Quaternary Tectonic Setting of the 1983 Borah Peak Earthquake, Central Idaho by William E

Bulletin of the Seismological Society of America, VoL 75, No. 4, pp. 1053-1066, August 1985

QUATERNARY TECTONIC SETTING OF THE 1983 BORAH PEAK EARTHQUAKE, CENTRAL IDAHO BY WILLIAM E. SCOTT, KENNETH L. PIERCE, AND M. H. HAIT, JR.

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 roughly V-shaped belt of latest Quaternary surface faulting that extends from the Wasatch fault, through the Yellowstone area, to the Lost River fault. The position of this belt may be related to the outward migration of a thermal front associated with the northeastward progression of late Cenozoic silicic volcanism along the Yellowstone- Snake River Plain axis. 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 main 1983 surface faulting occurred in an area of high structural relief along a central segment 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 perspective 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 28 October 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 (Figures 1 to 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 I covers the northeastern part of the Basin and Range province and shows the location of the surface faulting associated with the 1983 earthquake in relation to other areas of historic surface faulting, to major late Cenozoic normal faults, and to areas of historic seismicity that define the Intermountain Seismic Belt and the Idaho Seismic Zone of Smith and Sbar (1974). Without regard to historic seismicity, 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 (Figure 2). Domains 2 to 5 represent several structural settings and contain few and widely distributed faults that have moved in latest Quaternary time. In contrast, domain 1 contains many faults that have 1053

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0 100 KILOMETERS I t I Fro. 1. Neotectonic setting of the 1983 Borah Peak earthquake in the northeastern Basin-and-Range Province showing major, late Cenozoic normal-slip faults (modified from Howard et al., 1978 and Nakata et al., 1982). Those with latest Quaternary movement that lie at the base of high, steep range fronts are shown by a bold line; those with historic rupture are hachured. Light-shaded areas are parts of the Intermountain Seismic Belt (ISB) and Idaho Seismic Zone (ISZ) of Smith and Sbar (1974) that contain most epicenters of earthquakes from.1850 and 1974 (Arabasz et al., 1980; Figure 1-1). Asterisk indicates epicenter of the 1983 Borah Peak earthquake.

Although historic seismicity plays no role in defining the domains, domain 1 lies mostly within the historically active parts of the Intermountain Seismic Belt (Figures 1 and 2). Conspicuous exceptions to this relation include the area of domain 1 in which the Borah Peak earthquake occurred and which was nearly aseismic prior to 1983, and the part of the Intermountain Seismic Belt in west-central and northwest Montana in which there are few major latest Quaternary faults.

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O ~00 KILOMETERS I , I FIG. 2. Neotectonic domains (see text) of the same area as Figure 1 including latest Quaternary faults from Figure 1. Large arrows show trajectory of thermal activity along the Yellowstone-Snake River Plain (Y-SRP) axis. JC, Jim Sage and Cotterel Mountains; BFM, Blackfoot Mountains. (1) Wasatch (1A)- YeJlowstone-Lost River (1B) belt of latest Quaternary surface faulting. Area of greatest neotectonic activity based on Quaternary geologic evidence exclusive of historic seismicity is stippled. (2) South- western Montana. Many late Cenozoic faults, but only a few, widely distributed range-bounding faults with evidence of latest Quaternary surface faulting. Recent studies (e.g., O'Neill and Lopez, 1985) indicate that some additional latest Quaternary surface faults occur in this area, but the widely distributed pattern is maintained. (3) Idaho batholith. Relatively rigid block broken locally by Quaternary faults. (4) Eastern Snake River Plain. Little evidence of late Quaternary faulting except for rifts associated with basaltic volcanism. (5) Northeastern Basin and Range west of (1). Many late Cenozoic faults, but only widely distributed evidence of latest Quaternary surface faulting. shaped belt (1B) north of the Wasatch fault. These are: (a) a set of right-stepping faults that extend from Cache Valley, Utah, to Star Valley, Wyoming; (b) the Teton fault; (c) faults in the Yellowstone area; (d) the Deep Creek, Madison, Centennial, and Red Rock faults in Montana; and (e) the Beaverhead, Lemhi, and Lost River faults in Idaho.

