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Map of Lost River fault and location of the Borah Peak earthquake (M 7.3, star) in east-central Idaho. Solid line shows the 1983 surface rupture on the Lost River fault; dotted line shows unruptured parts of the fault. Field-trip stops are shown by numbered circles. Selected locations are Big Lost River Valley (BLRV), Round Valley (RV), Thousand Springs Valley (TSV), and Willow Creek hills (WC). Segments, from south to north, of the Lost River fault are bounded by double black line: Arco (A), Pass Creek (PC), Mackay (M), Thousand Springs (TS), Warm Spring (WS), and Challis (C).
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... and generally now coincide with geologic structures that may be effective barriers to rupture propagation. On this trip, we will travel from south to north along the Lost River fault and look at the geomor- phic and structural similarities and differences between the Arco, Pass Creek, Mackay, Thousand Springs, Warm Spring, and Challis segments ( fig. 1). The segments average 23 km in length, and all but the Challis segment have evidence of at least one surface-rupturing earthquake in the past 20-30 k.y. Schwartz and Coppersmith (1984) proposed a model for characteristic earthquakes in which a fault or fault segment tends to repeatedly generate approximately the same maxi- mum-size ...
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... Salt Lake City (Stover, 1985). The earthquake produced 34 km of surface rupture with a maximum vertical displacement (throw) of 2.7 m, indi- vidual scarps nearly 5-m high, and a component of left-lateral movement as much as 17 percent (Crone and others, 1987). In the weeks following the earthquake, field mapping defined a Y-shaped surface rupture (fig. 1); continuous ruptures formed along the entire 20.8-km-long Thousand Springs segment and discontinuous scarps formed to the north along the Warm Spring segment (14.2 km) and to the northwest across the Wil- low Creek hills (7.9 km) joining the west-dipping Lost River fault and the east-dipping Lone Pine fault (Crone and others, 1987). ...
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... the southeast, the mountain front swings eastward out of sight as the trend of the Lost River Range front changes direction by about 55°. At this location near Elkhorn Creek, low, uphill-facing scarps formed in 1983, probably due to gravitational failure in response to violent ground shaking (see fig. 11 in Crone and others, 1987). Wallace and Bonilla (1984) identified similar features near the crest of the Stillwater Range, in the general vicinity of the 1954 Dixie Valley earth- quake and the 1915 Pleasant Valley earthquake. Interestingly, uphill-facing scarps existed before the 1983 earthquake, and the scarps we see today are the ...
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... of this unique opportunity. Figure 10 shows simplified trench logs from the two studies. The logs show that faulting in 1983 closely mimicked the amount and style of prehistoric faulting ( Schwartz and Crone, 1985). ...
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... the Borah Peak earthquake, rupture propaga- tion slowed, and a large number of aftershocks concentrated near the Willow Creek hills. This segment boundary seems to be complex in the subsurface (Bruhn and others, 1991); the aftershocks sequence suggests that the overall geometry of the surface faulting directly reflects structure at depth. Detailed analysis of the aftershocks shows that they occurred in discrete zones less than 1 km in width between 6 and 10 km below the surface (Shemata, 1989). ...
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... view of the Challis segment ( fig. 11) illustrates the basis for assigning a low activity rate on the northern Lost River fault. Here the range is characterized by low topographic and structural relief. The fault is not marked by scarps on alluvium, and its location is poorly constrained by the bedrock-alluvium contact; likewise, the faulting history is poorly understood. ...
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... fault is not marked by scarps on alluvium, and its location is poorly constrained by the bedrock-alluvium contact; likewise, the faulting history is poorly understood. There is no evidence of late Quaternary Figure 11. View of Challis segment from Hot Springs road, south of Challis, Idaho. ...
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Citations
... Some of the discontinuities that we have highlighted as "major complexities" have been described by previous authors. In particular, these researchers segmented the entire structure in six segments based on changing geomorphic expression, structural relief, and age of last movement by dating faulted soils ( Figure 1) [6,7,[44][45][46][47][48]77]. DuRoss et al. [7] examined the branching Arentson Gulch Fault on the 1983Eq rupture and the two most recent paleoearthquakes [49]. ...
