The SCEC Community Rheology Model for the Southern California Lithosphere: Geological Framework, Ductile Flow Laws, and Preliminary Implications

Abstract The Southern California Earthquake Center (SCEC) Community Rheology Model (CRM) comprises a three‐dimensional geologic framework model (GFM) of southern California's lithosphere, and synthetic aggregate ductile flow laws applicable to each GFM rock type under low‐strain conditions. Given temperature, volatile content, pressure, and strain rate, the CRM may be used to calculate lithosphere effective viscosities and differential stresses. Here, we query the GFM and the SCEC Community Thermal Model (CTM) for rock type, province, and temperature, and use CRM flow laws for each rock type to compute effective viscosities throughout southern California. Assuming a simplified representation of geodetically inferred strain rates, we find that effective viscosities at 20 km depth or less exceed Pa s, except in the inner Salton Trough. Mean effective viscosities at 24–32 km depth are in the 1 to 5 × Pa s range, but lower where schist is present. Mantle viscosities below the lithospheric “lid” are consistent with values inferred from postseismic deformation studies. Our estimates of crustal viscosities are comparable to those inferred from seismic velocities, but spatial patterns of viscosity differ substantially, reflecting CTM temperature patterns and to a lesser extent, assumed strain rates and lithology. In general, CRM effective viscosities are more sensitive to temperatures and strain rates than to lithological variations. To be consistent with observed postseismic deformation at lower crust and upper mantle depths, especially in the Mojave and Walker Lane regions, high‐strain rate (and hence lower‐viscosity) shear zones should be represented in upcoming versions of the CRM.