HR: 09:30h
AN: T51A-04    [Abstracts]
TI: Seismic Behavior of OBS Data Recorded in the Southern Okinawa Trough
AU: * Chen, A T
EM: achen@mail.ntou.edu.tw
AB: A network of four ocean bottom seismographs (OBSs) deployed in the southern Okinawa Trough for 4 weeks were studied the seismic behavior of the regional earthquake events. We combined the OBS data with available land data in Taiwan to relocate the hypocenters with different models and picked the result with the overall minimum RMS residual for analysis. The locations determined with OBS data incorporated improve the locating quality and hypocenters are shifted toward OBS network while their foci are deepened. It implies that offshore seismicity based on land network data alone may be mislocated with shorter epicentral distances and shallower depths. Factors such as azimuth, depth, distance and site effect seem to affect the frequency contents of OBS signatures. Signals propagated along the axis of the Okinawa Trough generally show lower frequencies. Distance and depth affect the frequency content but not so prominently as does due to the azimuth factor. Many seismic signals missed at OBS1 at the north side of the trough axis are possible due to the faulted walls and fractured crust associated with the rifting which induces a shadow zone to prevent seismic waves from arriving at the station. Some of the remarkable low frequency signals might be caused by the magma chambers beneath the trough such that the shear waves even disappeared in most seismograms observed at OBS2. The microearthquakes exhibit two types of focal mechanisms: the rifting and the subducting types. Focal mechanisms of deeper events from the subducting slab are thrusts with maximum principal axes orientated NW-SE, consistent with the direction of the convergent plate motion between the Eurasia and the Philippine Sea plates. Shallow events within the rifting trough possess the normal faulting mechanisms related to the back arc spreading.
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 7230 Seismicity and seismotectonics
DE: 8150 Plate boundary--general (3040)
DE: 9320 Asia
SC: Tectonophysics [T]
MN: 2005 Joint Assembly