Evidence for the Influence of Fluids for Intraplate Earthquake Genesis: The 2001 Enola, AR Earthquake Swarm

HR: 0800h
AN: T51D-0763 [Abstracts]
TI: Evidence for the Influence of Fluids for Intraplate Earthquake Genesis: The 2001 Enola, AR Earthquake Swarm
AU: * Rabak, I
EM: irabak@memphis.edu
AF: Center for Earthquake Research and Information (CERI), at The University of Memphis, 3890 Central Ave, Memphis, TN 38152, United States
AU: Langston, C A
EM: clangstn@memphis.edu
AF: Center for Earthquake Research and Information (CERI), at The University of Memphis, 3890 Central Ave, Memphis, TN 38152, United States
AB: In 2001, almost 20 years after the occurrence of 20,000 earthquakes in the 1982 sequence, the Enola, AR, earthquake zone reactivated with another 2-month long sequence of 2,500 earthquakes. Waveform cross- correlation and double-difference relative hypocentral location methods reveal that the 2001 sequence occupied a crustal volume of 8 km³, at depths between 4-6 km. The swarm volume hosts two clusters of earthquakes at different depths that developed during two separate seismic episodes. We analyzed 57 SV/P & SV/SH amplitude ratios including 15 events for which focal mechanisms were calculated using available P, SV and SH wave polarities. Earthquake clustering in space, time and in the amplitude ratio domain is used to observe a change in source properties from deeper oblique to shallower strike-slip focal mechanisms. The focal mechanisms of the early, seismically deeper cluster show a strong thrust component but the later and shallower cluster source solutions show predominant strike slip motion. We examine an effective stress model to explain the observed change of minimum stress axis direction from the vertical to the horizontal. This behavior is consistent with excess fluid pressures at deeper hypocentral depths. Like artificially-induced earthquakes caused by fluid-injection, it is likely that some intraplate earthquake zones are influenced by natural perturbations in hydrologic state of the hypocentral crust.
UR: http://www.ceri.memphis.edu/people/rabak/tmp/agu07/ DE: 8168 Stresses: general
SC: Tectonophysics [T]
MN: 2007 Fall Meeting