Variable Normal-Fault Rupture Behavior, Northern Lost River Fault Zone, Idaho, USA GEOSPHERE

Research Paper

GEOSPHERE Variable normal-fault rupture behavior, northern Lost River fault zone, Idaho, USA

1 2 1 1 1,3 1, 2 GEOSPHERE, v. 15, no. 6 Christopher B. DuRoss , Michael P. Bunds , Ryan D. Gold , Richard W. Briggs , Nadine G. Reitman , Stephen F. Personius *, and Nathan A. Toké 1U.S. Geological Survey, 1711 Illinois Street, Golden, Colorado 80401, USA 2Utah Valley University, 800 West University Parkway, Orem, Utah 84058, USA https://doi.org/10.1130/GES02096.1 3University of Colorado–Boulder, UCB 399, Boulder, Colorado 80309-0399, USA

13 figures; 1 table; 3 sets of supplemental files ABSTRACT Springs section, and the southern half of the Warm Springs section, north CORRESPONDENCE: [email protected] of the Willow Creek Hills structure, a prominent hanging-wall bedrock ridge The 1983 Mw 6.9 Borah Peak earthquake generated ~36 km of surface where the LRFZ splits into multiple strands with differing strikes (Crone et al., CITATION: DuRoss, C.B., Bunds, M.P., Gold, R.D., Briggs, R.W., Reitman, N.G., Personius, S.F., and rupture along the Thousand Springs and Warm Springs sections of the Lost 1987) (Fig. 1). As one of the largest intraplate normal-faulting earthquakes Toké, N.A., 2019, Variable normal-fault rupture be- River fault zone (LRFZ, Idaho, USA). Although the rupture is a well-studied recorded historically and an example of the complex rupture of a multiseg- havior, northern Lost River fault zone, Idaho, USA: example of multisegment surface faulting, ambiguity remains regarding the ment normal fault system (Haller and Crone, 2004), the Borah Peak earthquake Geosphere, v. 15, no. 6, p. 1869–1892, https://doi.org /10.1130/GES02096.1. degree to which a bedrock ridge and branch fault at the Willow Creek Hills rupture offers an important opportunity to relate spatial and temporal patterns influenced rupture progress. To explore the 1983 rupture in the context of the of surface displacement to fault-rupture processes (e.g., Wesnousky, 2008; Science Editor: David E. Fastovsky structural complexity, we reconstruct the spatial distribution of surface dis- Nissen et al., 2014; Haddon et al., 2016; Delano et al., 2017; Personius et al., Associate Editor: Jose M. Hurtado placements for the northern 16 km of the 1983 rupture and prehistoric ruptures 2017; Johnson et al., 2018). in the same reach of the LRFZ using 252 vertical-separation measurements Although the subsurface rupture geometry (Boatwright, 1985; Doser and Received 29 November 2018 made from high-resolution (5–10-cm-pixel) digital surface models. Our results Smith, 1985; Smith et al., 1985; Richins et al., 1987) and slip (e.g., Ward and Revision received 16 May 2019 Accepted 15 August 2019 suggest the 1983 Warm Springs rupture had an average vertical displacement Barrientos, 1986), far-field crustal deformation (Stein and Barrientos, 1985; of ~0.3–0.4 m and released ~6% of the seismic moment estimated for the Barrientos et al., 1987), fault-zone structure (Janecke, 1993; Susong et al., 1990; Published online 8 November 2019 Borah Peak earthquake and <12% of the moment accumulated on the Warm Bruhn et al., 1991), and surface rupture extent and displacement (Crone et al., Springs section since its last prehistoric earthquake. The 1983 Warm Springs 1987) of the Borah Peak earthquake are well documented, uncertainty remains rupture is best described as the moderate-displacement continuation of pri- regarding the role the Willow Creek Hills structure played in controlling the mary rupture from the Thousand Springs section into and through a zone of length of the rupture (Crone et al., 1985; Bruhn et al., 1991). That is, did the structural complexity. Historical and prehistoric displacements show that the structure impede the lateral propagation of the 1983 rupture, where surface Willow Creek Hills have impeded some, but not all ruptures. We speculate faulting to the north along the Warm Springs section is secondary (nonseismo- that rupture termination or penetration is controlled by the history of LRFZ genic) in nature (Crone et al., 1987)? Or is the 1983 earthquake an example of moment release, displacement, and rupture direction. Our results inform the multisegment rupture in which the Willow Creek hills modulated, but did not interpretation of paleoseismic data from near zones of normal-fault structural fully stop slip propagation? Further, to what degree has the structure impeded complexity and demonstrate that these zones may modulate rather than the propagation of previous LRFZ ruptures? These questions are important in impede rupture displacement. recognizing how conditional probabilities of rupture through structural barriers (e.g., Oskin et al., 2015) may help explain evidence of multi-modal fault behavior (e.g., single-segment and multi-segment rupture; e.g., DuRoss et al., 2016), and ■■ INTRODUCTION ultimately help improve earthquake-rupture forecasts (e.g., Field et al., 2014). Here, we use high-resolution (5–10-cm-pixel) digital surface models (DSMs) The 1983 Mw 6.9 Borah Peak earthquake ruptured ~36 km of the ~130-km-long to improve our understanding of the 1983 rupture in the context of slip propa- Lost River fault zone (LRFZ, Idaho, USA) (Crone et al., 1987), one of several gation through the Willow Creek Hills structurally complex zone (Fig. 2). DSMs normal faults that accommodate dominantly SW-NE extension in the Centen- generated from low-altitude aerial photography derived from unmanned air- nial Tectonic Belt of the northern Basin and Range Province (Scott et al., 1985; craft systems (UAS) allow us to map the geometry and extent of deformation Stickney and Bartholomew, 1987; Payne et al., 2013) (Fig. 1). Surface rupture in the 1983 rupture, estimate the vertical displacement of geomorphic surfaces occurred along two structural fault sections, including all of the Thousand faulted by the LRFZ in both the 1983 and prehistoric earthquakes (Fig. 3), and This paper is published under the terms of the quantify trends in displacement along fault strike. We focus on the northern- CC‑BY-NC license. *Emeritus most 16 km of rupture, north and south of the Willow Creek Hills, using 252

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displacement observations (Supplemental Fig. S11). Measurements include 114°W 113°50' W geomorphic surfaces displaced by both the 1983 rupture (n = 196) as well as Idaho Montana prehistoric ruptures (n = 56). These observations highlight the complex sur- Challis face-rupture geometry near the Willow Creek Hills and clear and continuous section rupture of the southernmost Warm Springs section, and provide evidence BFZ W

that prehistoric ruptures of the Warm Springs section had displacements and a Challis r m along-strike extents distinctly different from the 1983 rupture. MC

LFZ

S N '

p 0

RC 2 r ° in Mackay 4 g LRFZ 4 s 1983 ESRP ■■ TECTONIC SETTING Mw 6.9 s e Arco c INL t Lone io Lost River Fault Zone n 20 Pine km fault The LRFZ is one of several NW-striking normal faults that accommodate SC dominantly SW-NE extension and terminate to the south near the northern margin of the eastern Snake River plain (Baldwin, 1951; Scott et al., 1985; Payne Willow Creek Hills et al., 2008) (Fig. 1). Six sections along the LRFZ have been proposed, includ- Dickey ing (north to south) the Challis, Warm Springs, Thousand Springs, Mackay, Peak

T Pass Creek, and Arco sections (Scott et al., 1985; Crone et al., 1987). These h o sections range from 15 km (Arco and Warm Springs sections) to 26 km (Pass u s a Creek section) in length (U.S. Geological Survey, 2018) and are defined using n Hwy 93 d PS

along-strike changes in fault geometry and geomorphology, structural relief

of the range front (including the presence of hanging-wall bedrock salients), '

1 DP ° and differences in the timing of most recent fault movement (Scott et al., 1985). 4 4 S Movement on the SW-dipping LRFZ since ca. 4–7 Ma has generated the p r in prominent SW-facing Lost River range front and ~2.7 km of maximum struc- g s Borah

tural relief, accounting for basin fill and range front topography (Scott et al., Peak

s 1985). This translates to a late Neogene average slip rate of ~0.4–0.7 mm/yr. e c

t i Scott et al. (1985) calculated a latest Pleistocene to present geologic slip rate o n of ~0.3 mm/yr for the Thousand Springs section, based on 3.5–4.5 m of vertical offset (including displacement from the 1983 earthquake) measured across

a ca. 15 ka fan surface (Pierce and Scott, 1982). Using the 1983 displacement EC (~1.5–2 m) and time since the previous surface-rupturing earthquake (~8 k.y.; Quaternary faults <1.6 Ma Scott et al., 1985), Hanks and Schwartz (1987) reported a single-event closed-in- <15 ka terval slip rate of ~0.2 mm/yr. Slip rates for fault sections north and south of 1983 AD the Thousand Springs section are not well known but are possibly <0.2 mm/yr 0 5 km Mackay based on geologic mapping and trench investigations (Scott et al., 1985; Olig 10 km section et al., 1995; U.S. Geological Survey, 2018). Figure 1. Surface-rupture extent of the 1983 Mw 6.9 Borah Peak earthquake (red), which ruptured the Thousand Springs and southernmost Warm Springs sections of the Lost River fault zone 1 Supplemental Figures. Lost River fault zone mapping, (LRFZ). The Willow Creek Hills are an area of hanging-wall bedrock and complex surface faulting topographic profiles, and vertical separation data 1983 Borah Peak Rupture that form a normal-fault structural barrier between the two sections. Yellow polygons show the (Fig. S1), aftershocks of the 1983 Borah Peak earth- extent of digital surface models generated in this study using low-altitude aerial imagery derived quake (Fig. S2), vertical separation in the Goosebury from unmanned aircraft systems. Fault traces and time of most recent faulting modified from graben (Fig. S3), vertical separation along the south- The 1983 Mw 6.9 Borah Peak rupture initiated at a depth of ~16 km (Doser U.S. Geological Survey (2018). Focal mechanism from Doser and Smith (1985); approximate lo- ern Warm Springs section (Fig. S4), vertical separa- and Smith, 1985) near the structural boundary between the Thousand Springs cation is 10 km south of figure extent (Richins et al., 1987). Triangles indicate paleoseismic sites: tion along the northern Arentson Gulch fault (Fig. S5). and Mackay sections (Susong et al., 1990) and propagated unilaterally NW RC—Rattlesnake Creek; SC—Sheep Creek; PS—Poison Spring; DP—Doublespring Pass; EC—Elk- Please visit https://doi.org/10.1130/GES02096.S1 or horn Creek; MC—McGowen Creek. Inset map shows regional context. LFZ—Lemhi fault zone; access the full-text article on www.gsapubs.org to (Boatwright, 1985; Doser and Smith, 1985; Richins et al., 1987), resulting in BFZ—Beaverhead fault zone; ESRP—Eastern Snake River Plain; INL—Idaho National Laboratory. view the Supplemental Figures. 36 ± 3 km of surface rupture (Crone et al., 1987) (Fig. 1). Finite fault models Base maps are National Elevation Data set 10 m and 30 m (inset map) digital elevation models.

114°W 113°55' W Topographic profile

0 4 km Surface projection 0 Historical rupture 0.5 m Scarp face projection profile Vertical separation 1983 surface rupture 0.6 m Prehistoric fault scarps N ' 0 1m 1983 Arentson Gulch fault 2 ° W 4

RC rupture (Crone et al., 1987) 4 a r 2 km 2.0 m m Cracks/unknown origin Eroded fault scarp (prehistoric)

1983 fault scarp & VS

VE=1.5x Eroded fault free face (historical)

Goosebury graben Regression Fig 4 Goosebury Scarp colluvium (historical) 4 km Creek domain Fig S3

S Scarp colluvium (prehistoric) pr in g s

Figure 3. Vertical separation measurement for a compound normal-fault scarp

formed by both prehistoric and historical fault rupture. Regression domains 6 km s e show points along profile used to calculate a linear surface projection. No ver- c t io n tical exaggeration (VE). Subset of profile, showing vertical separation for the historical rupture of the scarp, is enlarged by 50%.

