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Lidar Data Reveal New Faults in the Epicentral Region of the 2020 M 6.5 Stanley, Idaho Earthquake

The March 2020 M6.5 Stanley earthquake occurred in central Idaho, within the northern Basin and Range Province. The earthquake is enigmatic as it occurred ~18 km north-northeast of the Holocene-active Sawtooth fault, plots in an area devoid of Quaternary active structures and yields focal mechanisms with north-south or east-west dominantly lateral slip rather than normal faulting. Aftershocks occur within a north-northwest-aligned zone that is dominantly in the footwall of the east-dipping Sawtooth fault. These observations indicate that seismogenic structures in central Idaho are more complex than current mapping suggests.

We use high-resolution (0.5 m) post-event lidar data as a basis for geomorphic mapping at the northern end of the Sawtooth fault and in the Stanley earthquake epicentral area. Our preliminary mapping reveals a complex northern Sawtooth fault, which extends at least 6 km farther north than previously mapped and includes a ~10-km-long northeastern splay. We also identified evidence for two unrecognized late Quaternary faults. The first is ~5 km west of the Sawtooth fault and expressed as a nearly continuous linear alignment of scarps for ~10 km. From south to north, the fault cuts late Quaternary alluvial-fan and glacial deposits and a broad valley floor, then crosses Cape Horn Creek, climbs topography and cross cuts major valleys before becoming indistinct in steep glaciated terrain. The second fault is ~8 km north of the newly mapped northern extension of the Sawtooth fault in a basin at the head of Shake Creek. The fault is at least 1.8 km and possibly 3 km long and expressed as a nearly continuous linear alignment of scarps that cross steep colluvium and striated late Pleistocene deglacial surfaces and deposits. Both faults strike north-northwest, dip west—opposite the Sawtooth fault—and have unclear relationships to the March 2020 M6.5 earthquake.

Session: Advances in Earthquake Geology: Spatiotemporal Variations in Fault Behavior From Geology and Geodesy [Poster]

Type: Poster

Room: Evergreen Ballroom

Date: 4/22/2022

Presentation Time: 08:00 AM Pacific

Presenting Author: Mark Zellman

Student Presenter: No

Additional Authors