Performance of the hybrid method for waveform and travel-time analyses based on a comparison with the hypocenters calculated from the ocean-bottom-seismometer network

HR: 14:25h
AN: S13F-04 [Abstracts]
TI: Performance of the hybrid method for waveform and travel-time analyses based on a comparison with the hypocenters calculated from the ocean-bottom-seismometer network
AU: * Ito, Y
EM: yito@aob.geophys.tohoku.ac.jp
AF: Graduate school of Science, Tohoku University, 6-6, Aramaki-Aza-Aoba, Aoba-ku, Sendai, 980-8578, Japan
AU: * Ito, Y
EM: yito@aob.geophys.tohoku.ac.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tenno-dai, Tsukuba, 305-0006, Japan
AU: Yamamoto, Y
EM: yyama@aob.geophys.tohoku.ac.jp
AF: Graduate school of Science, Tohoku University, 6-6, Aramaki-Aza-Aoba, Aoba-ku, Sendai, 980-8578, Japan
AU: Suzuki, K
EM: suzuki@aob.geophys.tohoku.ac.jp
AF: Graduate school of Science, Tohoku University, 6-6, Aramaki-Aza-Aoba, Aoba-ku, Sendai, 980-8578, Japan
AU: Hino, R
EM: hino@aob.geophys.tohoku.ac.jp
AF: Graduate school of Science, Tohoku University, 6-6, Aramaki-Aza-Aoba, Aoba-ku, Sendai, 980-8578, Japan
AU: Matsubara, M
EM: mkmatsu@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tenno-dai, Tsukuba, 305-0006, Japan
AU: Obara, K
EM: obara@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tenno-dai, Tsukuba, 305-0006, Japan
AB: The locations and focal mechanisms of earthquakes provide basic information for understanding the features of earthquakes and their focal areas such as faulting, fault orientation, stress fields, etc. Ito et al. (GJI, 2006) estimated the accuracy of the centroid locations calculated by the regional CMT inversion method, in which the highly dense broadband seismic network in Japan was used. The accuracy in the horizontal and vertical directions was approximately 5 km when the seismic stations were located around the focal area. However, the accuracy of the horizontal distribution decreased by more than half when the distribution of the seismic stations was restricted azimuthally, i.e., for an analysis of offshore earthquakes. It should be noted that the depth accuracy did not decrease when the stations were restricted. In this paper, we first develop a new method for investigating the hypocenter and focal mechanism of offshore earthquakes occurring at a distance from the landward seismic network. Next, we apply this method to small or intermediate earthquakes occurring in the subduction zone along the Japan Trench. Finally, we discuss the reliability of our method by comparing them with accurate hypocenters obtained by the ocean-bottom-seismometer (OBS) network.
In our method, we determine a hypocenter and focal mechanism by using both the observed waveform and the travel times of P and S waves; this method is henceforth referred to as the hybrid method. First, we calculate the focal depth and focal mechanism at an initial epicenter by the moment tensor inversion method using the waveforms observed at the National Research Institute for Earth Science and Disaster Prevention (NIED) F-net broadband and Hi-net tiltmeter networks. Next, we fix the focal depth calculated by the moment tensor inversion method and determine the epicenter based on the travel times by using the least square method. With regard to the hybrid method, we repeatedly used the moment tensor inversion method and the least square method.
After 2002, we have repeatedly deployed the OBS network in the Miyagi-oki region, which is located on the landward slope along the middle part of the Japan Trench. In this region, interplate earthquakes with seismic magnitudes exceeding 7 repeatedly occur every 40 years. An interplate earthquake with a magnitude of 7.2 occurred in 2005. Using the data obtained from the OBS network, Hino et al. (EPS, 2007) accurately calculated the hypocenters of aftershocks. Here, we compare the hypocenters obtained by the hybrid method with those calculated by Hino et al. (EPS,2007); they are found to be consistent, and the accuracy in the horizontal and vertical directions is within approximately 6 km. One of the nodal planes of the aftershocks occurring near the plate interface exhibits a high consistency with the slope of the subducting plate. By using the bootstrapping method, the estimated depth error in the hybrid method is found to be approximately 3 km, and 10 degrees for fault parameters such as strike, dip, and rake, which are calculated from the moment solutions assuming a pure double-couple source. These results show the validity of the hybrid method for calculating the hypocenters and focal mechanisms of offshore earthquakes.
DE: 7200 SEISMOLOGY
DE: 7203 Body waves
DE: 7215 Earthquake source observations (1240)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 7255 Surface waves and free oscillations
SC: Seismology [S]
MN: 2007 Fall Meeting