HR: 0830h
AN: S21F-0382    [PDF]
TI: Seismogenic Layer and Seismic Reflectors in the Crust With Reference to Large Earthquakes
AU: * Ito, K
EM: ito@rcep.dpri.kyoto-u.ac.jp
AU: Ueno, T
EM: ueno@rcep.dpri.kyoto-u.ac.jp
AU: Yoshii, K
EM: yoshii@rcep.dpri.kyoto-u.ac.jp
AB: Most of large crustal earthquakes break out at the base of the seismogenic layer. Therefore, the heterogeneous structures, such as seismic velocity difference and/or seismic reflectors near the cutoff depth of seismicity are important to reveal the process of the large inland earthquakes. Seismic surveys in the source region of the 2000 Western Tottori earthquake of M7.2 (JMA-scale) in southwest Japan, show reflectors near the base of the seismogenic layer (Nishida et al., 2002) . Besides, high velocity patches are found nearly the same region of the aftershock area by using seismic tomography from data of a dense network observation of aftershocks just after the main shock (Shibutani et al., 2002).We analyzed seismic refraction and wide angle reflection data in southwest Japan to examine the lateral extension of the reflectors with relation to the cutoff depth of seismicity. As a result, we found reflectors at about 4-5s, 7-8s and 10-12s in two-way travel times along four measure lines. One of them passes the aftershock are of the 1995 Kobe earthquake. The 4-5s reflectors correspond roughly to the cutoff depth of seismicity at about 12-15km deep. The amplitudes of the reflected waves vary from place to place, though the reflectors exist in all areas in inland area of southwest Japan, where local seismicity of small earthquakes are relatively high. The 7-8s reflector is located in the middle crust and the 10s reflector at around the Moho discontinuity. In general, the middle to lower crust is reflective and the above described reflections are distinct. In addition, the cutoff depth changes its depth near the hypocenter of the main shock for the 2000 Western Tottori earthquake, 1995 Southern Hyogo prefecture earthquake and some other large inland earthquakes. Thus the seismogenic layer, in particular, the base of it plays an important role of the nucleation process of the large events. An fluid or gas seal-and-break effect may occur at the reflectors.
DE: 7200 SEISMOLOGY
DE: 7230 Seismicity and seismotectonics
SC: Seismology [S]
MN: 2003 Fall Meeting