HR: 1340h
AN: S53A-0185    [Abstracts]
TI: Global Outer-Rise/Near Trench Seismicity and Focal Mechanisms: Trends and Diversity
AU: * KITA, S
EM: kita@aob.geophys.tohoku.ac.jp
AF: Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Aramaki Aza Aoba, Aoba-ku, Sendai, 980-8576 Japan
AU: Nakajima, J
EM: nakajima@aob.geophys.tohoku.ac.jp
AF: Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Aramaki Aza Aoba, Aoba-ku, Sendai, 980-8576 Japan
AU: Hasegawa, A
EM: hasegawa@aob.geophys.tohoku.ac.jp
AF: Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Aramaki Aza Aoba, Aoba-ku, Sendai, 980-8576 Japan
AU: Kirby, S H
EM: skirby@usgs.gov
AF: U.S.Geological Survey, MS 977 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Engdahl, E
EM: engdahl@colorado.edu
AF: Department of Physics, University of Colorado, Campus Box 390 UCB , Boulder, CO 80309-0 United States
AB: Based on well-constrainted hypocenter information and focal mechanism solutions, Seno and Yamanaka (1996) identify two classes of outer-rise/near-trench (OR/NT) earthquakes: 1) Shallow tensional events and 2) Deeper compressional events. They indicate that subduction zones with compressional deep events (DCE) also tend to have double seismic zones (DSZ), and proposed a hypothesis that earthquakes in the lower plane of the DSZ represent reactivation of faults by dehydration embrittlement previously active as DCE_fs. They suggest that plates can be hydrated even at depths as great as 40 km by passing over the superplume volcanic centers. In this study we examine the characteristics of many more earthquakes at the outer-rise/near-trench region using the EHB hypocenter catalogue (Engdahl et al., 2002) and Harvard CMT focal-mechanism. We studied M$>$5.5 OR/NT events in the Circum-Pacific/Indonesian earthquake belts from 1977 to 2002 that occurred at depths shallower than 60 km depth and within 150 km from the trench axis. In order to select trench-outer-rise events, we checked carefully all event locations and CMT solutions in map- and cross-sectional-views superposed on the background seismicity. We also compared event locations with global maps of seafloor bathymetry, gravity and Kawakatsu_fs 1986 investigation of the DSZ in the Tonga subduction zone. Our results are as follows: 1) Eighteen compressional and 93 tensional events were found. 2) Solitary compressional events were found in two areas (Vanuatu and Guam) where a DSZ has not been observed. 3) Thirteen compressional events are concentrated in the Tonga-Kermadec reagion. 4) Tensional events occur at depths of less than about 33 km and the compressional group occurs at greater depths in the Tonga-Kermadec region, corresponding to Kawakatsu_fs upper an lower zones but with opposite focal mechanisms. 5) Many trench-outer-rise events occur where seamount/guyot volcanic chains are subducting. 6) Compressional OR/NT events tend to have mb_es that are larger that those for tensional and interplate events. This implies that DCE events at trench-outer-rise have higher amplitudes at high frequencies.
DE: 7230 Seismicity and seismotectonics
DE: 7200 SEISMOLOGY
DE: 7209 Earthquake dynamics and mechanics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting