Crustal geophysics and seismicity in southern California

S U M M A R Y The geographical distribution of the (1981–2005) seismicity in southern California forms a ±150 km broad zone adjacent to the Pacific–North America plate boundary, ranging from depths of ∼1–∼30 km, with the bulk of the focal depths in the range of 2–12 km. The distribution of the seismicity that includes both mainshock–aftershock sequences and background events is affected by both static and kinematic geophysical parameters of the crust. The static parameters include heat flow, topography, crustal density, Vp/Vs ratio, hypocentral faultdistance and crustal thickness from receiver functions. The tectonic loading is represented by kinematic parameters such as the crustal shear strain rate field, and the dilatational strain rate field. In our analysis, we normalize the seismicity relative to the areal density of the range of values of each of the parameters. Most of the seismicity occurs in areas of average heat flow, low to intermediate topography, average Vp/Vs and high late Quaternary fault density, and forms seismogenic zones that extend through the brittle crust. The location of late Quaternary faults, often described as zones of weakness, influences the geographical distribution of seismicity more than any other parameter. Although above or below average crustal properties such as high heat flow, thin crust or very low Vp/Vs values exist, these properties only influence the spatial distribution of seismicity in a minor way. As an example, the Salton Trough area of low topography, high heat flow, high Vp/Vs, high shear strain rate and thin crust has distributed seismicity within a thin seismogenic zone. Also, somewhat surprisingly, areas of high topography, low heat flow, low Vp/Vs, low shear strain rate and thick crust have low seismicity rates but a thin seismogenic zone. We determine an empirical relationship between heat flow and crustal thickness to show how the ∼400 ◦C temperature isotherm gradually deepens with crustal thickness and forms the base of the seismogenic zone for crustal thicknesses from 22 to 37 km. For crustal thickness ranging from 37 to 43 km, the ∼250 ◦C isotherm forms the base of the seismogenic zone, suggesting that seismic faulting in these regions is confined to the top of the upper crust (12–14 km), and thus does not accommodate plate motion.