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 north- eastward 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 wide- spread 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 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 105~4 WILLIAM E. SCOTT~ KENNETH L. PIERCE~ AND M. H. HAIT, JR. moved in latest Quaternary time and most of these have evidence of a high rate of Quaternary faulting. Also, of the three historic surface-faulting events in the Intermountain Seismic Belt, (1) 1934 in Hansel V~ley, Utah (ML = 6.6, Arabasz et al., 1980, (2) 1959 near Hebgen Lake, Montana (Ms = 7.5, Doser, 1985), and (3) 1983 near Borah Peak, Idaho (Ms = 7.3), the latter two occurred in domain 1. 114 o 111 ° 46 ~ 42 o 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. 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 1055 Domain 1 includes the Wasatch fault (1A, Figure 2), which has slip rates that locally exceed 1 m/1000 yr (Swan et al., 1980) and has accommodated much of the east-west regional extension between the Great Basin and stable interior in latest Quaternary time, and the following active neotectonic elements that form a V- 114 0 111 ° ~ I I \ \ MONTANA I 4BO_ 2 \ IDAHO I 'Ji \ MADISON CREEK FAULT 3 // RED ROCK LEMHIF? L,T! ~'."'-C~CENTENNIAL I ' -- .':.'.'Y.ELLOWSTONE i~: FA U L~'~' I •:"! '-"i~;~IVER FAULT BEAVERHEAD -~ I FAULT / I [ "! ~. ,~:~:,~-~ / / ETON FAULT LOST RIVER FAULT / / / , WYOMING \ ..../J/4 7 //BFM 6,R VALLEY FAULT / 7 Z I _-4 m BEAR LAKE/ FAU.:: m I °3O NEVADA UTAH 1 A > FAULT 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. 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 1056 WILLIAM E. SCOTT, KENNETH L. PIERCE, AND M. H. HAIT, JR. This neotectonic belt of major Quaternary faults has a remarkable spatial relation to the Yellowstone-Snake River Plain axis (Y-SRP axis; Figure 2). In the northern part of this belt, the most active portion based on Quaternary geologic evidence (stippled on Figure 2) lies generally from several 10 to 100 km beyond the margin 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