GrowClust and HypoDD Relocation and Characterization of the 2009 Earthquake Swarm Above the Socorro Magma Body, Central New Mexico, USA

The area above the Socorro Magma Body (SMB), a 150 m thick sill around 19 km depth in central New Mexico, experiences a higher level of seismicity than the surrounding areas. These earthquakes are concentrated primarily in the upper 10 km of crust above the SMB, in a zone dominated by extensional rifting due to the Rio Grande Rift (RGR). In recent years, the SMB area has experienced several seismic swarms, with an increase in earthquakes occurring over a short period of time. One prominent swarm was the 2009 earthquake swarm, which included 1280 earthquakes MD -1.91 to 2.36 that occurred over 26 days. The 2009 swarm earthquakes are primarily clustered in a relatively small area in the center of the SMB, making them a reasonable candidate for cross-correlation based earthquake relocation. Here, results of the relocation of 1280 earthquakes in the 2009 SMB earthquake swarm are presented using the earthquake relocation programs GrowClust and HypoDD (Waldhauser and Ellsworth, 2000; Trugman and Shearer, 2017). GrowClust combines cluster analysis and relocation of earthquakes within clusters from initial earthquake locations. HypoDD minimizes the travel time difference between a pair of earthquakes. Both GrowClust and HypoDD use properties of raypath and source mechanism similarity to minimize unmodeled crustal heterogeneity through cross-correlation, but differ in the relocation process. We will explore the use of these different relocation methods for the SMB network geometry to determine the best relocation method for other SMB swarms. Following relocation of the 2009 earthquake swarm, characteristics of the swarm are identified such as spatial and temporal patterns, the b-value, and a modified version of Omoris Law of aftershock decay. Correlation between swarm characteristics and pre-existing RGR faults has been noted in a previous study of a subset of the 2009 earthquakes through mainshock-aftershock fault sequences consistent with continental rifts and focal mechanisms (Ruhl et al., 2010). We use the larger relocated earthquake catalog to update frequency-magnitude statistics and spatial correlations, with the goal of understanding how the mid-crustal magma body interacts with the pre-existing fractured RGR crust to influence earthquake swarm behavior.