HR: 1340h
AN: S13A-1036    [Abstracts]
TI: Significant decrease of b-value of background seismicity prior to the M6.2 Northern Miyagi, Japan earthquake of 26 July 2003
AU: * Tsukakoshi, Y
EM: tsuka@eri.u-tokyo.ac.jp
AF: Earthquake Research Insitute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Shimazaki, K
EM: nikosh@eri.u-tokyo.ac.jp
AF: Earthquake Research Insitute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AB: Statistics of shallow crustal Japanese earthquakes shows that b-value of independent events is not necessarily equal to that of dependent events. The b-value of independent shocks, or background seismicity shows very significant decrease prior to the M6.2 Northern Miyagi earthquake of 26 July 2003. The b-value of the total activity including both dependent and independent shocks in a circular epicentral area with a radius of 30km decreased from 1.2 to 0.7 one year prior to the event. A comparison between models with and without a b-value change of background seismicity lasting one year, which are applied to the shallow crustal earthquakes, indicates that the b-value decrease prior to the Northern Miyagi event is second to the most significant among 3,900 cases. The independent events are stochastically separated from dependent events on the basis of OgataOs ETAS model. ReasenbergOs declustering algorithm gives similar but less significant result. Changes in the b-value in time or space can be clearly recognized by cumulative number vs. cumulative magnitude diagram. If we use the difference between magnitude and the lower threshold magnitude instead of magnitude itself, the slope in the diagram represents the maximum likelihood estimate of b-value.
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting