HR: 14:40h
AN: T13F-05    [Abstracts]
TI: Source duration of deep very low frequency earthquakes in the western Shikoku region
AU: * Matsuzawa, T
EM: tkmatsu@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba, Ibaraki, 305-0006, 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: Maeda, T
EM: maeda@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba, Ibaraki, 305-0006, Japan
AB: Recent studies (e.g. Obara, 2002; Rogers and Dragert, 2003) report various types of "slow" events (tremors, low frequency earthquakes, very low frequency earthquakes and slow slips) at the subduction zones. Source duration of such events follows a scaling relationship different from the regular earthquakes (Ide et al., 2007). Ito et al. (2007) found very low frequency (VLF) earthquakes at the subduction zone of the Philippine Sea plate. Assuming that the source duration is sufficiently shorter than the frequency band of 0.02-0.05 Hz, they determined the hypocenters, the seismic moments, and the focal mechanisms by the centroid moment tensor inversion (CMTI) approach. However, the actual source duration of VLF earthquakes is still unclear. In this study, we estimated the source duration of VLF earthquakes, using the seismic data of the high-sensitivity seismograph network (Hi-net) which is operated by National Research Institute for Earth Science and Disaster Prevention. In the middle of March 2007, deep low frequency tremors became very active associated with a slow slip event in the western Shikoku region. Applying the CMTI approach, we determined the hypocenters and the focal mechanisms of VLF earthquakes during this period. Obtained focal mechanisms are thrust type according to the geometry of the subducting plate boundary. The hypocenters of VLF earthquakes migrated from the north to the south. Mw 3.8 VLF earthquake at 19:29 on March 14 (UT) is the largest event that we and Ito et al. (2007) have ever detected. At this event, body waves are clearly recognized in the seismograms of the high-sensitivity horizontal accelerometer (Hi-net TILT) which is installed at each Hi-net station. We estimate the source duration of this event through the comparison between the observed and the synthetic waveforms. The synthetic waveforms are directly calculated if a source time function is given, because the hypocenter, the seismic moment, and the focal mechanism of this event are already obtained by the CMTI analysis. We assume that the source time function is expressed by a linear B-spline basis function whose seismic moment is Mw 3.8. Employing the forward modeling approach, we found that 12 s of source duration explains the observed displacement waves well. We note that the source duration is longer than the S-P time, because the all stations are located within 80 km from the epicenter. Such long duration smoothes out the contribution from the P-wave, S-wave, and near field terms. We also compared the waveform of several smaller VLF earthquakes (Mw 3.4-3.5) in the western Shikoku region, and the source durations of around 10 s are suitable to fit the waveforms, though the estimations for such small events are less reliable than the case of Mw 3.8 events. Our analysis also supports the previous results of CMTI approach in terms of direct body waves, and shows that the source duration of M3 "slow" events is around 10 s. This duration is several tens times longer than that of the regular M3 earthquakes.
DE: 7203 Body waves
DE: 7215 Earthquake source observations (1240)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting