HR: 0800h
AN: S21A-0198 INVITED     [Abstracts]
TI: Insights on the 1855 Ansei-Edo earthquake depth derived from attenuation tomography and comparison with the 23 July 2005 M=6 shock
AU: * Nakamura, R
EM: naka@tepsco.co.jp
AF: Tokyo Electric Services Co., Ltd., 3-3-3, Higashi Ueno, Taito-Ku, Tokyo, 110-0015 Japan
AB: The 3-D attenuation structure, Q, beneath the Kanto plain (Nakamura et al, 2003), Japan, has been obtained using short-period strong ground motion records from Kanto and Tohoku. Low Q is consistent with the Vp/Vs structure, and is associated with the active volcanic front (Nakajima et al, 2001). In addition, there is an unexpected low-Q zone oriented E-W beneath the Kanto plain. This low-Q band is coincident with high Poisson ratios, which have been interpreted as a serpentinized zone (Kamiya and Kobayashi, 2000). The heterogeneous attenuation structure must affect seismic ground motions for large events such as the damaging 1855 Ansei-Edo earthquake. First, we compare the distribution of seismic intensity between recent earthquakes and the Ansei-Edo shock. Despite the large and high quality intensity dataset for the 1855 shock, there are many difficulties in estimating its hypocenter. It is possible, for example, that different hypocenters produce the same seismic intensity distribution because of the complex attenuation structure. Considering these difficulties, we examine the ground motion of the Ansei-Edo and M=6 23 July 2005 earthquakes, and find out that the two intensity distributions are very similar. Second, we calculate the predicted ground motions using our 3-D Q structure for several trial 1855 hypocenters, principally in an attempt to infer whether the 1855 source was deep or shallow. Because of the observed high intensities characteristic of Kisarazu and the eastern Kanagawa prefecture shared by the Ansei-Edo and 23 July 2005 shocks, and the good match of these observations to the prediction using the 3-D Q structure, we infer a similar characteristic intensity distribution. We thus conclude that the 1855 Ansei-Edo earthquake may have occurred at the north end of Tokyo bay at a depth of about 70-80 km.
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7212 Earthquake ground motions and engineering seismology
DE: 7221 Paleoseismology (8036)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 7270 Tomography (6982, 8180)
SC: Seismology [S]
MN: Fall Meeting 2005