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 © 2019 The Authors GEOSPHERE | Volume 15 | Number 6 DuRoss et al. | Northern Lost River fault zone rupture behavior Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/15/6/1869/4876716/1869.pdf 1869 by guest on 27 September 2021 Research Paper 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 N of the range front (including the presence of hanging-wall bedrock salients), ' 0 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.