HR: 11:50h
AN: G12A-07    [Abstracts]
TI: Postseismic Deformations Following The 2003 Tokachi-oki Earthquake Detected By Dense GPS Observation
AU: Takahashi, H
EM: hiroaki@eos.hokudai.ac.jp
AF: Hokkaido Univ., Sapporo, Hokkaido, 060-0810 Japan
AU: Kato, T
EM: teru@eri.u-tokyo.ac.jp
AF: ERI, Univ. Tokyo, Tokyo, Tokyo, 113-0032 Japan
AU: Sagiya, T
EM: sagiya@seis.nagoya-u.ac.jp
AF: Nagoya Univ., Nagoya, Aichi, 464-8602 Japan
AU: * Hashimoto, M
EM: hasimoto@rcep.dpri.kyoto-u.ac.jp
AF: DPRI, Kyoto Univ., Uji, Kyoto, 611-0011 Japan
AU: Matsushima, T
EM: mat@sevo.kyushu-u.ac.jp
AF: Kyushu Univ., Shimabara, Nagasaki, 855-0843 Japan
AU: Kasahara, M
EM: mkasa@eos.hokudai.ac.jp
AF: Hokkaido Univ., Sapporo, Hokkaido, 060-0810 Japan
AB: A Mjma=8.0 earthquake hit Hokkaido, northern Japan, on September 26, 2003 (JST). This is the first great interplate earthquake since the deployment of the nation-wide continuous GPS observation network in Japan. Up to 1m coseismic displacements were detected in the southeastern Hokkaido by this network. Since there may be a seismic gap or unbroken asperity on the plate interface east of the source region of this earthquake, how released stress is redistributed is crucial. It is necessary to monitor the postseismic deformations as well as seismicity. Postseismic deformations can provide us with frictional characteristics of plate interface, which is critically important to the physics of earthquake. Postseismic movements have been observed by the nation-wide network as well. However this network is a bit sparser in Hokkaido than in other areas of Japan. We can not expect enough resolving power of slip distribution on plate interface, especially its deeper extension. Therefore, we newly established GPS sites just after the mainshock in the eastern part of Hokkaido to fill gaps of the network as a collaborative work of researchers from Japanese universities. Observation at some sites started on September 28. Thirty GPS sites with dual frequency receivers were established by the end of October in the region of about 250 km x 150 km in southeastern Hokkaido. Some sites are telemetrically monitored. Data are stored in receiver or PC and collected periodically at the rest of them. All the data are archived in the server of Hokkaido University. We have analyzed the data till March, 2004, by using Bernese 4.2 software, and obtained time series of coordinates of our temporary sites and GEONET sites. It is confirmed that postseismic deformation still continues 6 months after the main shock. Most sites moved SE-ward and their magnitude decay according to the distance from the epicenter. We found rate changes in mid-Oct. at some sites and reverse of motion in eastern Hokkaido around the New Year day. These characteristics in temporal variation in coordinates can be explained well by a logarithmic decaying function based on the rate and state-dependent friction law. Some sites suffered from snow cover and showed anomalous motion in winter. We tried inversion of the observed displacements to estimate spatio-temporal variation in afterslip on the interface between Pacific and North American plates. We divide whole observation period into three according to temporal changes. We can find large afterslip, which amounts to ~50cm for the first month, in the source region during all three period, but their magnitude is decaying according to time. So far we have not resolved separation of area of afterslip and the main shock. Afterslip in deeper part of plate interface seems to have died out a month after the main shock. In the eastern part, interplate coupling dominates during the whole period and afterslip may not have proceeded into this region.
DE: 8123 Dynamics, seismotectonics
DE: 7209 Earthquake dynamics and mechanics
DE: 1206 Crustal movements--interplate (8155)
DE: 1242 Seismic deformations (7205)
DE: 1243 Space geodetic surveys
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting