HR: 16:00h
AN: S24A-01    [Abstracts]
TI: Triggered Events due to the 2003 Tokachi-oki Earthquake (Mw 8.1), Japan
AU: * Miyazawa, M
EM: linen@rcep.dpri.kyoto-u.ac.jp
AF: Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto, 611-0011 Japan
AU: Mori, J J
EM: mori@rcep.dpri.kyoto-u.ac.jp
AF: Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto, 611-0011 Japan
AB: The 2003 Tokachi-oki earthquake, (Mw 8.1), one of the largest recent earthquakes in Japan, affected the seismicity of small shallow earthquakes within a few hundred kilometers and remotely triggered deep low frequency tremors at farther distances of 1000 km or more. The earthquake occurred offshore of southeast Hokkaido, where the Pacific plate subducts beneath the North American plate and large earthquakes have recurrently occurred. After the main event in 2003, shallow seismicity increased in the central area and decreased along the western coast of the Hokkaido region. The changes of seismicity could be explained by Coulomb failure stress changes ($\Delta CFF$). In the central area, where active volcanoes are located, $\Delta CFF$ values were 0.1--0.2 MPa if triggered earthquakes had mechanisms consisting of NW-SE compression on right lateral strike-slip faults. Off the western coast, seismicity in the aftershock area of the 1993 south-western Hokkaido earthquake (Mw 7.6) decreased, where $\Delta CFF$ had negative values of about -0.005 MPa for the thrust mechanism of the aftershocks. In order to detect the dynamically triggered events due to the seismic waves, we used continuous velocity waveform data at 642 stations of the High Sensitivity Seismograph Network (Hi-net) that covers most of Japan. The waveform data were filtered with pass-bands of 5--20 Hz so that the local signals in the vicinity of the observation stations could be detected. We constructed RMS envelopes of the filtered waveforms for three components. Two statistical parameters were adapted to quantitatively evaluate changes of amplitudes of 1000 sec envelopes before and after arrivals of the body waves. We examined the commonly used $z$-value and $\beta$-value to see the variant seismicity, substituting the amplitude of the envelope for the earthquake frequency. These obtained values indicated increases of the amplitude in three regions of western Japan, where the travel distances were 1000 km or more. The increased tremors are thought to be deep low frequency events, which occur at depths of 30--60 km near the subducting Philippine Sea plate. The transient stress perturbations in the furthest region were on the order of $10^{-3}$ MPa or more.
DE: 7230 Seismicity and seismotectonics
DE: 1734 Seismology
SC: Seismology [S]
MN: 2004 AGU Fall Meeting