8 km

suggest 1.5–2 m of dominantly dip-slip displacement occurred on a 40°–50° Fig S4 SW-dipping sub-planar normal fault (Doser and Smith, 1985; Stein and Bar- SC rientos, 1985; Barrientos et al., 1987). Surface faulting in the 1983 earthquake 10 km includes rupture of (1) the entire 24-km-long Thousand Springs section, (2) the

0.1 m southern 8 km of the ~15-km-long Warm Springs section to the north, and Sheep Creek (3) a fault that branches WNW from the range-front trace of the LRFZ at the

0 northern end of the Thousand Springs section (herein, the Arentson Gulch 12 km Willow Creek fault after Bruhn et al., 1991) and continues into the Willow Creek Hills (Crone N Fig S5 ' Hills 5

1 et al., 1987) (Fig. 2). The Willow Creek Hills are a prominent bedrock ridge ° 4 2 km 4 on the hanging wall of the LRFZ that form an apparent long-lived structural complexity between the Thousand Springs and Warm Springs sections (Figs. 1 Arentson Gulch fault 4 km 14 km and 2). Bruhn et al. (1991) suggest that the Willow Creek Hills are structur- Fig 5 ally perched (hung-up) on a SSW-plunging subsurface bedrock ridge in the 6 km LRFZ that increases in width with depth. Only a 9° change in the strike of the Dickey Peak range-front trace of the LRFZ occurs across the Willow Creek Hills (Crone et al., 1987) (Fig. 1). Aftershocks following the 1983 earthquake were most 16 km 8 km abundant near the Willow Creek Hills, eventually migrating to near the Warm Springs and Challis sections (Boatwright, 1985; Zollweg and Richins, 1985; Thousand Springs Richins et al., 1987). More recent (2010–2018) moderate-magnitude seismicity section distributed along the Challis section (Fig. S2 [footnote 1]) is likely part of a long-lived aftershock sequence (Pang et al., 2018) and highlights the area of Figure 2. Unmanned aircraft systems photography acquisition areas (yellow polygons), showing the 1983 and prehistoric ruptures of the Lost River fault zone (LRFZ (this study). The 1983 rupture increased Coulomb stress along the Challis section following the Borah Peak includes the southern 8 km of the Warm Springs section, the northern 6 km of the Thousand earthquake (Payne et al., 2004). Springs section, and the 8-km-long Arentson Gulch fault, which branches WNW into the Willow Surface rupture in the Borah Peak earthquake produced mostly continuous, Creek Hills structural barrier. The 1983 rupture had no net displacement within the 2-km-long Goosebury graben and did not break the ~5-km-long discontinuous trace of the LRFZ at the SW-facing normal-fault scarps with a minor (~17%) left-lateral component eastern boundary of the Willow Creek Hills (Crone et al., 1987). Ticks show along-strike fault (Crone et al., 1987). Vertical displacements are typically ~0.5–2 m and reach distance extending south from the northernmost rupture of the Warm Spring section. The 1983 a maximum of 2.5–2.7 m near the center of the Thousand Springs section rupture trace and 0.1 m vertical displacement of the Arentson Gulch fault outside of our Willow Creek Hills acquisition area is from Crone et al. (1987). Base map is National Elevation Data set (near Doublespring Pass Road; Crone et al., 1987). Measurements of laterally 10 m digital elevation model. RC—Rattlesnake Creek; SC—Sheep Creek; VS—vertical separation. offset cultural features (e.g., fences and roads) indicate 0.4–1.0 m of left lateral

strike-slip displacement along the Thousand Springs section (Crone et al., Stratigraphic evidence and geochronological results from an additional 1987). Along the northernmost Thousand Springs section near the Arentson trench and natural exposure suggest that the Thousand Springs paleoearthquake Gulch fault, complex faulting includes discontinuous normal and thrust faults may have occurred in the early Holocene. About 3 km northwest of Double (Crone et al., 1987) (Fig. 2). Displacement along the Arentson Gulch fault spring Pass Road, the Poison Spring (PS; Fig. 1) trench exposed faulted bedrock, decreases to the northwest: the southeastern part of the fault has 0.8–1.6 m hillslope colluvium, and scarp-derived colluvium from a prehistoric earthquake of vertical and 0.3–1.2 m of left lateral strike-slip displacement, compared to (Vincent, 1995). Charcoal from the uppermost part of the scarp-colluvial unit 0.3–0.6 m of vertical and 0.3–0.7 m of right lateral strike-slip displacement to yielded an age of 9045 ± 100 14C yr B.P. that provides a minimum constraint on the north (Crone et al., 1987). At the northern end of the Arentson Gulch fault the timing of the prehistoric rupture. A natural exposure at Elkhorn Creek (EC; (northern flank of the Willow Creek Hills), Crone et al. (1987) mapped zones Fig. 1) provides additional evidence of the prehistoric rupture of the Thousand of surface cracking and a single 0.1 m vertical displacement scarp (Fig. 2). Springs section (Vincent, 1995). At this site, a soil A horizon formed in allu- About 2–3 km west, two isolated and <0.05-m-displacement scarps correspond vial-fan sediments predates a prehistoric fault scarp; charcoal from the soil with the southern termination of the Lone Pine fault (Fig. 1), a Quaternary dated to 9710 ± 240 14C yr B.P. provides a maximum limit on the timing of the active, NE-dipping normal fault west of the LRFZ (U.S. Geological Survey, prehistoric rupture. Vincent (1995) used a similar pattern of surface displace- 2018). Crone et al. (1987) suggest a possible nontectonic (shaking-related) ment decreasing from the center (Doublespring Pass and Poison Spring) to origin for the 1983 Lone Pine scarps on account of the minor and isolated southern (Elkhorn Creek) parts of the Thousand Spring section to infer that nature of the slip. the Poison Spring and Elkhorn Creek sites record the same paleoearthquake. An important aspect of the 1983 rupture is the occurrence of prominent As a result, Vincent (1995) used the Poison Springs and Elkhorn Creek radio- gaps in surface displacement observed along the range-front-bounding LRFZ carbon ages to bracket the timing of the prehistoric Thousand Springs rupture (Crone et al., 1987). Northeast of the Arentson Gulch fault, the Willow Creek to between 9045 and 9710 14C yr B.P., or 10.2–11.1 ka, calendar calibrated here Hills abut the Lost River range front, coinciding with a 5-km-long portion of using OxCal (version 4.3; Bronk Ramsey, 2009). the range-front trace of the LRFZ that did not rupture in 1983 (kilometer marks Two trenches across the Warm Springs section (Fig. 1) provide evidence of 8–13; Fig. 2). Older, discontinuous scarps in this area (Crone et al., 1987) indi- a ~mid-Holocene earthquake. Schwartz and Crone (1988) excavated trenches cate previous ruptures of the LRFZ; however, the late Quaternary surfaces that across the section at two locations: an unnamed drainage ~2 km SE of McGowan are faulted have not been precisely dated. Northwest of the Willow Creek Hills, Creek (the Rattlesnake Canyon trench, RC; Fig. 2) and an unnamed drainage 1983 surface rupture resumes on the southeastern half of the Warm Springs within the northern part of the Willow Creek Hills near Sheep Creek (herein, the section (kilometers 0–8; Fig. 2). Displacements along this section decrease Sheep Creek trench, SC; Fig. 2). Although the RC and SC trench observations from ~0.5–1 m in the south to <0.2 m in the north, and only a few, small (<0.2 are unpublished, a summary by Schwartz and Crone (1988) indicates similar m) lateral offsets were measured (Crone et al., 1987). A nearly 2 km gap in the stratigraphic relations and ages. Charcoal derived from burn horizons within 1983 surface rupture occurs at the Goosebury graben in a zone of prominent basal scarp-derived colluvium at the sites yielded radiocarbon ages of 4940 prehistoric scarps (kilometers 3–5, Fig. 2). ± 200 14C yr B.P. (RC trench) and 5280 ± 240 14C yr B.P. (SC trench). Schwartz and Crone (1988) used the distance between the sites (~7.5 km) and degree of soil development to infer that the trenches record the same paleoearthquake on Paleoseismic Data the Warm Springs section, calendar calibrated here to shortly before 5.1–6.6 ka. Vertical displacement estimates are 0.75 m at the Rattlesnake Canyon trench Paleoseismic data for the northern LRFZ are from four paleoseismic trench and ~2.2 m at the Sheep Creek trench (Schwartz, written communication, 2016). sites and a natural exposure. Although constraints on event timing are sparse, we consider it most likely Trenches excavated near Doublespring Pass Road (DP; Fig. 1) exposed that earthquakes occurred at ca. 10–11 ka on the Thousand Springs section evidence of at least one paleoearthquake on the Thousand Springs section. and shortly before ca. 5–7 ka on the Warm Springs section. An initial trench excavated in 1976 by Hait and Scott (1978) found evidence for a single paleoearthquake expressed in alluvial-fan deposits postdating the most recent (Pinedale) glaciation (ca. 15 ka; see summaries by Crone, 1985; ■■ METHODS Schwartz and Crone, 1985; and Haller and Crone, 2004). A second trench, excavated immediately adjacent to the Hait and Scott (1978) trench follow- To investigate surface displacement along the LRFZ, we acquired low-altitude ing the 1983 earthquake, showed that 1983 rupture displacement at the site aerial photographs using UAS and a tethered balloon in May 2015 (Warm (1.7–2.0 m) had a similar magnitude compared to prehistoric displacement Springs section; Fig. 2) and May 2016 (Willow Creek Hills area, including the at the site (1.3–1.5 m) (Schwartz and Crone, 1985). Neither trench yielded age northern Thousand Springs section and Arentson Gulch fault; Fig. 2). Using constraints for the paleoearthquake. image-based (structure-from-motion) modeling, we generated 5–10-cm-pixel

DSMs for the northern 16 km of the 1983 rupture (Fig. 4), georeferenced using 2015 and 2016. UAS platforms and sensors included (1) a DJI Phantom that we ground control from kinetic differential global navigation satellite system customized to carry a Sony a5100 camera and 16mm prime lens, (2) a Falcon (GNSS) measurements of survey targets. We measured the vertical separation fixed wing with a Sony a5100 camera and 20mm prime lens, and (3) a 3DR (VS; Fig. 3) of geomorphic surfaces displaced by the LRFZ using topographic Solo with GoPro Hero4 Black (~3mm fisheye) camera. A Canon SX230 compact profiles extracted from the DSMs (e.g., Fig. 5; Fig. S1 [footnote 1]). camera set at a focal length of 28 mm was mounted on the Helikite. We used the Phantom and Helikite during the 2015 survey and the Phantom, Falcon, and Solo during the 2016 survey. Digital Surface Models For geographic control, we placed ground-control targets throughout the survey area. Targets consisted of ~0.3-m-diameter bucket lids, ~0.5-m-wide We generated DSMs from photographs acquired from low altitudes (mostly mesh construction flags, and, most commonly, ~1.5-m-square black and <100 m) using UAS and a Helikite tethered balloon during field campaigns in white (iron-cross pattern) vinyl targets. For our ~11.9 km2 acquisition area, we

0 0

Vertical separation (m) - prehistoric <2 m 2-3.5 m 3.5-4.5 m 1 Warm 1 Vertical separation (m) - 1983 <0.25 m 0.25-0.5 m 0.5-1.04 m 2 2 1983 Surface rupture certain uncertain

Springs concealed 3 3 unknown Prehistoric fault scarps Figure 4. Extent and magnitude of 1983 displacement certain along (A) the southernmost Warm Springs section, uncertain (B) the northern Thousand Springs section, and Ar- concealed entson Gulch fault near the Willow Creek Hills. Ticks Fig 8 4 4 Goosebury show kilometer marks along the Lost River fault zone

graben and Arentson Gulch fault. Scarp-profiles and vertical separation measurements are also shown in Figures 5 and 6 and Figure S1 (see text footnote 1). Base Fig S3 maps are slopeshade maps derived from low-altitude 5 5 unmanned aircraft systems photography (Bunds et al., 2019); point clouds available at Open Topography (https://doi.org/10.5069/G9222RWR). (Continued on following page.)

