994). Faulting and uplift of the northern Channel Islands, R. E. Powell, R. J. Weldon II, and J. C. Matti (Editors), Geol. Soc. California, W. L. Halvorson, and G. J. Maender (Editors), Proc. 4th Am. Memoir, Vol. 178, 107–159. Channel Islands Symposium: Update on the Status of Resources, Mellors, R. J., F. L. Vernon, G. L. Pavlis, G. A. Abers, M. W. Hamburger, Santa Barbara County Museum of Natural History, Santa Barbara, ס S. Ghose, and B. Iliasov (1997). The Ms 7.3 1992 Suusamyr, California, 281–296. Kyrgyzsatn, earthquake. I. Constraints on fault geometry and source Stein, R. S., and W. Thatcher (1981). Seismic and aseismic deformation parameters based on aftershocks and body-wave modeling, Bull. associated with the 1952 Kern County, California, earthquake and Seism. Soc. Am. 87, 11–22. relationship to the Quaternary history of the White Wolf fault, J. Morton, D. M., and J. C. Matti (1987). The Cucamonga fault zone: geologic Geophys. Res. 86, 4913–4928. setting and Quaternary history, U.S. Geol. Surv. Profess. Pap. 1339, Stuiver, M. and P. Reimer (1993). Radiocarbon Calibration Program Re- 179–203. vision 4.1.2, Radiocarbon 35, 215–230. Morton, D. M., and J. C. Matti (1993). Extension and contraction within Swan, F. H. (1988). Temporal clustering of paleoseismic events on the an evolving divergent strike-slip fault complex: the San Andreas and Oued Fodda fault, Algeria, Geology 16, 1092–1095. San Jacinto fault zones and their convergence in southern California, Toppozada, T. R., C. R. Real, and D. L. Parke (1981). Preparation of iso- in The San Andreas Fault System: Displacement, Palinspastic Recon- seismal maps and summaries of reported effects from pre-1900 Cali- struction, and Geologic Evolution, R. E. Powell, R. J. Weldon II, and fornia earthquakes, Calif. Div. Mines Geol. Open-File Rept. 81-11, J. C. Matti (Editors), Geol. Soc. Am. Memoir, Vol. 178, 217–230. 182. Munsell Soil Color Chart (1994). Macbeth Division of Kollmorgan Instru- Tsutsumi, H., and R. S. Yeats (1999). Tectonic setting of the 1971 Sylmar ments Corporation, New Windsor, New York, 10 pp., 10 plates. and 1994 Northridge earthquakes in the San fernando Valley, Cali- Oakeshott, G. B. (Editor) (1975). Geology of the epicentral area, in San fornia, Bull. Seism. Soc. Am. 89, 1232–1249. Fernando, California, earthquake of 9 February 1971, Calif. Div. U.S. Geol. Survey (USGS) Staff (1971). Surface faulting, in The San Fer- Mines Geol. Bull. 196, 19–30. nando, California, Earthquake of February 9, 1971. U. S. Geol. Surv. Okamura, Y., M. Satoh, and J. Miyazaki (1994). Active faults and folds on Profess. Pap. 733, 55–76. the shelf off Niigata and their relation to the 1964 Niigata earthquake, Vedder, J. G., H. G. Greene, S. H. Clarke, and M. P. Kennedy (1986). J. Seism. Soc. Japan, 46, 413–423 (in Japanese with English abstract). Geologic map of mid-southern California continental margin (Map Pentegoff, V. P., T. F. Thompson, and R. H. Jahns (1965). Geologic report 2A), California continental margin geologic map series (area 2 of 7; on the proposed Glendora Tunnel for the Foothill Feeder, [City of map sheet 1 of 4), H. Greene and M. Kennedy (Editors), U. S. Geol. Los Angeles] Metropolitan Water District Report No. 820, Los An- Surv., Menlo Park, California, and Calif. Div. Mines Geol., Sacra- geles, California, 17 pp., 2 app. mento, California, scale 1:250,000. Petersen, M. D., W. D. Bryant, C. H. Cramer, T. Cao, M. S. Reichle, A. D. Voelker, A. H. L., M. Sarnthein, P. M. Grootes, H. Erlenkeusser, C. Laj, Frankel, J. J. Lienkamper, P. A. McCrory, and D. P. Schwartz (1996). A. Mazaud, M.-J. Nadeau, and M. Schleicher (1998). Correlation of Probabilistic seismic hazard seismic hazard assessment for the state marine 14C ages from the Nordic seas with the GISP2 isotope record: Paleoseismologic Evidence for a Ͼ8 Ka Age of the Most Recent Surface Rupture on the Eastern Sierra Madre Fault 249

implications for 14C calibration beyond 25 ka BP, Radiocarbon 40, Whitcomb, J. H., C. R. Allen, J. D. Garmany, and J. A. Hileman (1973). 517–534. Focal mechanisms and tectonics—San Fernando earthquake Series, Wald, D. J., T. H. Heaton, and K. W. Hudnut (1995). The Slip History of 1971, Rev. Geophys. 11, 693–730. the 1994 Northridge, California, Earthquake determined from strong- Wright, T. L. (1991). Structural geology and tectonic evolution of the Los motion, GPS, and leveling-line data (supplement), Bull. Seism. Soc. Angeles Basin, in Active-Margin Basins, K. T. Biddle (Editor), AAPG Am. 86, no. 1B, S49–S70. Memoir 52, 35–106. Walls, C., T. Rockwell, K. Mueller, Y. Bock, S. Williams, J. Pfanner, J. Yeats, R. S. (1987). Late Cenozoic structure of the Santa Susana fault zone, Dolan, and P. Fang (1998). Escape tectonics in the Los Angeles met- in Recent Reverse Faulting in the Transverse Ranges, California, U.S. ropolitan region and implications for seismic risk, Nature 394, 356– Geol. Surv. Profess. Pap. 1339, 137–159. 360. Ziony, J. I., and L. M. Jones (1989). Map showing late Quaternary faults Weaver, K. D., and J. F. Dolan (2000). Paleoseismology and seismic haz- and 1978–84 seismicity of the Los Angeles region, U.S. Geol. Surv. ards of the Raymond fault, Los Angeles County, California, Bull. Misc. Field Studies Map MF-1964, scale 1:250,000. Seism. Soc. Am. 90, 1409–1428. Wells, D., and K. Coppersmith (1994). New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface Dept. of Earth Sciences displacement, Bull. Seism. Soc. Am. 84, 974–1002. University of Southern California Wesnousky, S. (1986). Earthquakes, Quaternary faults, and seismic hazard Los Angeles, CA 90089-0740 in California, J. Geophys. Res. 91, 12,587–12,631. (A.Z.T., J.F.D.) Working Group on California Earthquake Probabilities (WGCEP) (1995). Seismic hazards in southern California: probable earthquakes, 1994– 2024, Bull. Seism. Soc. Am. 85, 379–439. Manuscript received 23 June 2000.