FIG. 8. Major frontal faults in the Lost River-Beaverhead area. 1983 break from Crone and Machette (1984). Numbers along faults are estimates of minimum structural relief in kilometers from sum of thickness of basin fill (Crosthwaite et al., 1970; Ruppel, 1982) and relief of adjacent range, and, at north end of Lost River fault, from minimum offset of Challis Volcanics. Thick basin fills (stippled) interpreted from gravity lows with >15 mgal of closure (Mabey et al., 1974). Direction of dip of upper Tertiary volcanics are shown by strike-and-dip symbol. G, gravel transported from west of area that underlies 6.5-m.y.-old basalt (see text). Segments that have evidence of a left- or right-lateral component of slip in pre-1983 events are shown by L and R, respectively. DC, Devils Canyon; WSV, Warm Springs valley; WCH, Willow Creek hills; TSV, Thousand Springs Valley; WC, Willow Creek; BP, Borah Peak; EC, Elkhorn Creek; LCC, Lower Cedar Creek. The town of Challis, where the two earthquake-related fatalities occurred, lies just west of the north end of the north segment of the Lost River fault. of the eastern Snake River Plain, whereas the southern part lies from 100 to 200 km from the Plain margin in the west and merges to within 100 km of the Plain margin in the Teton area. Both parts join within the Yellowstone area. Major silicic volcanism started about 15 m.y. ago on the Y-SRP axis in southwest

Downloaded from https://pubs.geoscienceworld.org/ssa/bssa/article-pdf/75/4/1053/2705124/BSSA0750041053.pdf?casa_token=TSBt38b2JcoAAAAA:Sjnn_gY_cB_R2IMO456_ATbCsQoCOtNgovEXPd9DpGdIY1HaLTZojbOELizVZuKSELwns_JcrA by California Geological Survey, 19774 on 01 April 2020 THE 1983 BORAH PEAK EARTHQUAKE, CENTRAL IDAHO 1057 Idaho, migrated northeastward along the axis at an average rate of about 3.5 cm/ yr, and at present is centered in the Yellowstone area (Armstrong et al., 1975; Christiansen and McKee, 1978; Morgan et al., 1984). This volcanism was accompanied by crustal heating and uplift; subsequent cooling is reflected in the decrease in elevation southwestward along the Y-SRP axis (Brott et al., 1978; Smith et al., 1985). The passage of this thermal activity along the axis also produced an outwardly migrating thermal front with a geometry analogous to the wake of amoving boat. The belt encompassing the major latest Quaternary faults of the northeastern Basin and Range (Figure 2) also has a wake-like pattern about the Y-SRP axis and converges with the axis at Yellowstone suggesting the pattern of neotectonic activity may relate to the thermal front migrating outward from the Y-SRP axis. If this association between the thermal front and the active neotectonic belt is valid, it provides a testable late Cenozoic tectonic history for areas within the belt. That is, the passage of the thermal front produces first high rates of faulting as the crust is heated and thinned, followed by decelerating rates of deformation or relative quiescence as heating ceases and cooling occurs. Tectonic histories of range-front faults between the Snake River Plain and the belt of major latest Quaternary faulting (Figure 2) appear to confirm the above prediction of a high rate of faulting followed by a decelerating rate. Numerous basins and ranges south of the Plain attest to active late Cenozoic faulting, but in these areas there is little evidence of Quaternary faulting. For example, the Cotterel and Jim Sage Mountains are adjacent to the western part of the eastern Snake River Plain and consist of volcanics and sediments emplaced in a topographic low about 10 m.y. ago, at about the time silicic volcanism was centered along the adjacent part of the Y-SRP axis (Armstrong et al., 1975; Williams et al., 1982). Subsequently, the basin was extended greatly along an east-dipping, low-angle detachment, and the ranges were uplifted to produce about 1 km of structural relief on the 10-m.y.-old volcanics. High rates of deformation between 10 and 5 m.y. ago contrast with low rates during the last 5 m.y. (Covington, 1983, Figure 5; H. R. Covington, written communication, 1984) and low rates of Quaternary deformation (Williams et al., 1982; K. L. Pierce, unpublished data, 1984). Further northeast along the south margin of the eastern Snake River Plain, the Blackfoot Mountains were uplifted at high rates (0.8 m/1000 yr) between 5.9 and 4.7 m.y. ago to attain most of their present relief (Allmendinger, 1982). At about this time, major silicic volcanism was centered along the adjacent part of the Y-SRP axis (Armstrong et al., 1975). In contrast, there is no evidence of significant Quaternary surface faulting along the main range-front fault on the west side of the Blackfoot Mountains. Current geologic information from north of the Plain is not sufficient to determine if similar variations existed there in rates of uplift during late Cenozoic time. However, based on the height and morphology of range fronts and on the age of surface faulting discussed later, the parts of the range-front faults adjacent to the Plain appear to have had a lower rate of faulting during late Quaternary time than parts farther north. We do not intend to suggest that the cause of Basin and Range deformation marginal to the eastern Snake River Plain is solely the result of thermal activity associated with the Y-SRP axis, but rather that a thermal front may have localized faulting in a wake-like belt that has propagated outward through the northeastern Basin and Range from the Y-SRP axis. The Lost River, Lemhi, and Beaverhead faults define an area of similar structural pattern and neotectonic activity at the northwestern end of the latest Quaternary belt of surface faulting that is the focus of the rest of this discussion.

The present topographic relief of the Lost River, Lemhi, and Beaverhead ranges is the result of late Cenozoic faulting (Baldwin, 1951; Ruppel, 1982). Most of this faulting may have occurred during the last 4 to 7 m.y. based on: (1) the presence of fluvial-gravel clasts that must have been transported from west of this area and that now underlie 6.5-m.y.-old volcanic rocks in the Lemhi Range (G on Figure 3); and (2) uplift-induced east dips of basalts that overlie 4.3-m.y.-old ash-flow tufts near the southern ends of the ranges (Figure 3; M. H. Hait, Jr., unpublished data; dates from Morgan et al., 1984). Ruppel (1982) suggests that the Lemhi Range and Beaverhead Mountains are block uplifts of mostly Miocene age, and that the fault scarps on the southwest sides of the ranges reflect eastward tilting in late Pliocene to Holocene time. In contrast, Hait (1984) finds evidence for considerable extension during middle Cenozoic time. We favor an interpretation that the three ranges are parts of normal-fault-bounded blocks that range from structurally flat-topped (Ruppel, 1982) to eastward-tilted. Faulting during Quaternary time appears largely related to eastward tilting because: (1) fault scarps that cut Quaternary deposits occur mainly on the west sides of the ranges; (2) some parts of the ranges are asymmetric with steep west sides and gentler sloping east sides; and (3) upper Tertiary tufts and basalts in and between the southern parts of the ranges have easterly dips (Figure 3).

QUATERNARY SURFACE FAULTING IN THE LOST RIVER:BEAVERHEAD AREA Quaternary surface faulting has broken most of the Lost River, Lemhi, and Beaverhead range-front faults. Similar geometric and temporal patterns of breakage and structural relief are repeated along each fault (Figure 3). The mapped patterns of the faults are strikingly similar. All three generally hug the western range fronts, change trend by as much as 90 °, and, in places, diverge from range fronts and strike across basins. Quaternary offset on the faults is dominantly normal slip, but lateral components can be recognized locally by en- echelon patterns of faults (Figure 3). Structural relief along the three faults, as estimated from topographic and geophysical data, is generally greater along central segments, which are partly coincident with an arch north of and parallel to the Snake River Plain (see discussion in Ruppel, 1982), than along distal segments (Figure 3). These relations suggest that the central segments have higher long-term rates of faulting, which is consistent with estimates of short-term slip rates for various segments. In the Borah Peak area, which has about 2.7 km of structural relief, the 15,000-yr-old surface of the Willow Creek fan (Pierce and Scott, 1982) is offset 3.5 to 4.5 m, including the 1.5 to 2-m offset of 1983 (Crone and Machette, 1984). The mean slip rate there is about 0.3 m/1000 yr. Similar relations along the central segments of the Lemhi and Beaverhead faults suggest they have similar rates. In contrast, the Arco segment of the Lost River fault, which has about one-half the structural relief of the Borah Peak area, has a maximum mean slip rate of 0.1 m/1000 yr (see discussion of Arco segment; Pierce, 1985); the southern segment of the Lemhi fault probably has a similar rate. In addition to having greater structural relief, the central segments of the faults have moved more recently than the distal segments. The ages of latest faulting shown on Figure 3 are based on the stratigraphic relationship of fault scarps to surficial deposits whose ages are estimated by stratigraphic, geomorphic, and soil- development evidence (Malde, 1971; Hait and Scott; 1978; Pierce and Scott, 1982; Scott, 1982). The latest Quaternary scarps are typically 2 to 5 mhigh and displace