6 section 6

7 7

8 8

± Slope 0 2 km 0º ≥40º Fig S4

2 2 12

3 3 Slope 13 º º 0 ≥40 4 4 A ren Vertical separation tson 5 5 (m) - prehistoric G Fig S5 ulch 0-2 m f au 2-4 m lt 14 6 4-6 m 6 Fig 9 6-8 m

Vertical separation (m) - 1983 Springs section 0-0.5 m Thousand 7 0.5-1 m 7 15 1-2 m 1983 Surface rupture certain 8 uncertain 8 concealed unknown Prehistoric fault scarps certain 0 2 km uncertain concealed

Figure 4 (continued).

placed 186 control points, including ~28 control points/km2 along the Warm were recognized in individual photographs, (3) used the OPUS-corrected dif- Springs section (~4.6 km2) and ~8 control points/km2 near the Willow Creek ferential GNSS data to provide spatial control for the markers, (4) for 2016 data, Hills (~7.6 km2). Target locations were measured in the field using kinetic dif- removed sparse cloud points with large uncertainty via Photoscan’s gradual ferential GNSS systems and local geodetic-quality base stations (Trimble 5700 selection tools, and (5) optimized cameras (least squares bundle adjustment). receiver with Zephyr Geodetic I antenna) that were postprocessed in the NAD83 We merged 12 subregions that form the surveyed portion of the Warm Springs (2011) reference frame using the National Geodetic Survey’s Online Positioning section in Photoscan, re-optimized the camera models, and built a dense point User Service (OPUS; National Oceanic and Atmospheric Administration, 2018). cloud using high quality and aggressive depth filtering. Six subregions of Control-point positions were transformed into NAD83 (2011) UTM zone 12 and the Willow Creek Hills area were combined and processed in the same fash- orthometric heights using GEOID12A. ion as the Warm Springs section data except the merged sparse point cloud We used Agisoft Photoscan image-based photogrammetric modeling soft- was cleaned of high uncertainty points. We did not automatically classify the ware (versions 1.2.6–1.4.3) to generate two separate point clouds and DSMs dense point cloud because in some cases steep topography and locally heavy (available at Open Topography, http://opentopo.sdsc.edu/datasets; Bunds et vegetation prevented imaging of the ground surface. The DSM of the Warm al., 2019), one for the Warm Springs section (2015 data) and one for the Willow Springs section was generated in Photoscan from a mesh built in Photoscan. Creek Hills area (2016 data). Image-based modeling uses feature recognition The DSM of the Willow Creek Hills area was generated in Photoscan using and photo alignment to generate a point cloud for a 3-D surface (Matthews, its DEM generation tool (inverse-distance weighting method). The merged 2008; Harwin and Lucieer, 2012; Bemis et al., 2014; Johnson et al., 2014; Reitman DSMs (Bunds et al., 2019) are 10-cm-pixel resolutions. We used GIS software 2 2 Supplemental Text. Agisoft Photoscan digital surface et al., 2015). Our DSM workflow (Supplemental Text S1 ) consisted of the fol- to visualize the DSMs, create slopeshade and hillshade maps, and map fault model workflow (Text S1), vertical separation data lowing steps. The surveyed areas were initially subdivided into 18 subregions. traces and surficial geology. (Text S2), tutorial for Matlab code Scarp_VS.m (Text In Photoscan, for each subregion we (1) added photographs then generated a Error in our DSMs results from a complex interplay of factors such as cam- S3). Please visit https://doi.org/10.1130/GES02096.S2 or access the full-text article on www.gsapubs.org to sparse point cloud and aligned cameras using Photoscan’s highest accuracy era exposure settings, lens specifications, camera calibrations applied, flight view the Supplemental Text. setting, (2) manually placed markers on ground-control target centers that design (e.g., flight-line geometry and altitude), photograph overlap, sensor

2215 WS p38 regression 1.95 m antithetic: domain (1.40 m) 0.5–0.6 m 2210 Prehistoric (PH)

Elevation (m) 50 60 70 80 90 100 110 120 130 140 Distance (m) 2178 WS p26 2228 WS p40 4.10 m (2.88 m) 2177 2224 antithetic: 2176 0.26 m ~1.2 m PH 2220 2175 Elevation (m) 90 100 110 120 130 140 150 160 Elevation (m) 2174 1983 Distance (m) 2 3 4 5 6 7 8 9 10 11 Distance (m) WCH p56 2243

2241 1.83 m

2239 1983 Elevation (m) 0 5 10 15 20 25 30 35 40 45 50 Distance (m) 2248 WCH p58 2246 1.50 m 2244 Elevation (m) 2242 1983 40 50 60 70 80 90 Distance (m) 2238 WCH p54 2.95 m 2234

2230 PH+1983 Elevation (m) 0 20 40 60 80 100 Distance (m)

2256 WCH p61 3.49 m 2252 PH+1983

Elevation (m) 0 20 40 60 80 100 Distance (m) 2340 WS p74 WS p76 2339 2346 0.68 m 2338 2345 0.69 m excluded 1983 2337 1983 points 2344 Elevation (m) Elevation (m) 0 1 2 3 4 5 6 2350 0 1 2 3 4 5 6 7 8 9 Distance (m) Distance (m) 2350 2346 WS p77 WS p75 3.90 m 2342 3.82 m 2345 2338 Elevation (m) 2340 PH+1983 PH+1983 2334 10 20 30 40 50 60 70 0 10 20 30 40 50 Distance (m) Distance (m)

Figure 5. A subset of vertical separation (VS) measurements for scarps formed in the 1983 and prehistoric (PH) earthquakes along the Warm Springs and Thousand Springs sections to illustrate data quality and VS measurement method. For profiles U38 and U40, value in parentheses is net slip accounting for far-field antithetic faulting. Dotted boxes show regression domains, which were defined manually and used to determine linear surface projections (blue lines). Orange points represent vegetation points that we removed manually. No vertical exaggeration. Profile locations for all VS measurements shown in Figure S1 (see text footnote 1). WCH—Willow Creek Hills; WS—Warm Springs.

dimensions and ground sample distance, ground-control target placement (Caskey, 1995) (Fig. 5). For profiles with heavy vegetation, we manually selected and GNSS measurement, and effects from vegetation and topography. We and removed profile points above the ground surface prior to defining foot- applied a large number of strategies to reduce error, most important of which wall and hanging-wall surface projections. Because footwall fault exposures were (1) a dense network of ground-control targets, (2) placement of targets (preserved free faces) along the 1983 rupture are rare, we calculate VS rather on relatively flat ground away from vegetation to minimize any vertical and/or than fault throw, which requires assumptions about near-surface fault dip (e.g., horizontal movement during use, (3) use of high-resolution cameras, fast shut- Johnson et al., 2018). Although slope steepness and fault dip can enhance the ter speeds (>1/1000 s), and camera mounts that did not maintain perfect nadir discrepancy between actual fault throw and measured VS (e.g., Mackenzie camera direction, (4) varied flight altitudes, (5) GNSS base station occupations and Elliott, 2017), our VS measurements are likely ~80%–90% or greater of exceeding 8 h, and (6) application of best practices for processing in Agisoft throw because steep (60°–90°; Crone et al., 1987) near-surface faults dip in Photoscan (Matthews, 2008; James and Robson, 2012; Bunds et al., 2015). We the same direction as moderately (<15°) sloping geomorphic surfaces (Cas- assessed DSM errors using checkpoints and methods similar to those specified key, 1995). Differences between VS and throw along strike are small relative for U.S. Geological Survey Q1 airborne lidar (Heidemann, 2018). Checkpoints to along-strike changes in the VS and throw values themselves, making our are differential GNSS location measurements taken on bare, relatively flat VS measurements valid for evaluating along-strike surface-rupture behavior. ground across the surveyed area. We measured 82 checkpoints on the DSM Our VS measurements do not account for lateral slip and the possibility of of the Warm Springs section, and 28 on the Arentson Gulch fault and northern two-dimensional profiles sampling non-correlative surfaces because of later- Thousand Springs section. The misfit in elevation of the checkpoints relative ally offset geomorphic features (e.g., Mackenzie and Elliott, 2017). However, as to the DSMs was determined using GIS software, and root-mean-square-er- a whole, we expect the influence of lateral slip on VS to be minimal because ror (RMSE) was calculated for the checkpoints. RMSE for the Warm Springs of the minor (~17%) strike-slip component, steep fault dips, and moderate and section DSM is 6.4 cm and 5.8 cm for the Arentson Gulch fault and northern laterally continuous surface slopes. Thousand Springs section. DSM precision across distances of meters to tens For complex fault rupture patterns (e.g., multiple synthetic and/or antithetic of meters (i.e., common topographic profile lengths) is likely better than the traces), we adopted two approaches to measuring VS. For narrow zones of overall RMSE for the models as the fine topography of features such as cob- faulting (less than a few tens of meters wide), we accounted for complex bles, boulders, and small plants are well resolved in the DSMs. faulting (e.g., graben formation) by selecting upper and lower far-field points outside of the fault zone(s). For complex scarps, such as those consisting of multiple synthetic scarps and/or antithetic scarps several tens of meters or Vertical Separation Measurements more from the primary scarp, we subdivided the profile into shorter segments and summed the VS results. For compound scarps consisting of both 1983 We used topographic profiles extracted from the DSMs to estimate the ver- and prehistoric rupture, we used short profiles (generally <10 m in length) to tical separation (VS) of geomorphic surfaces displaced by the Warm Springs reconstruct 1983 VS, and longer profiles (tens of meters in length or more) to section (n = 133) and northern Thousand Springs section (n = 119), including extract the cumulative (prehistoric and 1983) VS. the Arentson Gulch fault (Fig. S1 [footnote 1] and Text S2 [footnote 2]). Profile To account for uncertainties in our measurements, we conducted multiple locations were manually chosen and drawn on hillshade and slopeshade maps (typically five to seven) VS-measurement iterations for each profile. Each iter- using GIS software and extracted from the DSMs using Quick Terrain Modeler. ation used a different subset of the original ground-elevation points to define The profile lengths are mostly <25 m long for recent and steep 1983 scarps and alternative upper and lower far-field surface projections, thereby standardizing <300 m long for the larger and more eroded, prehistoric compound scarps. For and facilitating multiple geomorphic interpretations, both factors that control some 1983 scarps, short (<10-m-long) profile lengths reflect the limited extent measurement error (e.g., Salisbury et al., 2015). This method is especially of correlative surfaces across the rupture (e.g., Figs. 3 and 5). We found that useful for non-bare-earth DSMs, where geomorphic surfaces adjacent to the randomly generated profiles or strictly imposed sample-distance intervals rupture can be partially obscured by vegetation. We use the VS results from resulted in poorly sited measurements and introduced unnecessarily inflated these iterations to define minimum and maximum VS as well as a subjective error estimates. With these limitations in mind, we subjectively chose profile best-fit preferred VS selected by assessing the linear fits to the geomorphic locations across correlative surfaces with the best geologic preservation and surfaces and the results of the multiple measurement iterations. We assume sparsest vegetation to ensure the highest-quality measurements. that the minimum and maximum VS bounds capture the range of possible For each profile, we measured VS using a Matlab script (Supplemental VS measurements (e.g., similar to Scharer et al., 2014 and Gold et al., 2015). Code S13 and Text S3 [footnote 2]) and the following process. For each The precision of our VS measurements is limited by DSM resolution, sur- 3 Supplemental Code. Matlab code Scarp_VS.m (Code topographic profile, linear approximations of the footwall and hanging-wall face texture, and vegetation height and density. For most of the 1983 rupture, S1). Please visit https://doi.org/10.1130/GES02096 .S3 or access the full-text article on www.gsapubs.org to geomorphic surfaces were projected to the midpoint of the scarp face; VS is the we were able to resolve displacements as small as ~0.2 m. Relatively smooth view the Supplemental Code. difference in elevation between the surfaces, measured at the scarp midpoint ground surfaces and a predominance of shortgrass vegetation allowed us to

resolve ~0.1 m displacements along the Warm Springs section (northernmost approximate the uncertainty in the moving average, rather than the full range extent of the 1983 rupture). VS values for compound scarps generally exceed of possible VS values. Mean VS values (and min-max values in parentheses) these lower limits. reported below for different ruptures or fault sections are based on the displacement curves, rather than the individual observations, which are unequally distributed along the fault trace. Along-Strike Displacement Curves

