Spatiotemporal Correlation Between Seasonal Variations in Seismicity and Horizontal Dilatational Strain in California

Abstract

We extract significant spatially coherent strain variations from horizontal seasonal Global Positioning System (GPS) displacements in the American Southwest. The dilatational strain is largest in northern California with maximum margin-normal contraction and extension in spring and fall, respectively, consistent with the Earth's surface going down and up at those times. The northern California signal has a phase shift with respect to that in southern California and the Great Basin. For northern and southern California the proportion of larger earthquakes are in-phase and the aftershock productivity out of phase with the inferred Coulomb stress on the San Andreas fault system. The intensity of mainshocks is in-phase in the north as well but not in the south. This suggests that a seasonal increase in fault-normal extension may or may not trigger mainshocks, but when an earthquake happens at those times, they grow larger than they otherwise would, which would cause a larger stress reduction and result in fewer aftershocks.