Geologic Context of the 2020 Mw 6.5 Stanley, Idaho Earthquake: Preliminary Paleoseismology of the Sawtooth Fault
Description:
The 2020 MW 6.5 Stanley, Idaho, earthquake occurred in a poorly understood area of oblique extension north of the 67-km-long Sawtooth fault, within the northwesternmost part of the Centennial Tectonic Belt. The dominantly left-lateral earthquake likely involved multiple, north-northwest (NNW) trending faults with blind rupture. To provide geologic context for this earthquake, we (1) mapped the surface traces of previously unknown faults in the epicentral region using 0.5-m-resolution lidar, and (2) excavated a trench across the northern extent of the Sawtooth fault at the Dutch Lake site, ~10 km south of the 2020 epicenter. Lidar mapping suggests that southwest–northeast oriented strain in the region is accommodated by a 5-km-wide overlapping left step between (1) NNW-trending and east-facing scarps at the northern terminus of the Sawtooth fault and (2) NNW- to north-trending and west-facing scarps along the 8–10-km-long Cape Horn fault. This pattern corresponds well with previous fault models for the 2020 earthquake showing lateral and normal slip on opposing, west- and east-dipping faults, respectively. We excavated a trench at the Dutch Lake site, within this left-stepping zone of slip transfer. The trench exposed postglacial alluvial-fan sediments vertically displaced 2.0–2.2 m by a single surface-faulting earthquake. Complex, likely normal-oblique faulting postdates surface formation at the site and predates charcoal-rich loess incorporated into surface sediments and scarp colluvium. Samples for 14C, luminescence, and 10Be cosmogenic dating will help resolve the timing of the prehistoric earthquake. We will explore scenarios of rupture length and displacement using the lateral continuity and vertical separation of scarps. Our preliminary results show that strain in the region is expressed along a complex, distributed zone of faults that have an unknown paleoseismic history compared to the rupture of the northern Sawtooth fault exposed at the Dutch Lake site.
Session: From Earthquakes to Plate Boundaries: Insights Into Fault Behavior Spanning Seconds to Millennia [Poster]
Type: Poster
Date: 4/20/2023
Presentation Time: 08:00 AM (local time)
Presenting Author: Christopher B. DuRoss
Student Presenter: No
Invited Presentation:
Authors
Christopher DuRoss Presenting Author Corresponding Author cduross@usgs.gov U.S. Geological Survey |
Zachery Lifton zlifton@uidaho.edu Idaho Geological Survey |
Alexandra Hatem ahatem@usgs.gov U.S. Geological Survey |
Richard Briggs rbriggs@usgs.gov U.S. Geological Survey |
Jessica Jobe jjobe@usgs.gov U.S. Geological Survey |
Nadine Reitman nreitman@usgs.gov U.S. Geological Survey |
Glenn Thackray glennthackray@isu.edu Idaho State University |
Mark Zellman mzellman@bgcengineering.ca BGC Engineering Inc. |
Camille Collett ccollett@usgs.gov U.S. Geological Survey |
Geologic Context of the 2020 Mw 6.5 Stanley, Idaho Earthquake: Preliminary Paleoseismology of the Sawtooth Fault
Category
From Earthquakes to Plate Boundaries: Insights Into Fault Behavior Spanning Seconds to Millennia