alluvial and glacial deposits of Pinedale age (as young as 12,000 to 15,000 yr). The late Pleistocene and older scarps are as high as 20 m. These do not displace latest Quaternary deposits, but do offset surficial deposits that are locally as young as 30,000 yr, but that are commonly more like deposits dated about 160,000 yr (see discussion of Arco segment; Pierce, 1985). A consistent pattern of ages of fault scarps on the western fronts of the ranges is evident on Figure 3. Late Pleistocene or older scarps occur along the southern sections of the range fronts within about 25 km of the Snake River Plain; scarps along the central and north-central sections of the ranges are latest Quaternary; and the absence of scarps along the northernmost range fronts suggests that in these areas the last surface-faulting event is older than late Pleistocene.

LOST RIVER FAULT AND 1983 SURFACE FAULTING Reconnaissance and detailed mapping of the Lost River fault defines 6 or 7 fault segments that are characterized by different geomorphic expression, structural relief, and ages of last movement that follow the pattern discussed previously (Figure 3). Fault scarps along the 20-kin-long Arco segment do not displace latest Quater- nary deposits, but do offset extensively preserved older fan surfaces by as much as 20 m. In the area of a trench across the fault (Malde, 1971, 1985), several stratigraphic datums allow estimates of the slip rate of this segment (Pierce, 1985). A surface estimated to be about 160,000 yr old based on uranium-series ages of layered carbonate coats on clasts in soils is offset about 19 m or possibly more, and a volcanic ash estimated to be about 70,000 to 110,000 yr old is offset about 8 m. Because of burial and limited exposure, the existence of back rotation or graben formation cannot be evaluated, so the slip rate of 0.07 to 0.1 m/1000 yr estimated from these datums is regarded as a maximum. Near the northern end of the Arco segment, the youngest faulting offsets fan gravels about 3 m (Pierce, 1985). The inner part of a carbonate coat on a clast from the soil in the faulted gravel has uranium-series dates of 23,000 and 30,000 yr (John Rosholt, written communication, in Pierce, 1985). Carbonate coats from the faulted gravel and slightly younger unfaulted gravel are about twice as thick as coats from a nearby surface estimated to be about 15,000 yr old (Pierce and Scott, 1982). Based on these uranium-series dates, carbonate-coat thicknesses, and the morphology of the scarp, Pierce (1985) concludes that the youngest faulting on the Arco segment occurred about 30,000 yr ago. South of the Arco segment, a zone of discontinuous scarps as high as 10 m that displace surficial deposits and lavas of late and middle Pleistocene age extends 12 km onto the Snake River Plain (Kuntz, 1978). Tlie exact age relationship of these scarps to the scarps along the Arco segment is not known, but both sets of scarps displace deposits of similar age. North of the Arco segment, the Pass Creek segment, which forms a marked dogleg in the range front, extends 30 km to just south of Lower Cedar Creek. Fault scarps are preserved only locally along this segment. The range front maintains a steep-faceted profile as in adjacent segments, has high structural relief, and is doubtless bounded by a fault. Surficial deposits older than about 30,000 yr are not well exposed along most of this segment, so the general lack of scarps is difficult to interpret, but faulting may not have occurred in the last 30,000 to 50,000 yr. The 22-km-long Mackay segment extends from Lower Cedar Creek, east of Mackay, to the prominent bend in the range front at Elkhorn Creek. Fault scarps

