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Mapping New Subsurface Faults in the Butte Valley, Northern California

Description: 

Butte Valley, in Siskiyou County, California, lies between the Western Cascade Range and the Mahogany Mountain Ridge, a prominent northwestward-trending fault block bounding the valley to the east. The region is dominated by block faulting structures, with normal faults characterized by almost vertical displacement ranging between few to several hundred meters along minor and major faults, respectively. The valley is a downfaulted basin, surrounded by well-preserved scarps of late Pleistocene to recent age. While structural elements are exposed on scarps and rocks surrounding the valley, their continuations are partly or fully concealed beneath the alluvium and lake deposits. No detailed investigations into the faults beneath the valley alluvium have been carried out, and their characteristics remain unknown.
In August 2024, we conduct a high-resolution seismic reflection survey in Butte valley, northern California, to investigate its subsurface structure. The survey acquired three profiles for a total of 9 km using 100 kg accelerated weight drop recorded by a 1.2 km-long linear array of 240 4.5 Hz vertical geophones, and achieving a nominal fold of 40. Across the surveyed area, the data image horizontally stratified reflectors above a ubiquitous high-amplitude reflector, traceable at a depth ranging between 120-150 meters. Based on correlation with the outcropping Butte Valley basalt, we interpret the high amplitude marker as a basalt sill of Pliocene age. Notably, this significant regional marker is displaced by ~40 and 25 m with a down to the northeast sense of motion at two locations along one of the profiles, revealing the presence of two northeast dipping normal faults. The data resolve no deformation in the overlying deposits. However, the surface projection of these faults is marked by northwest striking geomorphic features such as ponded alluvium and closed depressions indicative of latest Pleistocene and Holocene activity. We interpret these faults as part of the Cedar Mountain fault system resulting from the east-west extension at the boundary between the Cascade Ranges and the Modoc Plateau.

Session: Compiling Active Faults for Improved Hazard Modeling from Cascadia to Alaska [Poster]

Type: Poster

Date: 4/17/2025

Presentation Time: 08:00 AM (local time)

Presenting Author: Prajwal

Student Presenter: Yes

Invited Presentation: 

Poster Number: 76

Authors