SEISMOGENIC THICKNESS OF CALIFORNIA FROM EARTHQUAKE-LOCATION DATA: IMPLICATIONS FOR THERMAL STRUCTURE AND SEISMIC HAZARD

The seismogenic thickness of the crust, a proxy for brittle-crust thickness, is a geometric parameter that is related to crustal strength, seismic hazard, and the thermo-mechanical nature of the crust. Here we use published publicly available high-resolution earthquake-location data from California to construct a topographic map of the base of the seismogenic crust by calculating the depth above which 95% (D95) of seismicity is located for fixed width bins (i.e., ~20 km). Seismogenic thickness in California is highly variable, ranging from 10 to 20 km, with thicker D95 values in the Great Valley-Sierra Nevada and thinner values in the Walker Lane and northern coastal California. The depth to the brittle-ductile transition (BDT) is mostly controlled by the thermal structure of the crust, and the base of the seismogenic crust essentially represents a BDT isotherm (~300-350°C for quartz-dominated lithologies). Seismogenic thickness is inversely correlated with surface heatflow in most locations—parameters that can be related via steady-state heat conduction—and local deviations probably reflect non-steady-state conditions related to magmatism and/or hydrothermal circulation. Spatial variations of D95 depths across California can be used to evaluate or constrain the locations of future seismicity, propagation direction of earthquake ruptures, and maximum depth, rupture area, and magnitude of a future event. Thicker seismogenic crust should be stronger, and less small-magnitude events are observed in these regions. Seismogenic depth asperities, which represent mechanically stronger crustal patches, may focus and nucleate future earthquake events and/or impede rupture propagation. Comparison of the D95 maps with historical earthquakes can reveal important trends. For example, both the 1857 Fort Tejon and 1906 San Francisco earthquakes propagated from thicker to thinner brittle crust. Future work involves integrating seismogenic thickness maps with surface heatflow data and Moho depth/temperature inversions to more rigorously evaluate the thermal structure and rheology of Californian crust.