Downloaded from https://pubs.geoscienceworld.org/ssa/bssa/article-pdf/75/4/1053/2705124/BSSA0750041053.pdf?casa_token=TSBt38b2JcoAAAAA:Sjnn_gY_cB_R2IMO456_ATbCsQoCOtNgovEXPd9DpGdIY1HaLTZojbOELizVZuKSELwns_JcrA by California Geological Survey, 19774 on 01 April 2020 1060 WILLIAM E. SCOTT, KENNETH L. PIERCE, AND M. H. HAIT, JR. of latest Quaternary age occur throughout the entire segment, and generally are preserved even on steep (30 °) slopes at the base of the range front. A trench across the fault scarp at the mouth of Lower Cedar Creek in a pre-Pinedale fan deposit records several surface-faulting events, the last of which occurred after the deposition of a pod of Mazama ash (6800 14C yr old; Hait and Scott, 1978). A date of 4,320 + 130 14C yr B.P. (W-4427) from organic matter buried by colluvium derived from the free face formed during the last surface-faulting event indicates that this event probably occurred about 4,000 yr ago. The Mackay segment has less structural relief than the Thousand Springs segment to the north, suggesting that the Mackay segment has a lower long-term slip rate. However, the lack of evidence of pedimen- tation 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 alluvial-fan deposits by younger fan deposits (Scott, 1982) suggest that this segment has been very active in late Quaternary time. The Thousand Springs segment, which extends from Elkhorn Creek north to the Willow Creek hills, was the site of the greatest amount surface rupturing during the 1983 Borah Peak earthquake (Crone and Machette, 1984). This segment has the greatest structural relief (2.7 km; Figure 3) measured along the Lost River fault, and therefore probably has the highest long-term slip rate. Pre-1983 fault scarps of latest Quaternary age are best preserved at sites where the scarp lies a short distance out from the steep mountain front, such as at Cedar, Rock, and Willow Creeks. Trenches across the scarp at the mouth of Willow Creek (Hait and Scott, 1978; D. P. Schwartz and A. J. Crone, personal communication, 1984) where the scarp offsets an alluvial surface estimated to be about 15,000 yr old (Pierce and Scott, 1982) and a reconstruction of geomorphic surfaces across the fault (Vincent, 1985) indicate that one pre-1983 surface-faulting event occurred along the central part of this segment in latest Quaternary time. Based on the following limited evidence, the pre-1983 event on the Thousand Springs segment occurred probably in early Holocene or latest Pleistocene time rather than in late Holocene time as thought by Hait and Scott (1978) and Scott et al. (1985). A soil formed in the alluvium of the hanging wall and buried by colluvium derived from the pre-1983 fault scarp indicates that the 15,000-yr-old alluvial surface of Willow Creek had been exposed to soil-forming processes for some period of time prior to faulting. In the references cited above, we reasoned that because the buried soil and nearby relict soils formed in the same alluvium have similar degrees of development, most of the 15,000-yr-interval had passed prior to the pre-1983 event. On reexamination of the buried soil, we find that although the calcic C horizons of both soils are of similar thickness, the one in the relict soil contains much more secondary calcium carbonate, and therefore probably formed over a much longer period of time than the buried soil. Furthermore, the buried soil is at a shallow enough depth (several tens Of centimeters at most) that it may have continued to accumulate calcium carbonate after burial. We now favor a more conservative interpretation--the soil provides evidence of several thousand years passing between the stabilization of the alluvial surface and the pre-1983 event. Consistent with this interpretation, limited scarp-morphology data that was acquired along the pre-1983 scarp in the Willow Creek area (R. C. Bucknam, personal communication, 1985) suggest that it was morphologically similar to other scarps in the Basin and Range that formed in latest Pleistocene to early Holocene time, and that it was too degraded to be of late Holocene age. Therefore, we tentatively conclude that the pre-1983 event on the Thousand Springs segment is at least several thousand years

older than the last event on the Mackay segment, which occurred about 4000 yr ago. Near Willow Creek, many features of the 1983 break (Crone and Machette, 1984, Figure 4) closely mimic features of the pre-1983 scarp. The offset of the

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0 50 KILOMETERS 0 50 KILOMETERS f I I I Sites comparable to 1983 break and movement Segments adjacent to the ends of the main 1983 break triggered along faults linked at depth to the 1983 break

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0 50 KILOMETERS 0 50 KILOMETERS t. I I I Surface-faulting gaps Constant strain accumulation and characteristic offset

FIG. 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.

