Exploring the Brittle-Ductile Transition at the Base of the Seismogenic Zone of the Crust in Southern California: Implications for Crustal Rheology

Abstract:

We use 35 years of southern California waveform relocated seismicity and refined focal mechanisms to analyze the heterogeneity in seismicity in the context of geophysical crustal properties. First, we analyze the spatial heterogeneity in the seismogenic thickness across southern California. To determine the seismogenic thickness, we calculate the statistical properties of seismicity iso-surfaces near major faults as well as across southern California: 1) 5% shallow depth (D5%); 2) mean and median focal depths; 3) the D95% and D99% focal depths at the bottom of the seismogenic zone; 4) b-values; and 5) changes in the state of stress. Almost no seismicity occurs at shallow depths between ~1 and ~3 km which is likely caused by the predominance of velocity strengthening friction. As confining pressure increases with depth within the seismogenic zone, the rate of seismicity changes because the macroscopic behavior of crustal rocks becomes more ductile. At the base the seismogenic zone, where ductile behavior and velocity strengthening co-exist, the rate of decay of seismicity reflects changes in thickness of the brittle-ductile transition zone.

Second, to image the rheology of the brittle-ductile transition, we also analyze focal mechanisms of adjacent earthquakes that may exhibit a decrease in the internal friction angle as pressure increases with depth, near the bottom of the seismogenic zone. We search for such a weak zone in the lower crust using the horizontal moment tensor element of focal mechanisms to map decreasing horizontal shear tractions at the base of the seismogenic zone, especially beneath major Principal Slip Zones (PSZs).

We interpret our results in the context of the previously determined regional velocity structure, crustal thickness, proximity to major PSZs as well as geophysical parameters of the crust such as heat flow and tectonic strain rate. The new interpretation provides an image of the seismogenic thickness and brittle-transition zone rheology as they vary from one crustal block to the next and in the vicinity of major PSZs. In particular, the absence of significant stress field rotations within the brittle ductile transition zone beneath southern California suggests that the lower crust appears stronger than the lower crust beneath the Bay Area in northern California.