HR: 09:25h
AN: T11E-06    [Abstracts]
TI: Accretionary prism earthquake along the Nankai Trough, southwest Japan
AU: * Obara, K
EM: obara@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba, Ibaraki, 305-0006 Japan
AU: Ito, Y
EM: yito@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba, Ibaraki, 305-0006 Japan
AB: Recently, some seismic phenomena considered to be relevant to fluid in subduction zone have been discovered in the Nankai trough subduction zone according to the NIED Hi-net, dense seismic observation network in Japan. One is the non-volcanic tremor, which is distributed around the deeper extension to the seismogenic zone on the subducting Philippine Sea plate (Obara, 2002). The other is swarm activity of the very low-frequency (VLF) earthquake on shallower part of the seismogenic zone, close to the trench axis (Obara and Ito, 2005). The waveform of the VLF earthquake is characterized by the absence of higher frequency components and is composed of only a surface wave with a band-limited low-frequency content of around 10 seconds. Therefore, such seismic events are not recognized by routine seismic networks and there is no information about these events in any earthquake catalogs. The seismic event was discovered by Ishihara (2003) and we used the continuous seismogram data of high-sensitivity accelerometer (tiltmeter) installed in every NIED Hi-net station, which covers the whole Japanese Island with a spacing of about 20-30km, in order to detect and locate VLF events in the two years of 2003 and 2004. These VLF earthquakes are usually clustered in some groups spatially and temporally. In the two years we detected at least seven swarms landward of the trough axis. Each seismic swarm usually continues from a few days to weeks. Just after the 2004 off the Kii peninsula earthquakes (M7.2 and M7.5), the swarm activity of the VLF earthquake was excited by the intra-slab major earthquakes and continued for more than two months. The VLF earthquake is predominant in only low-frequency components, so it is easy to distinguish from regular aftershocks, which include both of high-frequency body waves and low-frequency surface wave. For large events, the centroid moment tensor analysis is available to determine the hypocentral location and focal mechanism based on the waveform inversion method. As the result, the source depth is ranging from 0 to 5km and the focal mechanism is the reverse fault type. Considering with the submarine structure, the source of the VLF earthquake is placed inside the accretionary prism. The accretionary prism is composed of sediment materials and the reverse fault system is well developed in the prism. The VLF earthquake is considered as a kind of slow earthquake on the reverse fault plane. The fault plane is regarded as the drain route for fluid liberated from the deeper portion by dehydration process. Therefore, the faulting on the reverse fault plane filled with fluid or unconsolidated materials should be characterized by low stress drop and predominant in low-frequency components seismogram. The VLF swarm activity might be a triggered phenomenon related to the diffusion or flow of fluid inside the accretionary prism.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7255 Surface waves and free oscillations
DE: 8045 Role of fluids
DE: 8150 Plate boundary: general (3040)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: Fall Meeting 2005