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Characteristics of the October 2005 Microearthquake Swarm and Reactivation of Similar Event Seismic Swarms over Decadal Time Periods near Socorro, New Mexico

Earth and Environmental Science Department, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, New Mexico 87801 janas{at}nmt.edu

Charlotte A. Rowe

EES-11 M.S. D408, Los Alamos National Laboratory, Los Alamos, New Mexico 87545

Richard C. Aster

Earth and Environmental Science Department, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, New Mexico 87801

Seismicity in the Rio Grande rift in central New Mexico, southwestern United States, has been dominated by microearthquakes occurring above the midcrustal Socorro magma body (SMB) for at least the past century. The SMB is a sill-like feature 3400 km² in area, with a top surface at 19-km depth and centered below the inner rift half-graben system. A recent swarm of microearthquakes above the magma body began on 20 October 2005. The activity intensified toward 30 October 2005, with the largest event being a shallow felt local magnitude (M_L) 2.4 earthquake at 02:57:35 (UTC). Seismicity increased in the region following this event, with over 1600 earthquakes detectable at a minimum of one local seismic station during the subsequent month. Seismic waveforms for the earthquakes in this sequence are remarkably similar, implying consistently similar location and faulting geometry. The time–space distribution of earthquakes indicates that this sequence is highly swarmlike, in contrast to tectonic mainshock–aftershock sequences. Given the local tectonic setting and shallow (<8 km) depth of individual events above the SMB, a likely explanation for the Socorro earthquake sequence is fluid flow- or pressure-induced triggering associated with the evolution of the SMB. Swarms such as the 2005 sequence are not uncommon in this area (e.g., prominent previous sequences occurred in May and July of 1983). Here we characterize the 2005 earthquake sequence by presenting composite focal mechanisms, magnitude estimates, and high-precision relative locations based on waveform cross-correlation techniques that capitalize on the similar waveforms. A new waveform scanning technique identifies over 1600 small similar earthquakes during this period from continuous seismic records. Waveforms and locations are also observed to be similar between the 2005 and 1983 sequences, suggesting consistency in source location, geometry, and thus mechanism over decadal time scales.