We constructed along-strike displacement curves using our VS data (Figs. 6 ■■ DISPLACEMENT ALONG THE 1983 BORAH PEAK RUPTURE and 7). We projected all VS data to a simplified fault trace with kilometer tick marks shown on Figure 2. For the Arentson Gulch fault, we also project VS 1983 Warm Springs Rupture measurements to an 8-km-long simplified trace, oriented WNW-ESE (Fig. 2). The mean along-strike displacement curve represents a 0.5 km moving average The Borah Peak earthquake ruptured ~8 km of the Warm Springs section applied to our preferred estimates of VS, linearly interpolated at 50 m intervals. north of the Willow Creek Hills (kilometers 0–8; Fig. 4A). We document rupture These parameters yield displacement curves that strike a balance between both north and south of the Goosebury graben (kilometers 3–5; Fig. 4A), an short-wavelength, hectometer-scale variability and longer-wavelength, kilo- ~2-km-long by ~0.1–0.3-km-wide zone of synthetic and antithetic faulting. Crone meter-scale trends. To define the uncertainty bounds in the curves, we applied et al. (1987) reported only minor 1983 rupture of the northernmost part of the the same linear interpolation and moving average methods to the minimum graben, including 0.03 m of synthetic displacement and 0.05 m of antithetic dis- and maximum VS values from each scarp profile. Thus, our uncertainty ranges placement. Mean VS for the Warm Springs rupture is 0.3 (0.2–0.4) m including the

1983 rupture of the Warm Springs section 1 This study (n=100): 0.3 (0.2-0.4) m

0.8 Crone et al. (1987) (n=25): 0.2 (0.2-0.3) m

0.6 Figure 6. Vertical separation (VS) along 0.4 the southern 8 km of Warm Springs section. (A) 1983 VS measured in this study (red) compared to those of Crone et al. Vertical separation (m) 0.2 Goosebury graben (1987) (blue) for the 1983 surface rupture. RC shows displacement measured at the RC 0 Rattlesnake Canyon trench (Schwartz, writ- 0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 ten communication, 2016). (B) Cumulative VS for prehistoric scarps along the Warm Willow Creek Springs section, showing scarps having VS Prehistoric ruptures of the Warm Springs section Hills of ≤2 m (PE1; blue line and shading) and PE2: Scarps >2m (n=26) 5 >2 m (PE2; magenta line and shading). Plus 3.3 (2.8-3.8) m signs (1983 rupture) and circles (prehistoric) 4 PE1: Scarps ≤2m (n=9) indicate preferred VS values; vertical lines 1.7 (1.3-1.9) m show min-max VS range based on multiple VS measurement iterations (see text 3 PE2 for discussion).

2 PE1 1 1983 Goosebury graben

Cumulative vertical separation (m) 0 0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 Distance SE along the Warm Springs section (km)

1983 rupture of the Arentson Gulch fault 2.0 This study (n=62): 0.7 (0.6-0.8) m Crone et al. (1987) (n=17): 1.5 0.4 (0.3-0.6) m

1.0 Figure 7. Vertical separation (VS) along the 8-km-long Arentson Gulch fault near 0.5 the northernmost Thousand Springs sec- Vertical separation (m) tion. (A) 1983 VS measured in this study (red) compared to those of Crone et al. 0 0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 (1987) (blue) for the 1983 surface rupture. Willow Creek (B) Cumulative VS for prehistoric scarps Hills (squares), including VS for compound (including 1983 and prehistoric displacement) and single-event (prehistoric displacement Prehistoric rupture of the Arentson Gulch fault only) scarps. Plus signs (1983 rupture) and 3.5 Compound fault scarp (prehistoric and 1983 rupture) squares (prehistoric) indicate preferred VS 3.0 prehistoric values; vertical lines show min-max VS Single-event (prehistoric) fault scarp + 1983 range based on multiple VS measurement 2.5 iterations (see text for discussion). Average vertical separation (n=14): 2.0 1.5 (1.2-1.6) m

1.5

1.0 1983

0.5 single-event (prehistoric)

Cumulative vertical separation (m) 0 0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 Distance SE along the Arentson Gulch fault (km)

displacement gap along the Goosebury graben (single, 8-km-long fault trace), or The 1983 rupture south of the graben includes a previously unrecognized, 0.4 (0.3–0.5) m, excluding the graben (two fault traces totaling 6.5 km in length). ~1-km-long zone of distributed splay faulting that continues 0.3–0.5 km into the The 1983 rupture is most prominent south of the Goosebury graben (kilo- hanging wall of the range-front trace of the LRFZ (kilometer ~5; Fig. 4A). The meters 5–8; Fig. 4A). Here, the rupture has a continuous, sinuous trace for northern limit of this zone of distributed scarps is unknown as it falls within ~3 km––a geometry that follows prehistoric fault scarps along the Warm a gap in DSM coverage, but it is likely <0.4 km northwest of scarps mapped Springs section and thus likely mimics previous ruptures (discussed below) in this study as we did not observe similar faulting in DSMs southwest of the (Fig. 4A). VS measurements have considerable scatter (~0.2–1 m) but exhibit Goosebury graben (Fig. 4A). Displacement along the splays corresponds with a clear displacement peak (~0.7–1.0 m) ~1–2 km north of the Willow Creek a section of the LRFZ south of the Goosebury graben that did not rupture in Hills (kilometers 7.0–7.5; Fig. 6A). VS decreases relatively steeply to the south, 1983. VS measurements for the splay faults decrease from south (~0.6–1.0 m) to toward the northern flank of the Willow Creek Hills, and more gently to the north (~0.2–0.3 m). These data form a cluster of VS observations at the southern north, toward the Goosebury graben. Our VS measurements broadly agree margin of the Goosebury graben (kilometer ~5.0–5.2; Fig. 6A), 0.2–0.4 km north with those made by Crone et al. (1987) (Fig. 6A). Only a single displacement of a previous observation made by Crone et al. (1987) (kilometer 5.4; Fig. 6A). recorded by Crone et al. (1987) is outside the range of our nearby VS data. North of the Goosebury graben, the 1983 rupture has a nearly continu- At kilometer ~6.2 (Fig. 6A), Crone et al. (1987) measured ~1 m of 1983 dis- ous and sublinear trace for at least 3 km (kilometers 0–3; Fig. 4A). Here, our placement on a compound scarp, whereas we report ~0.5–0.7 m of VS for six preferred VS measurements yield a VS curve that is consistently ~0.2–0.3 m profiles within ~0.1 km of this original measurement. It is possible that our (Fig. 6A); however, large uncertainties in the individual VS measurements allow estimates are minima or that the Crone et al. (1987) measurement includes a a range of values from ~0.1 to ~0.6 m. Although the scatter in these data reflect minor component of prehistoric displacement. the difficulty in using a non-bare-earth DSM to capture the small-displacement

rupture tail, we have confidence in the results because the coherent along- m near the SE end and ~0.4–0.8 m near the NW end to ~2.0 m near the cen- strike VS signal is generally consistent with small, less than ~0.3-m-high scarps ter (Fig. 7A). These measurements are similar to, and in some cases, slightly observed in the field and measured by Crone et al. (1987). (<0.5 m) larger than, previous estimates (Crone et al., 1987). Mean displacement Our VS measurements for the rupture of the Warm Springs section north of for the Arentson Gulch fault is 0.7 (0.6–0.8) m. the Goosebury graben generally exceed by as much as ~0.2 m those made by Near the northern end of the 1983 rupture of the Thousand Springs sec- Crone et al. (1987) (Fig. 6A). Although unrecognized prehistoric scarps along tion, mostly E-W–oriented fault scarps form a zone of faults that connect the this part of the Warm Springs section could account for the VS disparity, we Arentson Gulch and range-front-trace faults (kilometer ~13.5; Fig. 4B). This suspect that differences in scarp-measurement methods explain our greater ~1-km-long zone includes three parallel traces, each having ~0.1–0.4 m of VS VS values. For some Crone et al. (1987) measurements, a tape measure was (Fig. S1 [footnote 1]). A single fault trace that connects to the range-front trace used to measure the vertical height of the scarp free face (J. Lienkaemper, writ- of the Thousand Springs section has ~0.5–1.0 m of displacement. Based on ten communication, 2018). Our profiles, which ranged from meters to tens of the sum of the VS measurements for the separate traces and the maximum meters long, may have captured additional off-fault, but near-field distributed VS for the single strand, these faults accommodate ~1 m of down-to-the-south deformation and yielded larger VS values. displacement. Although right-lateral faulting is possible given the orientation of the faults, we did not find evidence of laterally offset geomorphic features.

1983 Thousand Springs Rupture 1983 Borah Peak Earthquake Mean Displacement Our analysis of the Thousand Springs section is focused on 7 km of the 1983 surface rupture along the section near the Willow Creek Hills (Fig. 2). The The mean vertical displacement for the entire Borah Peak rupture is import- 1983 surface rupture in this area is complex, including three zones of faulting: ant for estimating seismic moment release (discussed below) and placing our (1) displacement along the range-front trace of the LRFZ at the base of the Lost observations in the context of global regressions relating surface displacement River range, (2) the Arentson Gulch fault, which deviates to the WNW from the to earthquake magnitude (e.g., Wells and Coppersmith, 1994). We estimate range-front fault at an ~35° angle (Fig. 4B), and (3) a complex zone of E-W to ESE- a mean vertical displacement for the rupture of 0.93 (0.76–1.10) m, based WNW–trending faults between the range-front LRFZ and Arentson Gulch faults. on displacement curves (0.5 km moving average) fit to linearly interpolated We imaged ~1 km of the 1983 rupture along the range-front trace of the (50 m spacing) VS data reported in Crone et al. (1987) for the Thousand Springs Thousand Springs section (kilometers ~13.5–14.5; Fig. 4B). Here, 1983 scarps (~1 section and our VS measurements for the Warms Springs section and Arent- m VS) are superimposed on large (~5–8 m VS) scarps within latest Pleistocene son Gulch fault, which are generally denser and slightly larger than those glacial sediment (Pierce and Scott, 1982). For most of this 1 km section of rupture, reported by Crone et al. (1987). Although Crone et al. (1987) did not report a VS is ~0.8–1.0 m (~0.5–1.5 m with scatter and uncertainties; Fig. S1 [footnote 1]), mean displacement, we estimate a mean of 0.84 (0.67–1.01) m using displace- consistent with four previous observations of ~0.8–1.2 m (Crone et al., 1987). ment curves fit to their original VS data set for the entire rupture. Wesnousky VS decreases abruptly to the north to ~0.2–0.3 m, ~0.25 km north of where the (2008) reported 0.94 m based on linearly interpolated (>100 m spacing) data E-W transfer faults and NW-SE range front faults intersect (Fig. 4B). We did from Crone et al. (1987). We report a total length of 34.2 km, which reflects not construct displacement curves for this part of the rupture because of the surface displacements measured at the northern extent of our study area near complex deformation and limited along-strike extent of our VS measurements. McGowan Creek to the southern rupture terminus as mapped by Crone et al. The Arentson Gulch fault comprises an 8-km-long and <1-km-wide zone (1987) (Fig. 1). This length excludes an ~1-km-long zone of surface cracks and a of en echelon, mostly south-facing fault scarps that nearly cross the Willow single 0.04 m displacement scarp ~3–4 km northwest of McGowan Creek (Crone Creek Hills ~3.5 km west of the range-front trace of the LRFZ (Fig. 2). The rup- et al., 1987) as we cannot rule out a shaking related origin for these features. ture trace is complex near its SE end, forming a discontinuous zone of faults that terminate ~0.5 km west of the range-front LRFZ trace (Fig. 4B). The NW end is likely near the northern flank of the Willow Creek Hills (3 km NW of ■■ PREHISTORIC RUPTURES OF THE LRFZ the northernmost scarps measured in this study), based on small (~0.1–0.6 m VS) scarps measured by Crone et al. (1987) (Fig. 2). Although additional 1983 Warm Springs Section scarps roughly on trend with the Arentson Gulch fault are present ~5 km NW of the Willow Creek Hills (Crone et al., 1987), we exclude these disconnected Our high-resolution DSMs allow detailed inspection of 1983 and prehistoric and small (mostly ~0.05 m) displacement scarps in our 8 km length estimate fault scarps along the Warm Springs section. With the exception of the Goose- as we cannot rule out a secondary (i.e., shaking-related) origin. VS along the bury graben and the southernmost terminus of the Warm Springs section, the Arentson Gulch fault increases toward the center of the fault, from ~0.2–0.4 scarps are compound in nature with evidence of both the 1983 and at least

Prehistoric fault scarp T0 Young fluvial terrace with modern channel (unfaulted) vertical separation T1 Lowermost fluvial terrace 0.2 m 2.8 m measure along topographic profile 0.2 m T2 Intermediate fluvial terrace 1983 Borah Peak earthquake rupture T3 Uppermost fluvial terrace

0.1 m

0.2 m

2.8 m

3.9 m T3

T1 P38

T2 1.4 m T1

T0 T0 2.4 m

T1 2.9 m T2

0.2 m P40

Slope T3

0º ≥40º Not mapped 0 100 m 0 100 m

Figure 8. (A) Digital surface model for the NE part of the Goosebury graben (Fig. 4A). (B) Surficial geologic map showing four fluvial terraces. Scarp profiles (green and blue lines; Fig. 5 and text for explanations) show that terraces T2 and T3 record 2.4–3.9 m of VS compared to inset terrace T1 which records ~1.4 m. Terrace T0 is not faulted. Black lines depict prehistoric fault traces. The 1983 rupture (red) had no net displacement across the Goosebury graben. Labeled profiles (e.g., P38) are shown in Figure 5.

two prehistoric ruptures (e.g., Figs. 3 and 5). In total, VS measurements for 34 the largest VS measurements (~3–4 m) correspond with the stratigraphically prehistoric scarps range from 1.4 to 4.5 m (Fig. 6B). VS ranges are similar north oldest (highest elevation) alluvial-fan surfaces and likely include displacement of (1.4–3.2 m), south of (1.5–4.5 m), and within (1.4–4.1 m) of the Goosebury from the 1983, PE2, and PE1 ruptures. Inset surfaces exhibit less displacement graben (Fig. 6B). Displacements along the Warm Springs section generally (generally, ≤2 m), and likely only include faulting from the 1983 and PE1 rup- cluster within two groups: those ≤2 m (n = 8) and those >2 m, which are tures. For example, within the Goosebury graben (kilometers ~3–3.5; Fig. 4A), concentrated between ~2.5 and 4.0 m (n = 26). Based on these displacement the youngest faulted alluvial-fan surfaces have ~1.4–2.1 m of VS, whereas the populations as well as our geomorphic mapping discussed below, we infer highest (oldest) surfaces are displaced ~2.8–3.8 m (Figs. 8 and S3 [footnote 1]). that at least two earthquakes prior to the 1983 earthquake ruptured the Warm Beyond the southern extent of the 1983 rupture along the Warm Springs sec- Springs section since the latest Pleistocene (time of the last glacial maximum): tion (kilometer ~8; Fig. 4A), an inset fan surface has 2.0 m of VS compared to PE2 (VS >2 m) and PE1 (VS ≤2 m). older surfaces that are displaced 2.7–3.5 m (Fig. S4). At three areas along the Warm Springs section (kilometers 3, ~3.5, and 8; We computed along-strike displacement curves for scarps having VS val- Fig. 2), prehistoric scarps cut multiple geomorphic surfaces and support our ues of ≤2 m, including the 2.1 m VS measurement for the inset Goosebury interpretation of two previous earthquake ruptures (Fig. 4A). At these sites, graben surface (herein, ≤2 m curve) and >2 m (herein, >2 m curve) (Fig. 6B).

The curves are cumulative and include the 1983 rupture displacement. Both displacement tapering to the north and south (Fig. 7B). The mean (cumulative) curves have similar, broadly uniform along-strike shapes and barely overlap VS is 1.5 m (1.2–1.6 m). within their uncertainties. Although these curves reasonably approximate prehistoric displacement along the Warm Springs section, because we lack VS data for the northernmost part of the section, they may not capture the total Displacement Difference Curves variability or along-strike trend in prehistoric VS. The ≤2 m curve suggests a pattern of approximately uniform along-strike displacement and a mean VS Prehistoric displacement curves for the Warm Springs (Fig. 6B) and north- of 1.7 m (1.3–1.9 m). The >2 m curve has a mean VS of 3.3 m (2.8–3.8 m), with ern Thousand Springs (Fig. 7B) sections show the cumulative displacement peak displacement at the Goosebury graben. expressed along these faults. To evaluate plausible per-event displacement along the Warm Springs section, we differenced displacement curves for the 1983, PE1, and PE2 ruptures (Fig. 10). For each rupture, we differenced the mean VS curves, Northern Thousand Springs Section as well as the min-max curves separately to determine uncertainty bounds. For the Warm Springs section, we compute two difference curves that Along the northern Thousand Springs section, prehistoric scarps provide represent the displacement in at least two prehistoric earthquakes (Fig. 10A). evidence of multiple prehistoric ruptures of the range-front trace and at least The per-event displacement profile for PE2 is the >2 m displacement curve one previous rupture of the Arentson Gulch fault. minus the ≤2 m curve. The displacement for PE1 is the ≤2 m curve minus the Displacement along the range-front trace of the Thousand Springs section 1983 displacement curve. Although more than two paleoearthquakes could decreases to the north near its northern intersection with the Willow Creek be recorded in the prehistoric scarps, we interpret the PE2 and PE1 curves Hills (kilometer ~13.2; Fig. 4B). Late Pleistocene(?) surfaces along the western as each representing discrete events because the curves have similar along- flank of Dickey Peak (Fig. 2) record as much as 4.5–8.0 m of VS (this study; strike shapes, with mean per-event displacement varying between ~1 and 2 Fig. 4B). Displacement decreases to the north to ~2.0–3.5 m, where a promi- m (Fig. 10A), which is broadly similar to the 1983 rupture of the entire Thou- nent prehistoric scarp lacks evidence of 1983 rupture displacement (kilometer sand Springs section south of Willow Creek Hills. The PE2 curve has broader ~12.5; Fig. 4B) (Crone et al., 1987). This pattern of prehistoric displacement uncertainty because of more scatter in the >2 m data. The PE2 curve has an decreasing steeply toward the Willow Creek Hills is consistent with our VS approximately uniform along-strike shape, in contrast with the PE1 mean measurements for the 1983 rupture. Discontinuous scarps to the northwest curve that peaks at the Goosebury graben, where no net displacement was (east of the Willow Creek Hills; kilometers 8–13 km; Fig. 2) suggest prehistoric observed in 1983. Because we cannot rule out the possibility that graben faults faulting without reactivation in 1983. These scarps are outside of our DSM reactivated in 1983 with distributed faulting that was not expressed or detect- data set and thus we were unable to measure their VS. able at the surface, it is possible that the PE1 peak includes decimeter-scale Along the Arentson Gulch fault, compound (including 1983) and single-event displacement from the 1983 rupture. Mean displacement (VS) for PE2 is 1.6 m (prehistoric) scarps record at least one previous (prehistoric) rupture (Figs. 7B (0.8–2.5 m), compared to 1.4 m (0.9–1.7 m) for PE1. and 9; Fig. S5). Near Arentson Gulch (kilometer ~5.5; Fig. 4B), single-event To estimate the VS for the prehistoric rupture of the Arentson Gulch fault, we (1983) scarps that cross the lowermost (youngest) faulted alluvial-fan surface use single-event scarps at the northern and southern ends that record only a (T1) have 1.5–1.8 m of VS (Fig. 9). Compound scarps on older fan surfaces prehistoric event as well as compound scarps near Arentson Gulch (Fig. 7B). For record VS of ~1.9–3.5 m (n = 5), consistent with at least one previous rupture the compound scarps, we subtracted the VS measurements for the closest 1–2 of the Arentson Gulch fault. Less than 0.1 km to the southwest, a subparallel single-event (1983) scarps from the individual VS measurements for the com- compound scarp has 1.5–1.6 m of VS, compared to 1983 VS measurements that pound scarps, yielding per-event VS values that range from 0.8 to 2.0 m (Fig. 10B). are ~0.3–0.6 m. Near the northern extent of the Arentson Gulch fault within We then fit displacement curves to the corrected data, yielding a paleoearthquake the Willow Creek Hills (kilometer ~3; Fig. 4B), single-event (prehistoric) scarps curve with a mean VS of 0.9 m (0.6–1.0 m). This is similar to the 1983 displace- have ~0.2–0.5 m of VS (Fig. S5). A single-event (prehistoric) scarp near the ment curve that yields a mean VS of 0.7 m (0.6–0.8 m). Our Arentson Gulch fault southern terminus of the Arentson Gulch fault has 0.6 m of VS (Fig. 4B). We prehistoric displacement curve is constrained by 14 VS measurements and is suspect that additional compound scarps northwest of Arentson Gulch may similar in shape, length, and magnitude to that for the 1983 rupture. account for our slightly larger VS measurements than those of Crone et al. (1987). However, we were unable to resolve prehistoric scarps along this section of the Arentson Gulch fault because evidence of older, eroded prehistoric ■■ MOMENT CALCULATIONS scarp crests are not preserved in the steeply sloping geomorphic surfaces.

We computed a cumulative along-strike displacement curve for the Arent- We calculate seismic moment (M0) and moment accumulation rates (M0) for son Gulch fault. The curve suggests peak VS of ~3 m near Arentson Gulch, with the northern LRFZ (Table 1) to evaluate Warm Springs and Thousand Springs

T2 P54 T0

T1 3.0 m T2 T2 3.2 m T1 P56 1.8 m T1/T2 T0 T3a

1.5 m T3b 1.6 m T2 P61

T1/T2 T1

P58 1.9 m 1.9

1.9 m

3.5 m

0.3 m T2 T3b 1.5 m Prehistoric fault scarp T0 Young fluvial terrace with modern channel (unfaulted) vertical separation 1.6 m measure along T1 Lowermost fluvial terrace 2.8 m topographic profile 0.2 m T2 Intermediate fluvial terrace Slope 1983 Borah Peak ± earthquake rupture T3 Uppermost fluvial terrace Not mapped 0º >40º 0 100 m 0 100 m ±

Figure 9. (A) Digital surface model for the 1983 rupture of the Arentson Gulch fault at Arentson Gulch, Idaho, USA (Fig. 4B). (B) Surficial geologic map showing four fluvial terraces. Scarp profiles (green and blue lines; Fig. 5) show that terraces T2 and T3 record 1.9–3.5 m of VS compared to inset terrace T1 which records ~1.5–1.8 m. Terrace T0 is not faulted. Labeled profiles (e.g., P54) are shown in Figure 5.

section ruptures in the context of M0 accumulation and release. The main used for comparative purposes, we do not account for complex uncertainties

questions we seek to address with these simple M0 calculations are (1) what related to minor oblique slip, VS versus fault throw, and the assumption that

percentage of the total 1983 M0 release occurred on the Warm Springs section surface displacements can be used to approximate average displacement over

and (2) how does M0 in the 1983 rupture of the Warm Springs section compare the rupture at seismogenic depths. M0 estimates are converted to earthquake

to the M0 of prehistoric ruptures of the Warm Springs section? We treat M0 cal- magnitude (Mw) using the equation of Hanks and Kanamori (1979): culations in the standard way (Aki, 1966), where M is estimated as the product 0 2 of rupture area (based on rupture length and down-dip width), average coseis- M = [log M – 9.1]. (1) w 3 10 0 mic fault-parallel displacement, and the shear modulus of the crust (3.3x1010

Pa; Doser and Smith, 1985). M0 is the product of rupture area, shear modulus, Our observations yield a geologic M0 estimate for the total Borah Peak rupture

and fault-parallel slip rate (Brune, 1968). We use a fault dip of 45° (Doser and that is similar to previous seismic and geodetic M0 estimates. Using a surface-rup- Smith, 1985; Richins et al., 1987) and, in most cases, a maximum seismogenic ture length of 34.2 km and an average displacement of 0.93 m extending from depth of 15.5 km (midpoint of 15–16 km maximum depth of mainshock and the surface to 15.5 km depth (although see Discussion for uncertainties regarding

aftershocks reported in Doser and Smith, 1985 and Smith et al., 1985) to define 1983 coseismic slip along the Warm Springs section), we estimate a geologic M0 19 down-dip rupture width (Table 1). Because our M0 estimates are generally of 3.3 × 10 Nm (Mw 6.9) for the Borah Peak earthquake (Table 1). Seismic M0

