Relocating Seismicity in the 2014–2015 Sheldon, Nevada Earthquake Swarm

Since mid-July 2014, an earthquake swarm has been occurring in northwestern Nevada approximately 250 km north of Reno. Seismicity rates in the swarm slowly increased over a four month span culminating in a number of mid-magnitude 4 earthquakes in early November, 2014. Since that time, seismicity rates have slowed and, as of early April, the swarm now totals more than 4,500 earthquakes. The earthquake swarm is occurring in a region of the western basin and range that has seen very little historical seismicity, but is dominated by extensional features typical of the region. Focal mechanisms for the largest earthquakes show normal faulting but hypocenters appear to be poorly aligned with local tectonic structures. In order to assign these earthquakes to a regional structure, we relocated a total of 155 earthquakes using a joint hypocentral determination method. For the six largest earthquakes, we manually picked absolute P and S wave arrival times. In addition, we performed waveform cross-correlation on 153 earthquakes to get differential travel times. We then performed a joint hypocentral determination using both of our absolute and relative travel time catalogs. We found that the earthquake swarm is rupturing the normal faults bounding the western edge of the Sheldon Plateau. In addition, we found many highly correlated waveforms from earthquakes spanning the duration of the earthquake swarm, suggesting that the same regions of the fault are rupturing repeatedly