Seismogenic thickness of California: Implications for thermal structure and seismic hazard

Abstract

The seismogenic thickness of the crust, a proxy for brittle-crust thickness, is a geometric parameter related to crustal strength, seismic hazard, and the crust's thermo-mechanical nature. We use 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% of seismicity (D95) is located for fixed width bins. Seismogenic thickness is highly variable, ranging from ~5 km to >30 km, with thicker D95 values in the Great Valley-Sierra Nevada and thinner values in the Walker Lane and northern coastal California. Seismogenic thickness is inversely correlated with surface heat flow in most locations, consistent with a steady-state conductive crust, and local deviations probably reflect non-steady-state conditions related to magmatism and/or hydrothermal circulation. Such correlation suggests that, at regional scale, brittle-ductile transition (BDT) depth is mostly controlled by geothermal gradients, and the base of the seismogenic crust essentially represents a BDT isotherm (~300–350 °C for quartz-dominated lithologies). 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 future strike-slip earthquake events. Thicker seismogenic crust has a greater integrated strength. Seismogenic depth asperities, which represent mechanically stronger crustal patches, may focus and nucleate future earthquake events and/or impede rupture propagation.