Relocation and characterization of the August 2009 microearthquake swarm above the Socorro magma body in the central Rio Grande Rift

Abstract

Earthquake swarms, often attributed to magma or fluid movement in the uppermost crust, have differing statistics depending on tectonic setting, with volcanic swarms producing b-values of up to 2.5 and continental rift swarms having b-values ∼0.8-1. Along the Rio Grande continental rift (RGR) in the western US, earthquake swarms have occurred in the area of the Socorro Magma Body (SMB), including the 2009 swarm of 431 events (magnitudes -1.0 to 2.5) over 26 days. We characterize the 2009 swarm to define spatial and temporal patterns to help understand the role of the SMB on the swarm. Our relocations suggest a small seismogenic volume of 34.5 km 3 and focal mechanisms of the largest events are consistent with regional stress patterns. The b-value of 0.9 is more characteristic of continental rift zones than volcanic swarms, suggesting more emphasis on the preexisting highly fractured crust rather than direct influence of magmatic movement. © 2010 by the American Geophysical Union.