HR: 1340h
AN: S53A-0173    [Abstracts]
TI: Seismicity in the Oceanic Crust and the Uppermost Mantle of the Philippine Sea Plate off Kii Peninsula, the Central Nankai Trough Seismogenic Zone
AU: * Obana, K
EM: obanak@jamstec.go.jp
AF: Japan Agency for Marine-Earth Science and Technology, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, 236-0001 Japan
AU: Kodaira, S
EM: kodaira@jamstec.go.jp
AF: Japan Agency for Marine-Earth Science and Technology, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, 236-0001 Japan
AU: Ito, A
EM: iaki@jamstec.go.jp
AF: Japan Agency for Marine-Earth Science and Technology, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, 236-0001 Japan
AU: Fujie, G
EM: fujie@jamstec.go.jp
AF: Japan Agency for Marine-Earth Science and Technology, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, 236-0001 Japan
AU: Kaneda, Y
EM: kaneday@jamstec.go.jp
AF: Japan Agency for Marine-Earth Science and Technology, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, 236-0001 Japan
AB: Along the Nankai trough, southwestern Japan, the Philippine Sea plate (PHS) is subducting beneath the Eurasian plate, and large interplate earthquakes have occurred repeatedly. Recurrence intervals between the earthquakes were about 100-200 years. The latest large thrust earthquake at the eastern Nankai trough off Kii peninsula was the 1944 Tonankai earthquake. A previous ocean bottom seismograph (OBS) experiment showed an absence of the seismicity in the coseismic rupture area of the 1944 Tonankai earthquake and relatively active seismicity near the trough axis. Although the earthquakes near the trough axis were located in the oceanic crust, there were large uncertainties for the hypocenter locations, especially in depth. We conducted a microseismicity observation at the trough axis and the trough landward slope of the eastern Nankai trough off Kii peninsula to obtain accurate locations of the earthquakes near the trough axis and their focal mechanisms. We deployed 30 OBSs with shorter horizontal intervals than those in the previous OBS experiment. Hypocenters were determined using a 3-D seismic velocity structure, which was based on the first-arrival time tomography using the airgun shots as controlled sources. Obtained results show that the most of the earthquakes near the trough axis occurred within the oceanic crust of the PHS and made several clusters. The composite focal mechanisms of the earthquake clusters near the trough axis were strike slip faults with NW-SE direction T-axes. These earthquakes coincide with faults cutting the oceanic crust. The earthquakes in the oceanic crust at the trough axis are considered to indicate the intracrustal deformation of the incoming PHS. On the other hand, the coseismic rupture area of the 1944 Tonankai earthquake is characterized by an absence of the seismicity in the subducting oceanic crust. This may be a result of the uniform coupling between the subducting and overriding plates. Although the seismicity in the oceanic crust is absent in the coseismic rupture area of the 1944 Tonankai earthquake, earthquakes in the uppermost mantle of the subducting PHS were observed beneath the region from the trough axis to the Kii peninsula. The uppermost mantle seismicity may relate to the dehydration of the serpentinized mantle of the PHS.
DE: 7220 Oceanic crust
DE: 7230 Seismicity and seismotectonics
DE: 8150 Plate boundary--general (3040)
SC: Seismology [S]
MN: 2004 AGU Fall Meeting