HR: 1340h
AN: S53A-0174    [Abstracts]
TI: Seismic activity of very low-frequency earthquake on the subducting Philippine Sea plate near 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, 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, 305-0006 Japan
AB: The Nankai trough subduction zone in southwest Japan is characterized by some kinds of _gslow earthquake_h. Around the deep side of the seismogenic zone on the subducting Philippine Sea plate, non-volcanic tremor is distributed in a narrow belt along the strike of the plate (Obara, 2002). On the other hand, on shallower parts of the seismogenic zone, an anomalous seismic activity has been detected. The waveform of the earthquake is characterized by a band-limited low-frequency content of between 10 and 20 seconds. We call the earthquake _gvery low-frequency (VLF) earthquake_h. The filtered seismogram with a passband of 10 to 100 seconds periods operated on the output from high-sensitivity accelerometer (tiltmeter) installed in every NIED Hi-net station is used for detection and location analysis. The waveform of the VLF earthquake looks like to that of teleseismic event, however the spatial pattern of the amplitude and the apparent velocity of the VLF earthquake are quite different from those of teleseismic events. Because waveforms of the VLF earthquake are quite similar in neighbor stations, epicenters are estimated by using a cross correlation analysis. The Hi-net stations in southwest Japan are divided into some groups with a diameter of about 100km. The cross correlation is calculated for every pair of stations in each group in order to measure the time lag which gives the highest cross correlation coefficient. Then, the set of time lags obtained in each group are used to estimate the propagation direction and the apparent velocity. Finally, the epicenter of the VLF earthquake is estimated by focusing the back projection of the ray propagation calculated with good resolution in each group. In the year of 2003, there are two active clusters of the VLF event near the Nankai trough; off Cape Muroto and the Hyuga-nada region. The VLF seismic activity usually lasts for a month in each cluster. Both activities are located on the seaward updip portion of the seismogenic zone on the subducting Philippine Sea plate. Just after the occurrence of 2003 Tokachi Earthquake in northeast Japan on September 26, 2003, the VLF seismic activity in the eastern part of Hyuga-nada region became very active and continued for a month. Considering the reverse fault type mechanism and shallow depth estimated by the centroid moment tensor (CMT) analysis, the VLF earthquakes might occur in the accretionary prism or on the decollement. Moreover, both active clusters correspond to the extension of the chain-like sea mounts on the ocean floor of the Philippine Sea. Therefore, the occurrence of the VLF earthquake might be related to the existence of the subducting sea mount. At present, there is no clear relationship between the episodic tremor and the VLF earthquake activity in their time histories. However, both seismic phenomena are located on transition zones between the locked zone and decoupled aseismic zones at the deeper part and shallower part on the subducting plate boundary. Therefore, these low-frequency families must be representing the subduction process of the young oceanic plate.
DE: 7230 Seismicity and seismotectonics
DE: 7255 Surface waves and free oscillations
DE: 8123 Dynamics, seismotectonics
DE: 8150 Plate boundary--general (3040)
SC: Seismology [S]
MN: 2004 AGU Fall Meeting