Earth Planets Space, Vol. 56 (No. 3), pp. 329-334, 2004LETTER
Kazuki Koketsu1, Kazuhito Hikima1, Shin'ichi Miyazaki1,2 and Satoshi Ide3
1Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan
2 Department of Geophysics, Stanford University, 397 Panama Mall, Standford, CA 94305, USA
3 Department of Earth and Planetary Sciences, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
(Received January 6, 2004; Revised March 2, 2004; Accepted March 2, 2004)
The strong motion and geodetic data were individually inverted for the source process of the 2003 Tokachi-oki, Hokkaido, earthquake with a hypocenter 25 km deep and a fault plane above the subducting Pacific slab. Both the results show a simple slip distribution with a single major asperity, but the strong motion inversion may include a trade-off between slip location and rupture time and the geodetic inversion does not have sufficient resolution for far slips. We then carred out a joint inversion of the two datasets in order to overcome these weaknesses of the single dataset inversions. The resultant slip distribution still retains the simple pattern and has a seismic moment of 2.2 1021 N·m (Mw 8.2). The asperity, with a peak slip of 7.1 m, is located in the center of the fault plane 50 km away from the hypocenter in the down-dip direction. The slip rate functions on subfaults around the hypocenter and asperity indicate that the rupture propagated with a supershear speed on the upper part of the fault plane and slowed down to 100-90% of the S-wave velocity on the middle and lower parts. These simple slip patterns and near-supershear rupture may imply the maturity of the Hokkaido subduction zone around the source region.
Key words: Source process, joint inversion, strong motion data, geodetic data.