HR: 17:00h
AN: S52K-05    [PDF]
TI: Spatial Variation of Aftershock Stress Drops in the focal Area of the 2000 Western Tottori Earthquake (Mw=6.6)
AU: * Ito, Y
EM: yito@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1,Ten-no-dai, Tsukuba, 305-0006 Japan
AU: Obara, K
EM: obara@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1,Ten-no-dai, Tsukuba, 305-0006 Japan
AU: Hasegawa, A
EM: hasegawa@bosai.go.jp
AF: Research Center for Prediction of Earthquakes and Volcanic Eruptions Graduate School of Science, Tohoku University, Aoba, Aramaki, Aoba-ku, Sendai, 980-8578 Japan
AU: Yagi, Y
EM: yagi@kenken.go.jp
AF: Building Research Institute, 1,Tachihara, Tsukuba, 305-0802 Japan
AB: Stress drop is one of key parameters for understanding the source of an earthquake. Many researchers have been interested in the distribution of static stress drop on the fault plane of large earthquakes [e.g. Miyatake, 1992; Mikumo and Miyatake, 1995]. However, the relationship between the slip/stress drop distribution on the mainshock fault plane and spatial distribution of aftershock stress drops has not been studied well.\\ We estimated stress drops of 450 aftershocks of the 2000 Western Tottori earthquake using seismograms recorded at nearby NIED Hi-net stations. Observed velocity spectra are first corrected for radiation pattern obtained from focal mechanisms. Then, the optimal corner frequency, seismic moment, and the propagation effect (Q and site) at each station are calculated by grid-search using genetic algorithm by comparing the corrected observed spectrum with the synthetics calculated by BoatwrightOs (1978) method. A stress drop for each aftershock is calculated from its corner frequency and seismic moment according to Brune (1970).\\ We compare the obtained spatial distribution of aftershock stress drops with the slip/stress drop distribution of the mainshock proposed by Mikumo et al (2003). The results show that (1) the large slip and/or high stress drop area (the possible asperity) indicated by Mikumo et al. (2003) is surrounded by aftershocks with high stress drops. And in the contrast, aftershocks that occurred inside the asperity have relatively low stress drops. This maybe explained as that the occurrence of the main-shock released most of the stress concentrated inside the asperity, and caused the stress concentration in the surrounding area; (2) Aftershocks located in the northern part of the aftershock area show lower stress drops relative to those in the southern part. Surface geological studies (Sakamoto et al.,1982, and Teraoka et al., 1996) revealed that the northern part of the aftershock area is dominated by rhyolitic lava and pyroclastic rocks, whereas the southern part by granite. The difference of stress drops between the northern and southern parts may be attributed to the spatial variation of the strength of the rocks.
DE: 7209 Earthquake dynamics and mechanics
DE: 7215 Earthquake parameters
DE: 7230 Seismicity and seismotectonics
SC: Seismology [S]
MN: 2003 Fall Meeting