Can the Depth Distribution of Seismicity be Applied to Probe the Rheology of the Seismogenic Crust in Southern California?

Egill Hauksson, Men-Andrin Meier, & Zachary E. Ross

Published August 14, 2017, SCEC Contribution #7620, 2017 SCEC Annual Meeting Poster #231

We carry out joint analysis of waveform relocated seismicity (1981-2016), geophysical, and geological datasets to probe the rheology of the crust in southern California. First, we calculate earthquake depth histograms (EDH) to quantify the depth distribution of seismicity. In general the EDHs vary between regions with a shallower mean depth and more rapid decay with depth in the brittle-ductile transition zone for high heat flow regions, which results in a shallower 95% seismicity cut-off-depth.

Second, we determine depth profiles of crustal yield stress envelopes (YSE) by normalizing differential stress profiles determined with Byerlee’s law and a non-linear dislocation creep law. We analyze the EDHs and YSEs for each lithotectonic block to identify differences in shape that may be related to lithology or past tectonic history of the crust.
To model the YSE for each block we correct for measured average heat flow, strain rate, and state of stress. We determine YSEs of five different lithologies for each of the 16 lithotectonic blocks. We investigate if the EDHs are diagnostic of average lithology types. We find that the EDHs to first order follow the shape of the computed YSEs, and that the lithologies of the best matching YSEs conform with geological expectations. In particular, the blocks of the North America plate are mostly dominated by Quartzite lithology while the blocks of the Pacific plate are mostly of Diorite composition except for the Quartz dominated rifted margin in the Salton Trough. The extended San Gabriel block exhibits two-layer composition with Quartz underplated by Diorite. This suggests that EDHs can be used to infer the first order rheology of the seismogenic crust.

Key Words
Seismicity, seismotectonics, rheology, stress, strength

Citation
Hauksson, E., Meier, M., & Ross, Z. E. (2017, 08). Can the Depth Distribution of Seismicity be Applied to Probe the Rheology of the Seismogenic Crust in Southern California?. Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
SCEC Community Models (CXM)