Following observations made in a survey campaign along the Lost River Fault (Idaho, USA) in 2019, we integrate both original and previously published data to obtain a detailed segmentation of the fault sections that failed in the 1983 Borah Peak earthquake (Mw 6.9). The earthquake ruptured the topographic surface with an oblique-normal faulting mechanism, activating two SW-dipping fault segments (Thousand Springs and Warm Springs) and a branching SSW-dipping fault (Arentson Gulch Fault) and producing coseismic surface ruptures with up to 3 m of vertical separation. We augment the 1983 earthquake description by interpreting high-resolution topography and fault mapping. We use quality vertical separation data, rupture zone width measurements, and fault slip data to analyze major and minor structural-geometric complexities, highlighting a partition of the deformation and a fault segmentation into four detail levels (i.e., segments, sections, subsections, and sectors). Our work provides new details of the 1983 Borah Peak earthquake, constraints for paleoseismic and seismotectonic studies, and a methodological approach applicable in other areas of the world. Our fault-slip data show variations along fault-strike that we interpret as kinematic partitioning. In 1983, the main southern segment had a large rupture zone width, while the northern segment localized the deformation. The distributed ruptures accommodate a large portion of the rupture length (~19.5 km versus 31 km for the main rupture) and displacement (~66%). 83% of the surface faulting and 80% of the displacement are located at the hanging wall of the main rupture. There is a strong correlation between vertical separation, rupture zone width, rupture position (footwall or hanging wall), and fault geometry. We highlight the control of the obliquity and kinematic partitioning in the surface expression of the earthquake propagation. We interpret the coseismic (i.e., 1983) and long-term (i.e., Quaternary) behavior, showing that the two activated segments had similar cumulated behaviors in distributing the deformation between synthetic and antithetic ruptures, despite the different geometries. Our results have implications for fault rupture behavior with application to rupture hazard.
... Basemap is multi-directional hillshade (Esri, 2015 (Hait and Scott, 1978;Cochran, 1985) with 1.8-1.9 m of displacement, which is similar to that of the 1983 rupture (1.7-2.0 m) (Schwartz and Crone, 1985;Haller and Crone, 2004). The Doublespring Pass trenches did not yield limiting ages; however, the rupture occurred after deglaciation (ca. ...
We excavated trenches at two paleoseismic sites bounding a trans-basin bedrock ridge (the Willow Creek Hills) along the northern Lost River fault zone to explore the uniqueness of the 1983 Mw 6.9 Borah Peak earthquake compared to its prehistoric predecessors. At the Sheep Creek site on the southernmost Warm Springs section, two earthquakes occurred at 9.8−14.0 ka (95% confidence) and 6.5−7.1 ka; each had ∼1.9 m of vertical displacement. About 4 km to the southeast, across the Willow Creek Hills, two ruptures at the Arentson Gulch site on the northernmost Thousand Springs section occurred at 9.0−14.7 ka and 6.1−7.5 ka with ∼1.9 m of vertical displacement each. We synthesize these and previous paleoseismic results into a model of five postglacial (<15 ka) ruptures along a ∼65 km reach of the northern Lost River fault zone. Our results show that the Borah Peak earthquake (34 km; 0.9 m mean displacement) was unique compared to previous ruptures that had both longer and shorter rupture lengths (∼25−38 km), more displacement (mean of ∼1.3−1.4 m), and equal or greater magnitude (Mw 6.9−7.1) than that in the 1983 earthquake. These ruptures support a hypothesis of variable rupture length and displacement on the northern Lost River fault zone and show that predecessors to the 1983 rupture have passed unimpeded through the Willow Creek Hills. Our work demonstrates that normal faults are capable of producing variable spatial-temporal patterns of rupture that, together with comparisons of fault geometry and historical rupture length, improve our understanding of fault segmentation and help inform models of earthquake rupture probability.
... To our knowledge, this is the third time globally that a paleoseismic trench has been coseismically displaced. The first was during the Borah Peak earthquake of 1983 (Idaho, United States), when a normal fault (Lost River Fault) displaced a 7-yr old trench by 2 m (Haller et al. 2004). Parts of this trench were re-logged, and the location and style of the 1983 slip on the exposed wall was found to be similar to that which occurred during older paleoearthquakes. ...
