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The March 2020, Mw6.5 Stanley, Idaho Earthquake–Complex Faulting at the Northern End of the Basin and Range Province From Earthquake Relocation and Source Mechanisms

Session: Intermountain West Earthquakes in the Spring of 2020 I

Type: Oral

Date: 4/20/2021

Presentation Time: 03:30 PM Pacific

Description: 

The 31 March 2020 M_w6.5 Stanley, Idaho earthquake was the largest earthquake in the Intermountain West (IMW) region since 1983. Unlike other IMW earthquakes, which rupture prominent range-bounding normal faults, the Stanley mainshock was a left-lateral strike-slip event on a ‘blind’ fault. The epicenter is north of the Sawtooth normal fault in a region intersected by the Tertiary, northeast trending trans-Challis fault zone (TCFZ) resulting in complex fault interplay. We relocate M2.5+ aftershocks to identify active faults and perform waveform modeling of M3.2+ events to determine deformation styles. We use local and regional waveforms from the IRIS DMC for analysis. Initial relocations place the mainshock near the center of the first-day aftershocks suggesting a bilateral, 30-km long rupture on an NNW-SSE trending fault. The mainshock hypocenter is near the base of the aftershocks indicating rupture propagated upward. Aftershock activity near the mainshock epicenter remained vigorous throughout the sequence and shows a fault dipping ~70-75 towards west. Further north, aftershocks follow a linear trend on a near-vertical fault. Focal mechanisms for the two segments are strike-slip with a steeply dipping NNW-SSE trending nodal plane consistent with aftershocks. Deformation south of the hypocenter area near the terminus of the Sawtooth fault is complex. Aftershocks are widely distributed and involve normal and strike slip mechanisms with varying orientations suggesting several distinct faults are active. Starting one week after the main event, aftershocks expanded south-easterly resulting in an overall L-shaped aftershock distribution; some aftershocks possibly occurred close to the Sawtooth fault. Aftershocks at the boundary between the Sawtooth fault and the strike-slip fault illustrate complex stress transfer at the terminus of faults. The Sawtooth fault seems to terminate at the TCFZ suggestive of fault control by preexisting structures. We found no clear evidence for activity on the TCFZ; the broad aftershock distribution occurs south of the TCFZ.

Presenting Author: Jochen Braunmiller

Student Presenter: No

Authors