HR: 16:15h
AN: S32F-02    [PDF]
TI: Strong Tidal Modulation of Earthquakes by Strong Tides
AU: * Cochran, E S
EM: cochran@moho.ess.ucla.edu
AF: Department of Earth and Space Sciences, University of California, Los Angeles, CA 90095-1567 United States
AU: Vidale, J E
EM: vidale@ucla.edu
AF: Department of Earth and Space Sciences, University of California, Los Angeles, CA 90095-1567 United States
AU: Tanaka, S
EM: tanaka@zisin.geophys.tohoku.ac.jp
AF: Department of Geophysics, Tohoku University, Aoba-ku, Sendai, 980-8578 Japan
AB: We observe tidal triggering of shallow, thrust events by strong tidal normal stresses. The dataset consists of 9350 global earthquakes of M 5.5 or greater from 1977 to 2000 in the Harvard CMT catalog. A previous study of this dataset by Tanaka et al. (2002) using only phase information showed no significant tidal correlation over the entire catalog; however, slight tidal phase correlation was seen in some subsets of events grouped by tectonic regions and type of faulting (Tanaka et al., 2002; 2003). Other previous studies have seen, at best, a slight correlation of tectonic earthquakes with tidal phase, while most show no significant correlation. The majority of these studies only examine correlation of earthquake origin times with phase, i.e., peaks in tidal stress. Few studies include amplitude, in addition to phase, when examining tidal correlations. In this study, we examine the dependence of earthquake-tide correlation on the amplitude of tidal stress. Tidal stress is resolved into the normal and shear components acting on each of the two possible fault planes. We sort the catalog by the average of the peak stress amplitudes before and after each event. For the 1% of events with the highest averaged peak normal stress we find significant tidal correlation, with a corresponding phase near the peak extensional stress. A slight correlation was seen for shear stress, which tends to have amplitudes approximately half as large as normal stress. Tidal correlation is strongest for reverse faulting, and stronger still in the case that the shallower-dipping fault plane is assumed. Most of the high-tidal-stress events are in subduction zones near coastlines where ocean loading tends to be largest and low-angle thrust events are most common, so our final choice of the shallower-dipping fault plane is justified. We see high significance for the 20 events with highest average peak normal stress amplitude (above 2.7E+04 Pa, or 0.27 bars) and a slightly lower significance for the 40 events with average peak amplitude between 2.7E+04 Pa and 1.6E+04 Pa. These 60 events are all shallow thrust events above 25 km depth. A surprising 85% of the top 20 events and 63% of the next 40 occur in the half of the time with encouraging stress (-90\deg$ to 90\deg$ phase). In addition to the global dataset, 27,464 strike-slip events from California were found to have roughly 1-2% more events during times of encouraging tidal stress of a few hundreths of a bar. We have thus calibrated earthquake triggering over a wide range of tidal stress amplitudes. Tidal stress amplitudes required to strongly trigger earthquakes are similar to limits suggested for static and dynamic triggering of aftershocks of a few tenths of a bar. Tidal stress is static rather than dynamic, so we are better resolving conditions needed for effective trigger of earthquakes.
DE: 7209 Earthquake dynamics and mechanics
DE: 7223 Seismic hazard assessment and prediction
DE: 8164 Stresses--crust and lithosphere
SC: Seismology [S]
MN: 2003 Fall Meeting