HR: 0800h
AN: S51B-1002    [Abstracts]
TI: An Improved Double-Difference Earthquake Location Algorithm using sP Phases: Application to the 2004 Earthquakes Offshore of the Kii Peninsula
AU: * Bai, L
EM: bai@rcep.dpri.kyoto-u.ac.jp
AF: RCEP, DPRI, Kyoto Univ., Gokasho, Uji, Kyoto, Japan, Kyoto, 611-0011 Japan
AU: * Bai, L
EM: bai@rcep.dpri.kyoto-u.ac.jp
AF: Institute of Geophisics, China earthquake administration, No. 5, Minzu Xueyuan Nanlu, Haidian District, Beijing, Beijing, 100081 China
AU: Kawasaki, I
EM: kawasaki@rcep.dpri.kyoto-u.ac.jp
AF: RCEP, DPRI, Kyoto Univ., Gokasho, Uji, Kyoto, Japan, Kyoto, 611-0011 Japan
AU: Zhang, T
EM: zhangtz45@163.com
AF: Institute of Geophisics, China earthquake administration, No. 5, Minzu Xueyuan Nanlu, Haidian District, Beijing, Beijing, 100081 China
AU: Ishikawa, Y
EM: catfish@wa2.so-net.ne.jp
AF: Matsushiro, Japan meteorological Agency, Nishijyou 3511, Matsushiro tyou, Nagano city, Nagano, 3811232 Japan
AB: The Philippine Sea plate (PHS) is subducting beneath southwest Japan at the Nankai trough, where great earthquakes are expected to occur within 30-50 years. On September 5, 2004, an Mw 7.5 earthquake occurred offshore southeast of the Kii peninsula with significant foreshock and aftershock sequences. The earthquakes were more than 100 km from the onshore seismic network and thus their focal depths were poorly constrained and cannot related to the oceanic crustal and uppermost mantle structures. Clear later phases were observed about 7-11 seconds after the initial P phases with almost the same apparent velocity as for the P waves. Computation of ray-paths using the 2D ray-tracing program of Zelt and Smith (1992) shows that the arrivals times of the sP phases are sensitive to focal depths. A double-difference earthquake location algorithm (Waldhauser and Ellsworth, 2002) eliminates the errors introduced by crustal heterogeneity. Thus, our strategy to accurately determine the depths is to combine the use of the sP phases in the double-difference earthquake location algorithm. There were 547 earthquakes with magnitude greater than 3.0 during the period of Sep. 5 to Sep. 30, 2004 listed in the Japan Meteorological Agency (JMA) catalog. Among those events, 36 earthquakes have sP phases that can be identified on seismograms at more than six stations of the Japanese High Sensitivity Seismograph Network (Hi-net). These events are relocated using the improved double-difference earthquake location algorithm with sP phases. Fixing the 36 earthquakes as reference events, other earthquakes, including the foreshock, mainshock and two large aftershocks are relocated from a double-difference cluster analysis based on the P- and S-wave arrival times of the JMA catalog. Most of relocated hypocenters range between 10 and 35 km in focal depth, which is about 15 km shallower than JMA hypocenters and about 10 km deeper than hypocenters determined from Ocean Bottom Seismometers (OBS). The resolution is ~0.36 km in the east-west direction, ~0.63 km in the north-south direction, and ~1.26 km in depth. Our results are consistent with apparent velocity observed at Matsuhiro observatory, and a velocity model with an effective elastic thickness for the PHS estimated from topography. Two great earthquakes occurred offshore of southwest Japan in the past decades: the Tonankai Earthquake (M7.9) on December 7, 1944, and the Nankai Earthquake (M8.0) on December 21, 1946. Microearthquakes are not active in the areas of the starting points of the ruptures of the Nankai and Tonankai earthquakes, while they tend to take place close to the deformation front of the Nankai trough. The 2004 earthquakes offshore of the Kii peninsula occurred within the PHS slab beneath the trench-outer rise region, where the occurrence of compressional earthquake suggests the potential for future large underthrusting earthquakes.
DE: 1242 Seismic cycle related deformations (6924, 7209, 7223, 7230)
DE: 7220 Oceanic crust
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 9320 Asia
SC: Seismology [S]
MN: Fall Meeting 2005