Willow Creek fan accompanying the 1983 earthquake was 1.5 to 2 m, compared to the pre-1983 offset of 2 to 2.5 m. Other features include a right-stepping pattern of short faults, a broad complex graben with a conspicuous horst, and minor thrust features west of the graben.

LIKELY SITES OF FUTURE FAULTING IN THE LOST RIVER-BEAVERHEAD AREA In view of our understanding of the 1983 Borah Peak earthquake and its Quaternary tectonic setting, what are likely sites in the area for future extensive surface faulting accompanying large earthquakes? Widespread evidence of late Quaternary surface faulting and the morphology of the range fronts suggest that future ruptures could occur along any part of the Lost River, Lemhi, or Beaverhead faults. Prediction of specific sites of future surface faulting requires an understanding of several conditions at depth including patterns and rates of strain accumulation, physical properties of the rocks, and linkages between the range-front faults. Unfortunately, we know little about these. Nevertheless, one can view the history of surface faulting, the geomorphic and structural features of the area, and the 1983 break from the following perspectives to gain insight as to which of the segments are more likely to rupture than others. Although these perspectives do not allow the site of the next event to be predicted with confidence, they are useful for identifying potential sites for the next several surface ruptures in the area. Sites comparable to 1983 break. The 1983 surface faulting occurred along a segment with both high structural relief, which indicates high long-term slip rates, and latest Quaternary offset. Consequently, similar segments can be viewed as likely sites for future surface faulting. The part of the Lost River fault that broke in 1983 and the central segments of the Lemhi and Beaverhead faults have these characteristics (Figure 4A) and therefore are likely sites. Movement triggered along faults linke d at depth with the 1983 break. If the Lost River, Lemhi, and Beaverhead faults are linked to a single west-dipping detachment at depth, then the 1983 slip along the central segment of the Lost River fault, which is the westernmost of the three, might trigger movements along one or several of the central segments of the faults to the northeast (Figure 4A) that would lie higher on the assumed detachment plane. This is analogous to the displacement of blocks in a landslide, in which the movement of one block removes support for blocks higher on the failure plane and causes some of them to move.

