Median Earthquake Stress Drops of Local Earthquakes in Southern California in the Context of Crustal Geophysics: Implications for Fault Zone Healing

Egill Hauksson

Published 2014, SCEC Contribution #1923

We reanalyze the stress drop values for ~60,000 earthquakes in southern California which were originally determined by Shearer et al. (2006) using a spectral method. We only include stress drops that are derived from at least 10 spectra, and with corner frequencies between 3 and 30 Hz. After correcting the spectra for increasing S-wave speed with depth, there are no apparent changes in stress drops with depth, except for a small poorly constrained increase from 15 to 25 km. The geographical distribution of the grouped median stress drops includes very high stress drops near Ridgecrest, eastern California, as well as near fault jogs within the San Andreas fault system. Low stress drops dominate in trans-tensional regions. We use six crustal geophysics parameters to search for obvious correlations that may explain changes in the median values of the stress drops. None of the variables reduce the scatter but most can explain at most 10% to 20% variations in the median stress drops. Heat flow, and GPS measured shear strain rate have the largest influence on stress drops. In the range of low to medium heat flow, the stress drops increase with increasing heat flow. In contrast, at high heat flow, the stress drops decrease systematically with increasing heat flow. Increasing shear strain rate systematically correlates with decreasing stress drops. The crustal stress regime also influences the stress drops as demonstrated by lower in stress drops for north-northeast trending principal horizontal stress and in areas of dip-slip faulting. In general median stress drops decrease with high heat flow that usually coincides with thin crust. The median variations in stress drops with heat flow, stress regime, crustal thickness, and density can be explained in the context of fault healing (grain boundary growth) and corresponding increase in fault zone strengthening on time scales modulated by tectonic shear strain rate.

Citation
Hauksson, E. (2014). Median Earthquake Stress Drops of Local Earthquakes in Southern California in the Context of Crustal Geophysics: Implications for Fault Zone Healing. Pure and Applied Geophysics, 1-12. doi: 10.1007/s00024-014-0934-4.