3D P-wave velocity structure around the rupture area of the 2003 Tokachi-oki earthquake (Mw=8.0) in the margin of the Kuril trench

We determined the three dimensional P-wave velocity structure in the rupture area of the 2003 Tokachi-oki earthquake (Mw=8.0) by using P-wave travel time data of the aftershocks and relocated the hypocenters simultaneously. In the area off the southeastern coast of Hokkaido, Japan, the large earthquakes occurred repeatedly because the Pacific plate is subducting beneath Hokkaido island in the North American plate. After Yamanaka and Kikuchi (2003), the 2003 Tokachi-oki earthquake re-ruptured the same asperity on September 26, 2003, as the 1952 Tokachi-oki earthquake (Mw=8.1). In order to investigate the aftershock activity in the rupture area, we deployed 47 Ocean Bottom Seismometers (OBSs) immediately after the main earthquake. By using the travel time data from the OBS network, the accurate aftershock distribution was obtained [Shinohara et al (2004), Yamada et al (2005)]. In this study, we picked P arrival times of the aftershocks selected carefully as the best data set for a seismic tomographic inversion. And then we estimated 3D P-wave velocity structure by applying the P travel time data to the simultaneous inversion method [Thurber (1983, 1993), Eberhart-Phillips (1986, 1990)]. Although we couldn't image the overall structure of the subducting Pacific plate because of the shallow seismicity, the subducting plate is imaged as velocity anomalies dipping toward northwest. It is suggested from the result of the hypocenters relocated in the consequence of the inversion that the dip angle of the subducting plate increases at around the distance of 100km from the trench axis. It is also suggested that the dip angle is larger at the northeastern part than the southwestern part of the study area. The fault slip distribution of the 2003 Tokachi-oki earthquake seems to be unrelated to the P-wave velocity (Vp) structure obtained here. In the meantime, S-wave velocity (Vs) structure was not obtained because of the ambiguous S-wave forms and the undefined S-wave arrival times due to the surficial unconsolidated sedimentary layers at the stations. Vp/Vs structure in the rupture area is considered to be related to the source process of the large earthquake. In order to obtain Vp/Vs structure, we need to merge the arrival data of the OBSs with those of the land stations to lessen influence of sedimentary layers.