HR: 11:20h
AN: S22A-05    [Abstracts]
TI: Anomalous Intraslab Seismicity Beneath Hokkaido, Japan, Estimated From Double-Difference Hypocenter Locations
AU: * Kita, S
EM: kita@aob.geophys.tohoku.ac.jp
AF: Graduate School of Science, Tohoku University, Aza Aoba, Aramaki,Aoba-ku, Sendai, 980-8578 Japan
AU: Okada, T
EM: okada@aob.geophys.tohoku.ac.jp
AF: Graduate School of Science, Tohoku University, Aza Aoba, Aramaki,Aoba-ku, Sendai, 980-8578 Japan
AU: Nakajima, J
EM: nakajima@aob.geophys.tohoku.ac.jp
AF: Graduate School of Science, Tohoku University, Aza Aoba, Aramaki,Aoba-ku, Sendai, 980-8578 Japan
AU: Matsuzawa, T
EM: matuzawa@aob.geophys.tohoku.ac.jp
AF: Graduate School of Science, Tohoku University, Aza Aoba, Aramaki,Aoba-ku, Sendai, 980-8578 Japan
AU: Suganomata, J
EM: suga@aob.geophys.tohoku.ac.jp
AF: Graduate School of Science, Tohoku University, Aza Aoba, Aramaki,Aoba-ku, Sendai, 980-8578 Japan
AU: Hasegawa, A
EM: hasegawa@aob.geophys.tohoku.ac.jp
AF: Graduate School of Science, Tohoku University, Aza Aoba, Aramaki,Aoba-ku, Sendai, 980-8578 Japan
AU: Kirby, S H
EM: skirby@usgs.gov
AF: Graduate School of Science, Tohoku University, Aza Aoba, Aramaki,Aoba-ku, Sendai, 980-8578 Japan
AU: Kirby, S H
EM: skirby@usgs.gov
AF: US.Geological Survey, 345 Middlefield Road MS 977, Menlo Park, CA 94025-3591 United States
AB: 1. Introduction Generation process of intraslab earthquakes is one of important problems to be solved in seismology. In many subduction zones, intraslab earthquakes at 50-200km depth form the double seismic zone(e.q., Hasegawa et al., 1978). Dehydration embrittlement of the metamorphosed oceanic crust and oceanic mantle may be responsible for the occurrence of events in the double seismic zone(Kirby et al, 1996; Seno and Yamanaka, 1996; Peacock, 2001). Recent some large intraslab earthquakes ruptured off the usual seismic planes (e.g. the 1993 Kushiro-Oki earthquake (Ide and Takeo, 1996); the 2003 Miyagi-Oki earthquake (Sakoda et al., 2004) and the 2001 Geiyo earthquake (Suganomata et al., 2005b)). These large intraslab earthquakes may be caused by the reactivation of large hydrated faults within the subducting slabs. In the focal area of the 2003 Miyagi-Oki earthquake, anomalous seismicity between the upper and lower planes of the double seismic zone also had occurred before the earthquake (Sakoda et al., 2004; Suganomata et al., 2005a). This anomalous seismicity may be also caused by reactivation of the faults that are distributed within the subducting slab. Therefore the seismicity in the regions off the seismic planes may be a clue to understand the cause of intraslab earthquake. In this study we relocated microearthquakes and detect anomalous seismicity within the Pacific plate slab beneath Hokkaido, Japan. 2. Data and method We relocated events (0<M<9) in the JMA earthquake catalog at depths of 20-300km for the period from January 2002 to August 2005. Hypocenter parameters and arrival time data in the JMA catalog are used as the initial hypocenters and data for the relocation. We adopted the double-difference hypocenter location method developed by Waldhauser and Ellsworth (2000). 3. Results -Anomalous seismicity beneath Hokkaido- The relocation result is as follow : 1) The result shows some anomalous hypocenters that distributed between the upper and lower planes of the double seismic zone. This anomalous seismicity might indicate the location of highly hydrated mantle. One type of anomalous hypocenters is a horizontal hypocenter alignment, and the other is hypocenters aligned parallel to the double seismic zone. These anomalous seismicities are found particularly in eastern Hokkaido. The horizontal hypocenter alignment in Kushiro area is located at depths of about 100km, which corresponds to the aftershock distribution of the 1993 Kushiro-Oki earthquake. The inclined hypocenter alignments parallel to the double seismic zone area distributed at the middle of the upper and lower planes at depths of 70-100km. They are found at two locations in eastern Hokkaido. 2) Seismicity of upper and lower planes in the region where anomalous seismicity is observed is also active. 3) These anomalous seismicty is located along the deeper extension of the sea mounts alignment.
DE: 7200 SEISMOLOGY
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7220 Oceanic crust
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7240 Subduction zones (1207, 1219, 1240)
SC: Seismology [S]
MN: Fall Meeting 2005