Relocation of the 2009 Earthquake Swarm Above the Socorro Magma Body, Central New Mexico, USA

The Socorro Magma body (SMB), a thin sill approximately 150 m thick located at approximately 19 km depth in central New Mexico, USA, is one of the largest mid-crustal magma bodies currently known. Seismicity in the area has been monitored since the 1960s with 10-12 seismic stations currently operating in the Socorro region. Microearthquakes frequently occur in the SMB area, with several earthquake swarm sequences, including swarms in 1983, 2005, and 2009. The August 2009 swarm lasted about a month and included more than 469 earthquakes with the largest local magnitude M d of 2.5. Initial locations for events in this swarm suggested a rough linear trend propagating northward from the first events in the swarm (Ruhl et al., 2010). Because of the high cross-correlation values of many of the events in the swarm, we present relocations of events in this earthquake swarm using the GrowClust algorithm, which combines cluster analysis and relocation of events within clusters (Trugman and Shearer, 2017). We use P and S arrivals, cross-correlations of the P and S arrivals, initial hypocenter locations, and the same velocity model used for the initial locations. With the GrowClust algorithm, events are grouped into pairs based on waveform similarity and data quality. Relative locations of events are determined by minimizing the least squares 1-norm (L1 norm) of the residual between the observed and predicted differential travel times. Event pair locations are modified based on waveform similarity and then merged into larger clusters. It is expected that the relocated events will provide additional information on features associated with the SMB and reveal fault structures in the shallow crust. Our relative earthquake locations will also provide more detailed information about swarm migration from the August 2009 earthquake swarm.