HR: 14:25h
AN: S53C-04    [Abstracts]
TI: Application of the Coherence Collapsing relocation method to the aftershocks of 1995 Kobe earthquake
AU: * Asanuma, H
EM: asanuma@ni2.kankyo.tohoku.ac.jp
AF: Graduate School of Environmental Studies, Tohoku University, 6-6-20 Aramaki-aza-aoba, Aoba-ku, Sendai, 980-8579 Japan
AU: Izumi, T
EM: none
AF: Graduate School of Environmental Studies, Tohoku University, 6-6-20 Aramaki-aza-aoba, Aoba-ku, Sendai, 980-8579 Japan
AU: Hotta, A
EM: hotta@@ni2.kankyo.tohoku.ac.jp
AF: Graduate School of Environmental Studies, Tohoku University, 6-6-20 Aramaki-aza-aoba, Aoba-ku, Sendai, 980-8579 Japan
AU: Kato, A
EM: akato@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Niitsuma, H
EM: ni@ni2.kankyo.tohoku.ac.jp
AF: Graduate School of Environmental Studies, Tohoku University, 6-6-20 Aramaki-aza-aoba, Aoba-ku, Sendai, 980-8579 Japan
AU: Hirata, N
EM: hirata@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AB: The authors have developed a "Coherence Collapsing" method, which is a method for re-locating seismicity utilizing the spatial distribution of the residuals and the coherency of all the events in the data set (Asanuma et al., Joint Assembly, 2005). The Coherence Collapsing method aims to selectively relocate a group of multiplets to a common point and tries to optimize the whole error distribution. This differs from the original Collapsing method (Jones and Stewart, JGR 1997) which simply tries to reduce the spatial variation of the location by statistical processing. Because the inputs of the Coherence Collapsing method are the hypocenter and residual obtained by JHD, and correlation table of all the events at one station, the re-location procedure is much simpler than the double differential method and the other techniques where coherence of the events is used. Hence, this method can be used in semi-realtime to refine the locations of multiplets and seismic structure in large scale. The Coherence Collapsing method has been applied to induced microseismicity from gas/geothermal reservoir development where multiplets are dominantly observed, and the structure and dynamic behaviour of the fracture system have been successfully interpreted. We have also applied the Coherence Collapsing method to the 5000 aftershock events of the Kobe earthquake in 1995 collected by Hirata, et al. (JPE, 1996). Absolute picks of P and S waves are used as input to JHD and seismic traces from one of the stations that gave the highest signal-to-noise ratio were used to calculate the coherence table. The coherency among these events was relatively lower than the induced microseismicity from engineered reservoirs, suggesting a wider variation of source mechanisms of the aftershocks. The clusters of seismicity with higher mutual coherency were re-located to the edge of the aftershock zone by the Coherence Collapsing method consistent with the zones where the Poisson's ratio was reduced after the mainshock (Zhao and Negishi, JGR, 1998). Time series analysis of the magnitude did not show any obvious trend that may correlate to the postseismic fault slip. We also have not found distinctive source mechanisms of the coherent events, but the location of these events suggests that they are correlated to the boundary of the aftershock zone and may be related to existence of pore fluid.
DE: 7200 SEISMOLOGY
DE: 7219 Seismic monitoring and test-ban treaty verification
DE: 7299 General or miscellaneous
SC: Seismology [S]
MN: Fall Meeting 2005