Earth Planets Space, Vol. 56 (No. 3), pp. 377-382, 2004LETTER
Hiroaki Takahashi1, Shigeru Nakao2, Noritoshi Okazaki3, Junji Koyama4, Takeshi Sagiya5, Takeo Ito5, Fumio Ohya6, Kazutoshi Sato6, Yasuyoshi Fujita6, Manabu Hashimoto6, Yoshinobu Hoso6, Teruyuki Kato2, Takeshi Iinuma2, Jun'ichi Fukuda2, Takeshi Matsushima7, Yuhki Kohno7, and Minoru Kasahara1
1Institute of Seismology and Volcanology, Hokkaido University, Sapporo 060-0810, Japan
2Earthquake Research Institute, University of Tokyo, Tokyo 113-0032, Japan
3Geological Survey of Hokkaido, Sapporo 060-0819, Japan
4Division of Earth and Planetary Sciences, Hokkaido University, Sapporo 060-0810, Japan
5Research Center for Seismology, Volcanology and Disaster Mitigation, Nagoya University, Nagoya 464-8602, Japan
6Disaster Prevention Research Institute, Kyoto University, Uji 611-0011, Japan
7Institute of Seismology and Volcanology, Kyushu University, Shimabara 855-0843, Japan
(Received December 1, 2003; Revised January 30, 2004; Accepted February 10, 2004)
To investigate the postseismic crustal deformation associated with the Tokachi-oki earthquake (MJMA = 8.0) of 26 September 2003 in Japan Standard Time (JST), off southeastern Hokkaido, Japan, we newly established thirty GPS sites just after the mainshock in the eastern part of Hokkaido. Rapid data analysis for one month after the mainshock clearly indicated postseismic displacements only in the horizontal components. Observed maximum horizontal displacement was 6.6 cm from 28 September to 24 October, 2003. Absence of the vertical suggests that afterslip occurred in and around the coseismic fault rather than at downdip extension. Time series of coordinates are characterized by logarithmic decay functions with 4-11 days relaxation times. This suggests that postseismic deformation was due to afterslip on the fault following the large earthquake.
Key words: The 2003 Tokachi-oki earthquake, GPS, postseismic deformation, afterslip.