Willow Creek Hills 2.0

1.5 PE2: >2m minus ≤2m: 1.6 (0.8-2.5) m 1.0 PE1: ≤2m minus 1983: Figure 10. (A) Displacement difference curves for 1.36 (0.9-1.7) m plausible Warm Springs section paleoearthquakes 0.5 PE1 (blue) and PE2 (magenta). Along-strike displace- 1983 Goosebury graben ment in PE1 is based on the 1983 mean displacement 0 curve (Fig. 6A) subtracted from the ≤2 m mean dis- 0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 placement curve (Fig. 6B). Displacement in PE2 is Distance SE along the Warm Springs section (km) based on the ≤2 m mean curve subtracted from the >2 m mean curve (Fig. 6B). (B) Along-strike displacement in the prehistoric rupture of the Arentson Gulch 2.0 fault (green). Prehistoric displacement computed by Arentson Gulch fault prehistoric displacement: Thousand Springs subtracting 1983 displacement(s) (Fig. 7A) from the 0.9 (0.6-1.0) m section Vertical separation (m) 1.5 VS for compound (including 1983 and prehistoric dis- 1983 placement) scarps and using per-event (prehistoric displacement only) scarps along the fault (Fig. 7B). 1.0 1983 prehistoric 0.5

0 0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 Distance SE along the Arentson Gulch fault (km)

TABLE 1. SEISMIC MOMENT M0 AND MOMENT MAGNITUDE M CALCULATIONS FOR THE WARM SPRINGS AND THOUSAND SPRINGS SECTIONS, IDAHO, USA

1 2 3 5 6 8 Fault rupture Fault length Fault area VS FPD M0 M M0 Net accum M0 km km2 m m Nm 1019 M Nm 1019 0 Nm/yr 1016 Warm Springs section, 1983 8.1 115 0.35 0.9 0.19 6.1 Warm Springs section, PE1 1.0 329 1.20 1.0 2.09 6.8 Warm Springs section, PE2 15.0 329 1.1 2.00 2.1 6.8 Arentson Gulch fault,1983 8.0 11 0.68 0.96 0.3 6.3 Arentson Gulch fault, PE 8.0 11 0.8 1.23 0.8 6. Thousand Springs, 1983 21. 69 1.11 1.5 2.3 6.9 1983 Borah Peak earthuake 3.2 50 0.93 1.32 3.25 6.9 Warm Springs section 15.0 329 3.0 1.38–1.8

1 Rupture scenario considered for seismic moment M0 calculation. Borah Peak earthuake geologic M0 is based on vertical separation data from this study and Crone et al. 198. Net M0 accumulated on the Warm Springs section since PE1. 2Rupture length based on kilometer marks shon on Figure 2; see tet for discussion. 3Calculated using fault length, a seismogenic depth of 15.5 km, and a fault dip of 5. For the 1983 rupture of the Warm Springs section, e apply a seismogenic depth of 10 km; for the Arentson Gulch fault, e take to-thirds of the rupture area calculated using a 15.5 km seismogenic depth see tet for discussion. Average vertical separation VS based on rupture curves shon on Figures 6 and . Fault-parallel displacement FPD calculated using a 5 fault dip. For PE1 and PE2, FPD reflects a tapered-slip model 50 of mean slip for the northern and southern 2 km of the ruptures. 5 10 M0 calculation is product of FPD, rupture area, and the shear modulus of the crust 3.3 10 Pa; Doser and Smith, 1985 Aki, 1966. 6 Moment magnitude M calculation is 2/3 log M0 – 9.1 Hanks and anamori, 199. 10 M0 accumulation rate M0 is product of slip rate, rupture area, and shear modulus 3.3 10 Pa, after Brune 1968. We use a Warm Springs section vertical slip rate of 0.2 mm/yr 0.3 mm/yr fault-parallel. 8 Net accumulated M0 since the time of Warm Springs earthuake PE1 5.1–6.6 ka, less the M0 release in the 1983 rupture of the section.

15 (including main shock and 3.5 weeks of aftershocks; Smith et al., 1985) and geo- a Warm Springs M0 of 3.1 × 10 Nm/yr. The Warm Springs section M0 rate, as

detic M0 (based on a 70-km-long leveling line surveyed in 1933, 1948, and 1984; well as the M0 estimates for PE1 and PE2, suggest that the Warm Springs Stein and Barrientos, 1985) estimates are 2.9 × 1019 Nm (Mw 6.9) and 3.2 × 1019 Nm section is capable of generating large (Mw ~6.8) earthquakes approximately (Mw 7.0), respectively (Table 1). By comparison, the Thousand Springs section every 7 k.y. Assuming that at least 5.1–6.6 k.y. have elapsed since earthquake 19 alone, excluding the Arentson Gulch fault, yields a M0 of 2.4 × 10 Nm (Mw 6.9), PE1 and subtracting our M0 estimate for the 1983 rupture of the section, the 19 assuming a length of 21.4 km and average displacement of 1.1 m. While moment net M0 accumulated on the Warm Springs section since PE1 is 1.4–1.8 × 10 calculations from average surface slip are only a rough approximation of slip Nm, enough to generate a Mw 6.7–6.8 earthquake. at depth, these calculations underscore the simple observation that the bulk of moment in 1983 occurred south of the Willow Creek Hills.

To more closely examine the spatial distribution of M0 release north of the ■■ DISCUSSION

Willow Creek Hills in 1983, we find that M0 for the 1983 rupture of the Warm Springs section is 1.9 × 1018 Nm (Mw 6.1) based on our 1983 scarp mapping 1983 Rupture of the Northern LRFZ and VS data and assuming a seismogenic depth of the rupture of 10 km, consistent with the slip-inversion models of Ward and Barrientos (1986), Mendoza Surface rupture in the 1983 Borah Peak earthquake propagated northward

and Hartzell (1988), and Du et al. (1992). M0 estimates for the Arentson Gulch from the southernmost Thousand Springs section toward the Willow Creek fault are poorly constrained because of uncertainty in its down-dip extent and Hills (Crone et al., 1987; Richins et al., 1987). Our observations, based primarily intersection with the Thousand Springs section. Assuming that the Arentson on remapping of 1983 surface displacement, allow us to closely examine how Gulch fault lies entirely within the hanging wall of the LRFZ, and using the slip was modulated by the Willow Creek Hills structural complexity in 1983. orientation of the two fault planes, we use two-thirds of the rupture area calcu- Within ~1 km of the Willow Creek Hills, displacement on the Thousand lated using the 8 km length and 15.5 km seismogenic depth. Using an average Springs section in the 1983 rupture decreased abruptly to the north and a 18 displacement of 0.7 m (1983 rupture) results in a M0 of 3.7 × 10 (Mw 6.3) for complex pattern of surface faults transferred slip to the 8-km-long Arentson the 1983 rupture of the Arentson Gulch fault. In summary, we estimate that Gulch fault (Fig. 4B), which continues into and nearly through the Willow Creek the Warm Springs section and the Arentson Gulch fault only contributed 9% Hills (Fig. 2). The rupture did not break the surface in the ~5-km-long portion of

and 11% of 1983 M0, respectively (Table 1). the range-front trace of the LRFZ along the eastern edge of the Willow Creek To gain insight on prehistoric ruptures centered on and north of the Willow Hills (Fig. 2). Surface rupture occurred along the southernmost Warm Springs

Creek Hills, we also estimate M0 for the Arentson Gulch fault and PE1 and section, with less displacement and a more discontinuous trace than along the PE2 along the Warm Springs section. For the Arentson Gulch fault, we use an Thousand Springs section (Fig. 4A). Although the direction of the Warm Springs

average displacement of 0.9 m for the prehistoric rupture to obtain a M0 of rupture propagation is uncertain, it likely continued south to north, based on 18 4.8 × 10 (Mw 6.4). To estimate M0 for PE1 and PE2 along the 15-km-long Warm the unilateral rupture propagation inferred for the Borah Peak earthquake Springs section, we use along-strike displacement curves for PE1 and PE2. For (e.g., Doser and Smith, 1985) and the northward-decreasing displacement tail PE1, we speculate that the rupture includes the range-front trace of the LRFZ observed north of the Goosebury graben (consistent with Ward, 1997). through the Willow Creek Hills (discussed below) and increase the length by 2 km (total of 17 km). Because the displacement curves for PE1 and PE2 do not cover the entire length of the Warm Springs section, likely excluding a decrease Prehistoric Rupture of the Northern LRFZ in displacement toward the rupture ends, we apply a tapered-slip model for the Warm Springs section which arbitrarily reduces the mean displacement Our displacement measurements fall into two clear populations that allow by 50% for the northern and southern 2 km of the ruptures, consistent with us to infer that at least two prehistoric earthquakes (PE1 and PE2; Fig. 11A) the displacement taper along the northern and southern ends of the Thousand have ruptured the Warm Springs section. PE2 likely occurred after last glacial Springs section. As a result, overall mean displacement is reduced from 1.4 maximum deglaciation (ca. 15 ka) as scarps are formed in extensive surfaces 19 m to 1.2 m for PE1 and 1.6 m to 1.4 m for PE2. M0 for PE1 and PE2 is 2.1 × 10 related to this period (Pierce and Scott, 1982). PE1 occurred shortly before ca. Nm (Mw 6.8) and 2.2 × 1019 Nm (Mw 6.8), respectively. 5.1–6.6 ka based on radiocarbon ages from the Rattlesnake Canyon and Sheep

M0 calculations for the Warm Springs section allow us to calculate the total Creek trenches (Schwartz and Crone, 1988). Although per-event displacement

M0 accumulated on the section since the most recent earthquake (PE1). We patterns for PE1 and PE2 are similar (Figs. 10A and 11B), peak displacement in calculate a vertical slip rate for the section of ~0.2 mm/yr using the average VS PE1 at the Goosebury graben could explain the 1983 displacement minimum of PE2 scarps (~3.3 m) and the approximate age of the displaced geomorphic within this 2-km-long structure. Alternatively, the PE1 displacement peak could surface (ca. 15 ka; Pierce and Scott, 1982). This rate is similar to vertical slip signal and include decimeter-scale displacement across the graben in 1983 rates of 0.2–0.3 mm/yr calculated for the Thousand Springs section and yields that was not observable at the surface.

8 ? Prehistoric vertical separation Western section Thousand Springs section 6 PE2 Figure 11. Summary of vertical separation (VS) ~15 ka(?) DP ? trench along the Warm Springs and Thousand Springs ~10–11 ka 4 sections. (A) Cumulative VS, showing Warm Arentson Gulch fault Springs section scarps (magenta and blue) and SC the 1983 rupture (red). Prehistoric scarps along trench the northern Thousand Springs section (gray cir- 2 cles; this study) show a pattern of VS decreasing PE1 ~5–7 ka 1983 toward the Willow Creek Hills that is similar to

Cumulative vertical separation (m) the 1983 (red) and prehistoric (green) VS curves 1983 0 for the Arentson Gulch fault. The VS curve for the 1983 rupture of the Thousand Springs section Warm Springs section Willow Creek Thousand Springs section Hills (kilometers 13–34) is fit to data reported in Crone 3 et al. (1987). (B) Per-event vertical displacement DP based on mean displacement difference curves PE2 SC trench (see text for discussion). Along the Warm Springs trench ~15 ka(?) section, prehistoric ruptures PE2 (magenta) and ? Arentson Gulch fault ~10–11 ka PE1 (blue) show significantly more displacement 2 PE1 than the 1983 rupture (red). Green line shows 1983 prehistoric VS along the Arentson Gulch fault. Gray box shows extent of the Willow Creek Hills 1 1983 structure along the Lost River fault zone. Trian- PE1 PE2 ~5–7 ka gles show paleoseismic sites. SC—Sheep Creek; DP—Doublespring Pass.

1983 ? ~10–11 ka?

