COSEISMIC ORIGIN OF TRANSGRAVEL FRACTURES NON-LITHIFIED CLASTIC DEPOSITS

Earthquake-rupture parameters are typically derived from large-offset faults. As a result, small faults are often ignored in seismic hazard assessment of continental interiors. We mapped the brittle process zone of a small bedrock fault recorded in a thin muddy-gravel deposit, and document permanent textural and structural evidence for near-surface rupturing. Gravels, which were systematically rotated and fractured, indicate particulate to cataclastic flow mechanisms at high strain-rates, consistent with the regional stress field. The contrasting sense of gravel-rotation invokes elastic rebound of underlying crustal rocks. Coseismic stress is transmitted across the basement-sediment interface kinematically and dynamically, and then dissipated in the sediments, documenting the fault's arrest. This provides a mechanism for the "shallow slip-deficit" above blind faults. Mixed-gravel textures are a high-resolution paleoseismological tool to extend mappable limits of active faults, which leads to improved earthquake magnitude, hazard, and zoning estimates.