During the Kaikōura earthquake, a paleoseismic trench was dextrally displaced ∼9 m and shortened by 1.3 ± 0.4 m – the largest globally recorded displacement of a trench. Analysis showed that two processes accommodated subequal amounts of slip at the surface: (1) discrete dextral-slip on two steeply-dipping faults bounding a <3.5 m wide central deformation zone; and (2) coseismic clockwise rotation of turf rafts and pervasive sediment deformation in that zone. The second (successive) process resulted in upward (<1 m) and outward (<2 m) bulging along low-angle thrusts, creating horizontal fault-perpendicular shortening that exceeds the heave (∼1.3 m). This discrepancy results from coseismic rotation of rafts, that shorten upon approaching perpendicularity with the fault – creating extra apparent shortening (in fault-orthogonal view). Comparison of pre- and post-earthquake trench logs indicates that strike-slip ruptures at the same site can be expressed differently over time; fault strands carrying major displacement in 2016 were not the locus of deformation in the previous earthquake(s), suggesting temporal unpredictability is important in defining fault zones. The last several paleoearthquakes at the trench produced cm-dm scale normal-sense dip separations across faults; however, the 2016 earthquake created compressive structures including up-bulging and low-angle reverse faulting, as well as fissuring. This contrast in deformation style likely resulted from an >8° clockwise rotation of the local slip vector in 2016 (becoming transpressive), highlighting that small changes in slip kinematics may affect rupture zone structures.
... The earthquake resulted in the deaths of two young children and $12.5 million in damage (Stein and Bucknam, 1985;Bucknam and Stein, 1987). The earthquake initiated along a central segment of the Lost River fault at a depth of about 16 km and propagated northward across a segment boundary (e.g., Crone et al., 1987;Haller and Crone, 2004;DuRoss et al., 2019). Unlike the Stanley earthquake that produced no surface rupture, this is an example of a multisegment rupture, with a surface scarp that preserves the earthquake's slip. ...
We report on the tectonic framework, seismicity, and aftershock monitoring efforts related to the 31 March 2020 Mw 6.5 Stanley, Idaho, earthquake. The earthquake sequence has produced both strike-slip and dip-slip motion, with minimal surface displacement or damage. The earthquake occurred at the northern limits of the Sawtooth normal fault. This fault separates the Centennial tectonic belt, a zone of active seismicity within the Basin and Range Province, from the Idaho batholith to the west and Challis volcanic belt to the north and east. We show evidence for a potential kinematic link between the northeast-dipping Sawtooth fault and the southwest-dipping Lost River fault. These opposing faults have recorded four of the five M≥6 Idaho earthquakes from the past 76 yr, including 1983 Mw 6.9 Borah Peak and the 1944 M 6.1 and 1945 M 6.0 Seafoam earthquakes. Geological and geophysical data point to possible fault boundary segments driven by pre-existing geologic structures. We suggest that the limits of both the Sawtooth and Lost River faults extend north beyond their mapped extent, are influenced by the relic trans-Challis fault system, and that seismicity within this region will likely continue for the coming years. Ongoing seismic monitoring efforts will lead to an improved understanding of ground shaking potential and active fault characteristics.
... Surface rupture occurred along two structural fault sections, including all of the Thousand Springs section, and the southern half of the Warm Springs section, north of the Willow Creek Hills structure, a prominent hanging-wall bedrock ridge where the LRFZ splits into multiple strands with differing strikes (Crone et al., 1987) (Fig. 1). As one of the largest intraplate normal-faulting earthquakes recorded historically and an example of the complex rupture of a multisegment normal fault system (Haller and Crone, 2004), the Borah Peak earthquake rupture offers an important opportunity to relate spatial and temporal patterns of surface displacement to fault-rupture processes (e.g., Wesnousky, 2008;Nissen et al., 2014;Haddon et al., 2016;Delano et al., 2017;Personius et al., 2017;Johnson et al., 2018). ...
... 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 Schwartz and Crone, 1985;and Haller and Crone, 2004). A second trench, excavated immediately adjacent to the Hait and Scott (1978) trench following the 1983 earthquake, showed that 1983 rupture displacement at the site (1.7-2.0 m) had a similar magnitude compared to prehistoric displacement at the site (1.3-1.5 m) (Schwartz and Crone, 1985). ...