Per-event vertical separation (m) 0 2 6 10 14 18 22 26 30 34 Distance SE along Lost River fault zone (km)

Although we document evidence for pre-1983 rupture of the Arentson Gulch moderate-displacement (~0.2–1.0 m VS) scarps are locally superimposed on fault (Fig. 10B), we lack information on displaced surface ages and thus cannot larger (~1.5–4.5 m VS) prehistoric scarps (Fig. 4A; Fig. S1 [footnote 1]). Although constrain the timing of the rupture. However, it is possible that the rupture is the 1983 mean displacement is small (~0.35 m VS), peak displacement near contemporaneous with the previous rupture of the Thousand Springs section, the Willow Creek Hills is significant (~0.7–1.0 m VS), similar to coseismic dis- constrained to ca. 10–11 ka from paleoseismic trenches (Crone, 1985; Vincent, 1995). placement observed along the Thousand Springs section and Arentson Gulch fault (Fig. 11). A shallow, shaking-related origin of displacements along the Warm Springs section is unlikely as there is a lack of complex, isolated, arcuate, Primary versus Secondary Origin of 1983 Scarps along the Warm and/or compressional scarps along the main fault trace and especially within Springs Section the hanging wall, that might be expected if only shaking-induced gravitational failure or consolidation of near-surface deposits had occurred (e.g., Caskey et A central motivation for this study is to help interpret the significance of the al., 1996). Instead, the 1983 rupture appears to accurately track the prehistoric northern extent of the 1983 rupture pattern, addressing two related questions. rupture trace, just with less displacement and possibly in more discontinuous First, is the Warm Springs rupture primary or secondary in nature? We define fashion than prehistoric ruptures. primary rupture as surface slip related to coseismic rupture propagation at As a result of the geometrically simple fault pattern and moderate dis- seismogenic depths, whereas secondary surface slip may reflect a range of placements, Crone et al. (1987) and Crone and Haller (1991) concluded that processes such as nonseismogenic gravitational failure, shallow triggered the rupture of the Warm Springs section is not directly related to the primary coseismic slip, or postseismic slip (Fig. 12). Second, how is normal-fault rup- rupture on the Thousand Springs section, but may be the result of insignif- ture arrested or modulated as it approaches a major trans-basin structural icant, shallow faulting triggered by strong shaking and the directivity of the complexity such as the Willow Creek Hills? primary rupture (Fig. 12A). Slip inversions using teleseismic waveforms and Several lines of evidence point toward coseismic, tectonic slip as the geodetic observations have poor resolution along the Warm Springs section, driver for surficial displacement along the Warm Springs section. Continuous, but these models suggest that slip in the 1983 Warm Springs rupture occurred

at least to depths of ~5 km (Ward and Barrientos, 1986) or ~8–10 km (Mendoza Willow +11 km Warm Springs section Creek Thousand Springs SE and Hartzell, 1988; Du et al., 1992). Predictions of deep slip in the finite fault Hills section 1 m models and decimeter-scale surface displacements observed along the Warm Goosebury graben Springs section make it unlikely that surface displacement along the Warm 1983 surface VS

Springs section reflects secondary, dynamically triggered shallow slip. Trig- 0 gered slip would likely produce shallow, sub-centimeter displacements (e.g., ? Rymer, 2000; Wei et al., 2011) that would likely be undetectable at the surface. 1983 schematic ? Although the 1983 surface displacement of the Warm Springs section is coseismic slip Arentson Gulch

20 km fault

likely deeply (at least 5–10 km) rooted (Fig. 12B), it is possible that some portion Distance down dip 0 5 km of the observed displacement represents postseismic slip adjacent to coseismic slip patches on the Thousand Springs section and Arentson Gulch fault (Fig. 12C). Postseismic slip is commonly observed following large earthquakes,

including normal fault ruptures (e.g., D’Agostino et al., 2012). Unfortunately, ? we lack the field and remote sensing observations necessary to evaluate a 1983 schematic ? postseismic slip signal, especially since the 1983 Warm Springs section scarps coseismic slip were not visited in the field on the day of the earthquake (Ostenaa et al., 1984). Postseismic slip that continued after the Crone et al. (1987) measurements along 1983 Warm Springs section could explain some of the greater values we obtain for VS measurements for the northernmost part of the rupture as the Crone et al. (1987) measurements were made between a day to several years ? following the earthquake (A.J. Crone, written communication, 2018). However, 1983 schematic ? this explanation for the VS difference is unlikely for those field measurements postseismic slip obtained years after the rupture, as rates of postseismic slip typically decay within days to months following the main shock (e.g., Cheloni et al., 2014; Gua- Figure 12. Conceptual models of subsurface rupture on the Thousand Springs and Warm Springs sections during the 1983 Borah Peak earthquake. (A) Insignificant, landi et al., 2014; DeLong et al., 2016). Average and maximum displacements shallow faulting along the Warm Springs section triggered by strong shaking and for the 1983 rupture of the Warm Springs section are both <40% of those for the directivity of the primary rupture of the Thousand Springs section and Arentson the Thousand Springs section rupture, but global postseismic-coseismic slip Gulch fault (subsurface slip not depicted for the Arentson Gulch fault). Red indicates relations show that the ratio of coseismic to postseismic slip can vary sub- areas of peak coseismic slip. (B) Continuous primary rupture beneath the Willow Creek Hills structural complexity and Warm Springs section. In this model, subsur- stantially: postseismic slip (or M0) is most commonly ~10%–30% or less of face but not surface slip occurs along the Goosebury graben and Willow Creek Hills coseismic slip (Wdowinski et al., 1997, Reilinger et al., 2000; Ozawa et al., 2012; (range-front Lost River fault zone trace). (C) postseismic slip on the Warm Springs Lin et al., 2013), but in some earthquakes exceeds 30% (Wang et al., 2015) or section, adjacent to coseismic slip on the Thousand Springs section. Yellow indicates areas of peak postseismic slip. VS—vertical separation. could be as much as 100% (DeLong et al., 2016) of coseismic slip. However, the magnitude of 1983 Warm Springs section displacement (maximum of ~1 m) is less consistent with postseismic slip observations, which are mostly 0.1–0.5 m occurring within 24 h of the 1983 mainshock are distributed along the entire (Wdowinski et al., 1997; Reilinger et al., 2000; Cheloni et al., 2014; DeLong et length of the Thousand Springs and Warm Springs section ruptures (Richins al., 2016). In summary, we cannot rule out the possibility that postseismic slip et al., 1987), suggesting the continuation of mainshock slip north of the Willow contributed to the observed surface displacement on the Warm Springs section. Creek Hills for at least 8 km. We note that none of these criteria are absolute Given the unknown postseismic slip contribution to 1983 scarps along and all allow some combination of primary coseismic slip plus secondary the Warm Springs section, we prefer the interpretation that primary coseis- afterslip as the origin of the Warm Springs scarps. mic slip is the main cause of these features (Fig. 12B). This is the simplest explanation and is consistent with (1) recognition of the scarps within about a day of the Thousand Springs section rupture, (2) the magnitude of the sur- Ruptures prior to 1983 along the Warm Springs Section face displacement, and (3) the northward-decreasing displacement profile,

which is suggestive of a gradual (if discontinuous) taper in displacement at We combine our vertical separation observations with M0 and M0 estimates the primary rupture terminus (e.g., similar to rupture tails for the Dixie Valley, to compare the 1983 and prehistoric ruptures of the Warm Springs section and Nevada, USA [Caskey et al., 1996], and Edgecumbe, New Zealand [Beanland place the 1983 rupture in the context of prior ruptures. We find that the 1983 et al., 1989] earthquakes and models of [Ward 1997]). Further, aftershocks Warm Springs rupture is fundamentally different from prehistoric earthquakes

PE1 and PE2 (Fig. 11B). Our M0 estimate for the 1983 Warm Springs rupture is as a conditional barrier to rupture propagation “as a result of proximal fault 18 1.9 × 10 Nm (equivalent to a Mw 6.1 earthquake), or ~9% of the potential M0 geometry, rupture direction, and prior earthquake history” (Oskin et al., 2015). released in PE1 and PE2 (2.1–2.2 × 1018 Nm; Mw ~6.8 earthquakes) (Table 1). Neither the segment boundary nor earthquake gate analogies are perfect, This implies that even though the 1983 earthquake ruptured ~50% of the Warm because the 1983 rupture was not fully arrested by the Willow Creek Hills, but Springs section, the moment release is minor compared to possible previous instead the rupture style and slip amount were significantly modulated (Fig. 13). ruptures of the section. If we assume a simplistic earthquake cycle with similar That is, the Borah Peak earthquake did not pass through the Willow Creek Hills strain accumulation and release, we can calculate the Warm Springs section without penalty; rupture continued, but without sufficient energy to initiate a 15 M0 of 3.1 × 10 Nm/yr and an elapsed time since PE1 of at least 5.1–6.6 k.y. to full (displacement and length) rupture of the Warm Springs section. There are

infer that the M0 accumulation on the Warm Springs section at the time of the several plausible reasons for modulation of rupture at a structural complexity. 19 Borah Peak earthquake was ~1.6–2.0 × 10 Nm, or about ~74%–95% of the M0 First, complex, interlocking faults within a structural barrier have been shown release estimated in PE1 and PE2 (2.1–2.2 × 1019 Nm). This suggests that the to impede rupture progress (King, 1983; King and Nabelek, 1985; Bruhn et al.,

1983 Warm Springs rupture released only ~9%–12% of this accumulated M0 1987). Limited rupture north of the Willow Creek Hills in 1983 may also relate to (Table 1), delaying the approximate 7 k.y. Warm Springs clock (based on the the Warm Springs section’s prior stress conditions; it was further from failure

section’s M0 and prehistoric M0 release) by ~0.6 k.y. than the Thousand Springs section because of its more recent paleoearthquake.

Only a minor percentage of the total M0 release in the 1983 Borah Peak

earthquake occurred along the Warm Springs section. Although the 1983 Warm DP Willow ? Springs rupture contributed 24% to the total Borah Peak rupture length (8 of Creek Hills 34 km total), the M for the rupture is only 6% of our estimate for the entire 0 TS PE Borah Peak earthquake (Mw 6.9) (Table 1). In contrast, M0 release on the Warm ~10-11 ka 1 m WS PE2 Springs section in PE1 and PE2 was ~65% that of the Borah Peak earthquake. <~15 ka

VS In relative terms, the 1983 rupture of the Thousand Springs section (Mw 6.9) Arentson released ~13 times the M for the 1983 Warm Springs rupture, or ~75% of the Gulch fault N 0 0 5 km total Borah Peak M0. Taken together, the surface faulting pattern, displacement profile, and moment comparisons to prehistoric events suggest that the 1983 Warm Springs rupture is the moderate displacement continuation of a rupture SC whose propagation energy dissipated during complex, branch-fault rupture RC into the Willow Creek Hills structure. WS PE1 >=6-7 ka ?

Rupture Modulation at the Willow Creek Hills The earliest post-1983 earthquake studies recognized that 1983 Borah Peak Goosebury 1983 rupture propagation was profoundly affected by a structural discontinuity at graben rupture direction the Willow Creek Hills (Crone et al., 1985), and this was interpreted as a rupture ction S (WS) se Thousand prings (TS) section “barrier” in the emerging terminology of the day, which focused on fault seg- Warm Springs mentation and characteristic earthquake models (Schwartz and Coppersmith, 1984). The observations we present here show that per-event and cumulative displacement data for the northern LRFZ suggest a complex history of rup- Figure 13. The Willow Creek Hills rupture structural complexity (gray-shaded areas), ture at and through the Willow Creek Hills. Multisegment ruptures during which has impeded some but not all surface ruptures along the Warm Springs and Thousand Springs sections of the Lost River fault zone. (A) Warm Springs (WS) prehis- historical earthquakes on strike slip faults (e.g., Sieh et al., 1993; Treiman et toric rupture PE2 (WS PE2) may have terminated at the Willow Creek Hills, compared al., 2002; Klinger et al., 2005; Oskin et al., 2012; Hamling et al., 2017), complex to (B) WS PE1, which possibly continued across the structure. The southern extent prehistoric slip histories across historical rupture terminations in subduction of WS PE1 and WS PE2 are poorly resolved. Paleoseismic displacements for WS PE1 are from the Rattlesnake Canyon (RC) and Sheep Creek (SC) trenches (see text for settings (e.g., Shennan et al., 2009; Briggs et al., 2014), and prehistoric “spill- discussion). The Thousand Springs prehistoric rupture (TS PE) occurred at the Dou- over” ruptures along the Wasatch normal fault zone (Personius et al., 2012; blespring Pass (DP) road trench and possibly the Arentson Gulch fault. The extent of DuRoss et al., 2016) are evidence of variable rupture response to along-strike this rupture is not well constrained but may be similar to the (C) 1983 rupture, which crossed the Willow Creek Hills with moderate-displacement spillover rupture. Fault structural complexities in several tectonic settings. Such features have been traces shown in map view; surface vertical separation is depicted above possible recently termed “rupture gates,” defined as fault complexities that can act ruptures. Triangles show vertical separation (VS) measured at paleoseismic sites.

Other possibilities are the interruption of dynamic rupture propagation by fault suggest at least one prehistoric rupture of the branch fault with an along- varying fault geometry (Ando et al., 2017), the reduction in rupture velocity strike displacement pattern similar to that for the 1983 rupture of the fault. We approaching a fault branch (Templeton et al., 2010), and energy loss to off- speculate that reactivation of the Arentson Gulch fault is more likely during fault deformation in a damaged zone (Andrews, 2005). Regardless, the Warm northward propagating ruptures on the Thousand Springs section based on Springs rupture is best described as a spillover rupture (DuRoss et al., 2016), its proximity to the Arentson Gulch fault trace and the geometric configura- or rupture across a “leaky” boundary (Crone and Haller, 1991), which added tion more favorable for simple branching. However, this geometry does not significant length to the rupture, but only minor displacement and moment. preclude simultaneous rupture of the Arentson Gulch fault and Warm Springs It is useful to place the 1983 rupture in the context of prehistoric ruptures to section, which could depend on unresolved structural relationships between appreciate the range of slip behaviors observed across the Willow Creek Hills. the two at depth. Using the 1983 and possible prehistoric ruptures, it appears Paleoearthquakes PE1 and PE2 resulted in a significantly different pattern of that the Willow Creek Hills structural complexity is not a hard barrier to rupture, displacement compared to the 1983 rupture (Fig. 11). These earthquakes may but effectively modulates or limits the displacement in ruptures breaching have spanned the entire ~15 km length of the Warm Springs section based on the structure (Fig. 13). This is consistent with the conclusion of Bruhn et al. ~2–3 m VS scarps identified at the northern edge of our study area. We infer (1991) that prehistoric scarps along the eastern margin of the Willow Creek that the PE1 rupture continued south into the Willow Creek Hills based on dis- Hills (along the range-front trace of the LRFZ) are evidence that the structure placements measured at the Sheep Creek trench (Schwartz and Crone, 1988). acts as a nonpersistent (after Wheeler and Krystinik, 1992) rupture barrier with If our along-strike correlation of PE1 with the >5–7 ka Warm Springs rupture only occasional full-displacement, throughgoing ruptures (Crone et al., 1987). (Schwartz and Crone, 1988) is correct, PE1 displacement may increase toward From our observations of 1983 and previous surface-rupturing earthquakes, the structure, from ~1–1.5 m to ~2.2 m (Fig. 11B). Thus, we speculate that the we speculate that ruptures through the Willow Creek Hills contribute only minor PE1 ruptured the Willow Creek Hills and possibly the northernmost Thousand moment and cumulative vertical separation in the context of large ruptures Springs section. The ~2 m VS scarp on the northernmost Thousand Springs to the north and south (Table 1; Fig. 13). There is only limited evidence for section, east of the Willow Creek Hills, that did not break in 1983 (kilometer throughgoing ruptures with significant displacement and moment. Additional ~12.5; Figs. 2 and 4B) could be related to earthquake PE1. A mid-Holocene paleoseismic data for the southern Warm Springs and northern Thousand prehistoric rupture of the Willow Creek Hills may partly explain why the 1983 Springs sections would serve to refine the ages and displacements of prehis- rupture did not continue northwest along the range-front trace of the fault, but toric ruptures of these sections and further test the models of rupture at and instead took a complicated path along the Arentson Gulch fault. We suspect through the Willow Creek Hills suggested here. that PE2 terminated at or just north of the Willow Creek Hills (e.g., kilometer Analogs to the Willow Creek Hills can be found along the Wasatch fault zone ~8–9; Figs. 2 and 11B), based on an apparent decrease in PE2 displacement (WFZ), Utah, USA, where paleoseismic data provide evidence of ruptures mod- from ~1.5–2 m in the north to <1 m toward the Willow Creek Hills and the lack ulated by long-lived normal-fault structural complexities. For example, near of evidence for PE2 in the Sheep Creek trench. However, it is possible that the the center of the Holocene-active part of the WFZ, a prominent hanging-wall Sheep Creek trench did not expose strata predating and displaced by the PE2 bedrock ridge and fault bend separate the Salt Lake City and Provo sections and event. Although ruptures PE2 and PE1 may have influenced the northward form the Traverse Mountains structural boundary (Bruhn et al., 1992; Machette propagation of Thousand Springs ruptures (including 1983), an explanation et al., 1992). Although a complex, distributed zone of fault scarps connect the for their southern extents remains unresolved. segments (Toké et al., 2017), similar to faults at the eastern edge of the Willow Rupture into and through the Willow Creek Hills may depend on several Creek Hills, this structure has previously been considered a hard barrier to factors, including rupture propagation direction, displacement magnitude, rupture (e.g., Schwartz and Coppersmith, 1984; Machette et al., 1992; Wheeler and the history of past strain release on the Thousand Springs and Warm and Krystinik, 1992). Paleoseismic data from near this structure indicate that it Springs sections (Fig. 13). PE1 and PE2 could have ruptured into, and possibly has also impeded some but not all surface-faulting earthquakes and may also through the structure, but PE2 was not observed at the Sheep Creek trench be a source of <30-km-long ruptures centered near the structure (Bennett et (southernmost Warm Springs section) and the southern terminus of PE1 is al., 2018; DuRoss et al., 2018). To the north along the WFZ, the Pleasant View only constrained by its absence in the Doublespring Pass trench (no ruptures salient consists of a hanging-wall bedrock ridge and fault step that have long younger than ca. 10–11 ka on the central Thousand Springs section). How- been considered a barrier to ruptures on the Brigham City and Weber segments ever, peak displacement in PE1 at the Willow Creek Hills could be evidence of (Machette et al., 1992). However, Personius et al. (2012) used paleoseismic rupture penetration, and we speculate, north to south propagation, into the and structural data to infer a small-displacement rupture through the barrier, Willow Creek Hills. In contrast, prehistoric ruptures of the Thousand Springs similar to that observed in the Borah Peak rupture. Spillover rupture across section (e.g., TS PE; Fig. 13) may have stopped short of the Willow Creek Hills the barrier, possibly from the Weber to Brigham City segment, broke ~8 km

based on the clear decrease in scarp VS along the Thousand Springs section of the 36-km-long Brigham City segment and reduced M0 accumulated since as it nears the structure (Fig. 11). Prehistoric scarps along the Arentson Gulch the most recent rupture on the segment by ~11%–13%.

Implications for Normal-Fault Paleoseismology within the Willow Creek Hills (e.g., the Arentson Gulch fault) includes evidence of at least one prehistoric rupture; (2) although the 1983 rupture of the Warm Our results should be taken into account when interpreting rupture length Springs section is part of the primary Borah Peak rupture, it is a moderate-dis-

and M0 along complex multisegment normal faults from paleoseismic data. For placement spillover rupture (defined by significant rupture length but modest

example, for geometric fault sections separated by a trans-basin high such as M0 release) across a structural complexity; (3) two prehistoric ruptures of the the Willow Creek Hills, complex spillover and single-section ruptures may be Warm Springs section (PE1 and PE2) had larger (~1–2 m) displacements and

more common than unimpeded rupture through the structural complexity with local M0 release than the 1983 spillover rupture; and (4) a variable history of peak per-event displacement centered near the structure. Our results suggest rupture termination and penetration through the Willow Creek Hills suggests that although structural barriers can be long lived, they may act as “rupture that the structure impeded some, but not all earthquakes, possibly depending

gates” and conditionally allow ruptures to penetrate, but not without modulat- on the history of M0 release along the fault (prior stress conditions) as well ing rupture length and displacement. Spillover ruptures suggest that structural as fault displacement and rupture direction. Ultimately, our VS data and dis- barriers are not hard limits to rupture but may moderate rupture displacement placement distributions help improve our understanding of the role structures and progress and be regions of frequent strong ground shaking. For example, such as the Willow Creek Hills play in influencing rupture length, displacement,

based on our rupture model (Figs. 11 and 13), the Willow Creek Hills structure and M0 release. Our results have broad implications for the interpretation of has experienced more earthquake ruptures than either the Warm Springs or normal-fault paleoseismic data and can help inform the range and weights of Thousand Springs sections. Our conclusion is similar to that for the Traverse rupture models in regional seismic-hazard assessments. Mountains structure on the Wasatch fault zone, which has recorded at least six late Holocene ruptures compared to about four ruptures on the segments bordering the structure (Bennett et al., 2018; DuRoss et al., 2018). This lends ACKNOWLEDGMENTS support to higher probabilities assigned to single-segment and spillover rup- We thank Kendra Johnson, Lia Lajoie, and Edwin Nissen for assistance completing Helikite surveys tures than full two-segment ruptures in normal-fault rupture forecasts (e.g., of the Warm Springs section and the U.S. Geological Survey (USGS) Unmanned Aircraft Systems (UAS) Project, including Jeff Sloan, Mark Bauer, Joe Adams, and Todd Burton, for discussions of Working Group on Utah Earthquake Probabilities, 2016). UAS photography and for conducting a Falcon flight near the Willow Creek Hills. Brad Koeckeritz The fortuitous 1983 spillover rupture of the Warm Springs section offers provided aerial images (3DR Solo and GoPro) of the northern Arentson Gulch area. The remaining essential lessons on the interpretation of sparse paleoseismic data obtained UAS flights were conducted by M.P.B. and N.A.T. Digital surface models (DSMs) used in this study are available at USGS ScienceBase (https://doi.org/10.5066/P9CH0IQ4); DSMs, point clouds, and near normal fault structural complexities. First, because of moderate displace- metadata are available at Open Topography (Bunds et al., 2019; https://doi.org/10.5069/G9222RWR). ments in spillover ruptures, their preservation potential is limited as they can Thanks to David Schwartz for discussions of this work and for providing unpublished paleoseismic easily be overprinted by large-displacement ruptures. That is, it is not clear data for the Warm Springs section. We also thank Utah Valley University (UVU) students Jeremy that the small, decimeter-scale displacements in 1983 will be preserved at the Andreini, Bret Huffaker, Kenneth Larsen, Rick Lines, Ephram Matheson, Brittany Ungerman, and Alexandra Valenzuela for able assistance in the field. Finally, we thank the UVU College of Science century or millennial scale of paleoseismic observations. This may explain why Scholarly Activities program for financial support to M.P.B. and N.A.T., and Nvidia Corporation such ruptures are rarely interpreted from normal-fault paleoseismic data sets. for computing support to UVU through their Graphics Processing Unit (GPU) Grant Program. If spillover displacement onto a fault section adjacent to the primary rupture is Idaho State University provided accommodations at the Lost River field station. We thank Jaime Delano (USGS), Austin Elliot, and one anonymous reviewer for their constructive peer reviews. significant enough to be observed in a typical trench setting (i.e., >0.25–0.5 m, Any use of trade, product, or firm names is for descriptive purposes only and does not imply depending on stratigraphy), the moderate M0 event could be misinterpreted endorsement by the U.S. Government. as the complete and full-displacement rupture of the fault section hosting the spillover. The collection of high-resolution topographic data and dense paleo seismic data from multiple sites approaching structural barriers may help REFERENCES CITED resolve the extent and timing of these types of ruptures compared to ruptures Aki, K., 1966, Generation and propagation of G waves from Niigata earthquake of June 16, 1964: that terminate cleanly at structural barriers. 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