Aftershocks of the 2014 M6 South Napa Earthquake: Detection, Location, and Focal Mechanisms

The aftershock sequence of the South Napa earthquake is notable both for its low productivity and for its geometric complexity. The aftershocks do not clearly define a fault plane consistent with the NNW-striking vertical plane implied by the mainshock moment tensor and the mapped surface rupture, but instead seem to delineate multiple secondary structures at depth. We investigate this unusual sequence by identifying additional aftershocks that do not appear in the network catalog, relocating the combined aftershock catalog using waveform cross-correlation arrival times and double-difference techniques, and determining focal mechanisms for individual events and event clusters. Additional aftershocks are detected by applying a matched filter approach to the continuous seismic data at nearby stations, with the catalog earthquakes serving as the waveform templates. In tandem with new event detections, we measure precise differential arrival times between events, which we then use in double-difference event location. We detect about 4 times as many well-located aftershocks as in the network catalog. We relocate the events using double-difference in both a 1D and a 3D velocity model. Most of the aftershocks occur between 8 and 11 km depth, similar depth to the mainshock hypocenter and deeper than most of the slip imaged seismically and geodetically. The aftershocks form a diffuse NNW-trending structure, primarily to the north of the mainshock hypocenter and on the west side of the main surface rupture. Within this diffuse trend there are clusters of aftershocks, some suggesting a N-S strike, and some that appear to dip to the east or west. Preliminary single-event and composite focal mechanisms also imply N-S striking strike-slip structures. The mainshock hypocenter and many of the aftershocks occur near the intersection of a sharply defined NE-dipping seismicity structure and the probable location of the West Napa fault, suggesting that stress is concentrated at a fault junction at depth. While the mainshock occurred on the West Napa fault, the majority of the aftershocks occur on other structures in a complex deformation zone.