Downloaded from https://pubs.geoscienceworld.org/ssa/bssa/article-pdf/75/4/1053/2705124/BSSA0750041053.pdf?casa_token=TSBt38b2JcoAAAAA:Sjnn_gY_cB_R2IMO456_ATbCsQoCOtNgovEXPd9DpGdIY1HaLTZojbOELizVZuKSELwns_JcrA by California Geological Survey, 19774 on 01 April 2020 THE 1983 BORAH PEAK EARTHQUAKE, CENTRAL IDAHO 1063 Segments adjacent to the ends of the main 1983 break. The Mackay and Warm Springs segments have less structural relief than the Thousand Springs segment, which accounted for the main part of the 1983 surface rupture, and therefore probably have lower long-term slip rates. However, the similar height and morphology of the range along these segments and the position of latest Quaternary fault scarps on the lower slopes of the steep faceted spurs of the range front suggest that all three segments have had comparable rates of Quaternary faulting. In addition, the similarity in the height of the latest Quaternary fault scarps along these segments indicates all three probably had similar amounts of surface faulting in latest Quaternary time. Therefore, future surface-faulting events along the adjacent segments seem likely if these sections of range front are to keep pace with slip along the Thousand Springs segment (Figure 4B). From the evidence presented earlier, the pre-1983 event on the Thousand Springs segment (early Holocene or latest Pleistocene) probably preceded the last surface- faulting event on the Mackay segment (about 4000 yr ago) by at least several thousand years. The 1983 event on the Thousand Springs segment might then be expected to be followed by offset on the Mackay segment. We do not know the age relation of the pre-1983, latest Quaternary event(s) on the Warm Springs segment with those on the Mackay and Thousand Springs segments; however, the Warm Springs segment did have a small amount of surface rupture in 1983, which may act to delay its next event by having released the strain accumulated since its last rupture. Surface-faulting gap. A seismic gap is a fault segment that has not ruptured in historic time and that lies between historic fault scarps; such a gap is thought to be a likely site for future rupture (Wallace and Whitney, 1984). By analogy, a fault segment that lies between ones having evidence of more recent, but prehistoric, breakage can be called a surface-faulting gap, and be considere(] a likely site for future faulting. The Pass Creek segment of the Lost River fault has no evidence of late Pleistocene or Holocene movement; however, it lies in an area having high structural relief and an imposing range front (Figure 4C), which suggests that it probably has a long-term rate of faulting similar to that of adjacent segments that have ruptured in late Pleistocene or Holocene time. A few segments of the Lemhi and Beaverhead faults also fall in this category. Other possible gaps are shown in Figure 4C by the dashed bold lines along latest Quarternary faults; however, so little is known about the age relations between individual segments of these latest Quaternary faults that surface-faulting gaps cannot be defined with certainty. Characteristic offset assuming constant strain accumulation. Schwartz and Cop- persmith (1984) propose that a given fault segment ruptures when a certain strain threshold is reached and results in a characteristic surface offset. If we assume that slip rates estimated from geologic relations reflect mean strain rates on a segment, then an offset is likely when the product of the slip rate and the time since the last surface-faulting event approaches the characteristic offset. The Arco segment has an estimated maximum long-term slip rate of about 0.1 m/1000 yr and has not ruptured in the past 30,000 yr, which suggests a potential strain accumulation of as much as 3 m (Pierce, 1985) provided that the rate of strain accumulation in the last 30,000 yr is equal to the long-term rate. As the surface offset accompanying the 1983 earthquake and prehistoric offsets estimated from trench studies are less than this, movement on the Arco segment can be considered overdue (Figure 4D). The potential strain accumulation on other fault segments is not as well known; however,

limited geomorphic information on the southern segment of the Lemhi fault suggests that surface-faulting events may have a similarly long (104 yr) recurrence interval. The time since the last surface-faulting event on this segment is >15,000 yr, which implies a potential strain accumulation of >1.5 m by using the slip rate for the Arco segment. Grouping of events. Surface faulting along segments may not occur at uniform rates, but rather as intervals of activity along one segment or a belt of several segments separated by periods of diminished or no activity during which surface faulting is concentrated in another area (e.g., Wallace and Whitney, 1984; Wallace, 1985). If such grouping in space and time occurs in the Lost River-Beaverhead area, some of the preceding perspectives may simply be manifestations of it. For instance, grouping provides an explanation for surface-faulting gaps; the gaps are segments that are in a relatively inactive interval. Likewise, the occurrence of latest Quater- nary offsets on the central segments of the faults may be a functin of grouping. In addition, nonuniform rates of surface faulting for a given segment would negate the assumption of uniform strain accumulation for the characteristic offset perspective. The demonstration of grouping is a poorly understood but key component in understanding the kinematics of late Cenozoic deformation in the Lost River- Beaverhead area (as well as other sites in the Basin and Range) and thereby being able to better predict sites of future surface faulting. Whether or not grouping occurs, we regard several fault segments as the most likely candidates for future surface faulting in the Lost River-Beaverhead area. These sites are: (1) segments of the Lemhi and Beaverhead faults that ruptured in latest Quaternary time and have high structural relief, and that perhaps had some support removed by the 1983 displacement on the Lost River fault {Figure 4A) and (2) the Mackay segment of the Lost River fault (Figure 4B) because of the apparent similarity in the latest Quaternary fault activity along the Thousand Springs and Mackay segments.

ACKNOWLEDGMENTS We thank S. S. Oriel for encouraging and supporting our research during the late 1970's on Quaternary faulting in the Borah Peak region, as part of the U.S. Geological Survey's Snake River Plain Project. We appreciate the constructive reviews of the manuscript provided by K. J. Coppersmith, A. J. Crone, S. S. Oriel, M. W. Reynolds, and E. T. Ruppel.

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U.S. GEOLOGICALSURVEY DENVER FEDERAL CENTER DENVER, COLORADO80225

Manuscript received 27 December 1984