HR: 0800h
AN: T41A-1167    [Abstracts]
TI: An Earthquake Swarm on the Galapagos Transform Fault: Implications for Earthquake Triggering
AU: * Roland, E
EM: eroland@mines.edu
AF: Department of Geophysics Colorado School of Mines, 1500 Illinois St., Golden, CO 80401
AU: Boettcher, M S
EM: mboettcher@whoi.edu
AF: MIT-WHOI Joint Program, Dept of Geology and Geophysics, MS24, Woods Hole, MA 02543
AU: McGuire, J J
EM: jmcguire@whoi.edu
AF: Woods Hole Oceanographic Institution, Dept. of Geology and Geophysics, MS24, Woods Hole, MA 02543
AB: Transform faults on the East Pacific Rise spreading system often have large amounts of seismicity in short periods of time. The magnitude vs. time distribution of these sequences ranges from what would be classified as similar to a typical continental earthquake but with an elevated foreshock to aftershock ratio to sequences that would traditionally be classified as an "earthquake swarm" closer to those seen in volcanic regions (e.g. Forsyth et al., 2003). Analysis of declustered earthquake catalogs (i.e. ignoring the swarms) suggest that the anomalous foreshock to aftershock ratio on EPR transforms requires different triggering processes in the oceanic regime from those that explain continental seismicity. Here we investigate the best recorded earthquake swarm on an EPR transform to evaluate whether it has similar implications. Due to the lack of oceanic earthquake catalogs with low detection thresholds and reliable magnitude estimates, the unique characteristics of transform sequences have been difficult to quantify. This study uses the earthquake catalog derived from NOAA's hydroacoustic array in the equatorial Pacific Ocean and data from a land-based seismometer array on the Galapagos Islands (from Toomey et al.) to examine the temporal and size distribution of an earthquake swarm which occurred on the Galapagos Transform in 2000. Seismic moment estimates are determined with a high degree of accuracy using an Empirical Green's Function based, cross-correlation method. Characteristics of the Galapagos swarm can be compared to that normally observed in continental strike-slip faults by applying the Epidemic Type Aftershock Sequence (ETAS) Model [e.g. Helmstetter and Sorenette, 2002], which is a common earthquake triggering model used to explain continental seismicity. This model assumes aftershocks are triggered from all large earthquakes with a triggering rate that decays in time following the Omori Law and increases with triggering event magnitude. The utility of the ETAS model to characterize the Galapagos Transform swarm is examined using both common continental time decay and triggering exponent parameters and parameters appropriate for the abundant foreshocks and few aftershocks often observed in oceanic settings. ETAS can explain the Galapagos swarm only with relatively low values of the triggering exponent, $\alpha \sim 0.3$ and relatively high values of the Omori exponent, p$\sim$1.8. We will discuss the feasibility of simultaneously explaining both the swarm and ordinary EPR events with the ETAS model as well as explore explanations for the low value of $\alpha$ in terms of the Coulomb stress model.
DE: 8499 General or miscellaneous
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8155 Plate motions--general
DE: 8164 Stresses--crust and lithosphere
DE: 7218 Lithosphere and upper mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting