HR: 17:40h
AN: NG24B-03 INVITED     [Abstracts]
TI: Foreshock Sequences and Short-Term Earthquake Predictability on East Pacific Rise Transform Faults
AU: * McGuire, J J
EM: jmcguire@whoi.edu
AF: Woods Hole Oceanographic Institution, Dept of Geology and Geophysics, MS24, Woods Hole, MA 02543 United States
AU: Boettcher, M S
EM: mboettcher@whoi.edu
AF: MIT-WHOI Joint Program, Dept of Geology and Geophysics, WHOI, MS24, Woods Hole, MA 02543 United States
AU: Jordan, T H
EM: tjordan@usc.edu
AF: Department of Earth Sciences, University of Southern California, Southern California Earthquake Center, Los Angeles, CA 90089-0740 United States
AB: A predominant view of continental seismicity postulates that all earthquakes initiate in a similar manner regardless of their eventual size and that earthquake triggering can be described by an Endemic Type Aftershock Sequence (ETAS) model [e.g. Ogata, 1988, Helmstetter and Sorenette 2002]. These null hypotheses cannot be rejected as an explanation for the relative abundances of foreshocks and aftershocks to large earthquakes in California [Helmstetter et al., 2003]. An alternative location for testing this hypothesis is mid-ocean ridge transform faults (RTFs), which have many properties that are distinct from continental transform faults: most plate motion is accommodated aseismically, many large earthquakes are slow events enriched in low-frequency radiation, and the seismicity shows depleted aftershock sequences and high foreshock activity. Here we use the 1996-2001 NOAA-PMEL hydroacoustic seismicity catalog for equatorial East Pacific Rise transform faults to show that the foreshock/aftershock ratio is two orders of magnitude greater than the ETAS prediction based on global RTF aftershock abundances. We can thus reject the null hypothesis that there is no fundamental distinction between foreshocks, mainshocks, and aftershocks on RTFs. We further demonstrate (retrospectively) that foreshock sequences on East Pacific Rise transform faults can be used to achieve statistically significant short-term prediction of large earthquakes (magnitude $\ge$ 5.4) with good spatial (15-km) and temporal (1-hr) resolution using the NOAA-PMEL catalogs. Our very simplistic approach produces a large number of false alarms, but it successfully predicts the majority (70%) of M$\ge$5.4 earthquakes while covering only a tiny fraction (0.15%) of the total potential space-time volume with alarms. Therefore, it achieves a large probability gain (about a factor of 500) over random guessing, despite not using any near field data. The predictability of large EPR transform earthquakes suggests that a fault preparation process with a characteristic times scale of hours exists and is observable through foreshocks. The predictability of EPR transform earthquakes, their abundant foreshock activity, and their predominance of aseismic fault slip are all consistent with a model in which slow slip transients trigger ordinary earthquakes, enrich their low-frequency radiation, and accommodate much of the subseismic plate motion.
DE: 7220 Oceanic crust
DE: 7223 Seismic hazard assessment and prediction
DE: 8123 Dynamics, seismotectonics
DE: 7209 Earthquake dynamics and mechanics
DE: 3902 Creep and deformation
SC: Nonlinear Geophysics [NG]
MN: 2004 AGU Fall Meeting