The 1983 Mw 6.9 Borah Peak earthquake generated ~36 km of surface rupture along the Thousand Springs and Warm Springs sections of the Lost River fault zone (LRFZ, Idaho, USA). Although the rupture is a well-studied example of multisegment surface faulting, ambiguity remains regarding the degree to which a bedrock ridge and branch fault at the Willow Creek Hills influenced rupture progress. To explore the 1983 rupture in the context of the structural complexity, we reconstruct the spatial distribution of surface displacements for the northern 16 km of the 1983 rupture and prehistoric ruptures in the same reach of the LRFZ using 252 vertical-separation measurements made from high-resolution (5–10-cm-pixel) digital surface models. Our results suggest the 1983 Warm Springs rupture had an average vertical displacement of ~0.3–0.4 m and released ~6% of the seismic moment estimated for the Borah Peak earthquake and <12% of the moment accumulated on the Warm Springs section since its last prehistoric earthquake. The 1983 Warm Springs rupture is best described as the moderate-displacement continuation of primary rupture from the Thousand Springs section into and through a zone of structural complexity. Historical and prehistoric displacements show that the Willow Creek Hills have impeded some, but not all ruptures. We speculate that rupture termination or penetration is controlled by the history of LRFZ moment release, displacement, and rupture direction. Our results inform the interpretation of paleoseismic data from near zones of normal-fault structural complexity and demonstrate that these zones may modulate rather than impede rupture displacement.
... The resulting recurrence estimates were filtered to eliminate values less than 195 ± 165 yr (2), which DuRoss et al. (2011) used as an estimated minimum time required to degrade a fault-scarp free face and begin to deposit scarp-derived colluvium along the rupture in a semiarid environment. The minimum time likely ranges from approximately a few tens to a few hundred years based on the elapsed times since the Borah Peak, Idaho, earthquake rupture (~30 yr) (which is now forming colluvial wedges; Crone and Haller, 2004) and the most recent earthquake on the NS (less than ~360 yr). The filtered inter-event recurrence intervals are similar (less than 10-yr difference) to those determined without a minimum time, with the exception of estimates for B4-B3, W2-W1, P3-P2, and N4-N3, where the filtered recurrence estimates are about 20 to 70 yr longer than the unfiltered results because of overlapping segment PDFs . ...
... The resulting recurrence estimates were filtered to eliminate values less than 195 ± 165 yr (2), which DuRoss et al. (2011) used as an estimated minimum time required to degrade a fault-scarp free face and begin to deposit scarp-derived colluvium along the rupture in a semiarid environment. The minimum time likely ranges from approximately a few tens to a few hundred years based on the elapsed times since the Borah Peak, Idaho, earthquake rupture (~30 yr) (which is now forming colluvial wedges; Crone and Haller, 2004) and the most recent earthquake on the NS (less than ~360 yr). The filtered inter-event recurrence intervals are similar (less than 10-yr difference) to those determined without a minimum time, with the exception of estimates for B4-B3, W2-W1, P3-P2, and N4-N3, where the filtered recurrence estimates are about 20 to 70 yr longer than the unfiltered results because of overlapping segment PDFs . ...
... The resulting recurrence estimates were filtered to eliminate values less than 195 ± 165 yr (2), which DuRoss et al. (2011) used as an estimated minimum time required to degrade a fault-scarp free face and begin to deposit scarp-derived colluvium along the rupture in a semiarid environment. The minimum time likely ranges from approximately a few tens to a few hundred years based on the elapsed times since the Borah Peak, Idaho, earthquake rupture (~30 yr) (which is now forming colluvial wedges; Crone and Haller, 2004) and the most recent earthquake on the NS (less than ~360 yr). The filtered inter-event recurrence intervals are similar (less than 10-yr difference) to those determined without a minimum time, with the exception of estimates for B4-B3, W2-W1, P3-P2, and N4-N3, where the filtered recurrence estimates are about 20 to 70 yr longer than the unfiltered results because of overlapping segment PDFs . ...
... The resulting recurrence estimates were filtered to eliminate values less than 195 ± 165 yr (2), which DuRoss et al. (2011) used as an estimated minimum time required to degrade a fault-scarp free face and begin to deposit scarp-derived colluvium along the rupture in a semiarid environment. The minimum time likely ranges from approximately a few tens to a few hundred years based on the elapsed times since the Borah Peak, Idaho, earthquake rupture (~30 yr) (which is now forming colluvial wedges; Crone and Haller, 2004) and the most recent earthquake on the NS (less than ~360 yr). The filtered inter-event recurrence intervals are similar (less than 10-yr difference) to those determined without a minimum time, with the exception of estimates for B4-B3, W2-W1, P3-P2, and N4-N3, where the filtered recurrence estimates are about 20 to 70 yr longer than the unfiltered results because of overlapping segment PDFs . ...
... Relief of the LRR has resulted from movement along the Lost River Fault, a west-dipping normal fault that bounds the western flank of the mountain range. Bedrock is primarily Precambrian and Paleozoic limestones and quartzarenites (Haller and Crone, 2004;Skipp and Hait, 1977). ...
