Investigating Triggering of the Aftershocks of the 2014 Napa Earthquake

The occurrence of the Mw 6.0 South Napa California earthquake, on 24 August 2014 at 03:20 a.m. local time, triggered discussion in the seismological community about the level of damage associated with such a moderate-magnitude event and about the geometry and orientation of the causative fault. In addition, coulomb static stress change mapping does not seem to be able to fully explain near-source aftershock distribution. Here, we find clear evidence of a north-northwest source directivity from the analysis of the spatial distribution of peak ground motion. The area of the highest values of the estimated peak dynamic strain field, computed accounting for fault extent and source directivity, agrees with the near-source aftershock distribution. This might suggest that, in addition to coulomb static stress change, dynamic strain also contributed to the triggering of near-source Napa earthquake aftershocks. The approach used here might be useful to identify areas likely prone to aftershock occurrence.