HR: 1330h
AN: S52B-0138    [PDF]
TI: Detailed Structure of Plate Boundary Revealed by Seismic Reflection Survey and Spatial Relation With Interplate Seismic Activity Indicated by Converted Waves, East off Boso Peninsula, Japan
AU: * Kimura, H
EM: kimura@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, Tennodai 3-1, Tsukuba, Ibaraki, 305-0006 Japan
AU: Kasahara, K
EM: kasa@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, Tennodai 3-1, Tsukuba, Ibaraki, 305-0006 Japan
AB: Tokyo metropolitan area of Japan is located near Sagami trough where Philippine Sea(PHS) plate subducts beneath Kanto district and large earthquakes including Kanto Earthquake, 1923 took place repeatedly. Thus, for the purpose of damage mitigation in Tokyo metropolitan area, it is important to study the characteristics of plate boundary and relation with seismic activity. The area eastern off Boso peninsula is one of regions where interplate earthquakes are occurring at shallower part around Kanto district. We researched the detailed structure of plate boundary and spatial relation with seismic activity using seismic reflection survey data and waveform data of microearthquakes. In this region, multi-channel seismic surveys were conducted by NIED and JNOC(Japan National Oil Corporation). From the analysis of those data (Kimura and Kasahara, 1997; Kimura and Kasahara, 2001), the following results are obtained. 1)There exists clear sediment layer which consists of Quaternary and Tertiary layer(Kazusa, Shimousa and Miura group) with thickness of 0.2 - 2.0 sec in TWT. 2)Major deep reflectors(MDR) have been found dipping northward along the survey line coincident to the plate boundary between PHS plate and landward plate. 3)From the analysis of cross-correlation between reflected waves at acoustic basement and MDR, it is revealed that the polarity of upper reflector is reverse and that of lower is normal. 4)Crustal layer between acoustic basement and MDR is transparent. The candidates of MDR may be the upper and lower boundary of oceanic crust or other surface structure of PHS plate. Taking into consideration polarity of reflectors and results of past survey, it is thought that this layer would correspond to shallowest layer of Izu peninsula with acoustic velocity of 4.1 - 5.0 km/sec and with thickness of 1.0 - 2.0 km revealed by the explosion seismic survey(Asano et al., 1982). In the southern portion of survey line, TWT of MDR is 6.0 - 8.0 sec and suggesting that depth of the reflector is about 14 - 18 km which is close to earthquake hypocenters. From the analysis of observed converted waves recorded by microearthquake observation network of NIED, it is revealed that seismic activities around this region is occurring just beneath some major acoustic impedance discontinuity (Kimura and Kasahara, 2003) and from the analysis of polarity of converted waves, it is more probable that this discontinuity corresponds to lower reflector of MDR. From these result, it is indicated that thin layer within MDR would play important role on the characteristics of seismic activity.
DE: 0900 EXPLORATION GEOPHYSICS
DE: 1744 Tectonophysics
DE: 7230 Seismicity and seismotectonics
DE: 8015 Local crustal structure
DE: 8150 Plate boundary--general (3040)
SC: Seismology [S]
MN: 2003 Fall Meeting