HR: 10:50h
AN: T41E-03    [PDF]
TI: Seismic activity in the Japan Trench forearc from network observation in the seafloor
AU: * Araki, E
EM: araki@jamstec.go.jp
AF: DTM, Carnegie Institution of Washington, 5241 Broad Branch Rd, NW Washington, DC 20015
AU: Sacks, S
AF: DTM, Carnegie Institution of Washington, 5241 Broad Branch Rd, NW Washington, DC 20015
AU: Linde, A
AF: DTM, Carnegie Institution of Washington, 5241 Broad Branch Rd, NW Washington, DC 20015
AU: Kanazawa, T
AF: ERI, Univ. of Tokyo, 1-1-1 Yayoi, Bunkyo, 1130032 Japan
AU: Shinohara, M
AF: ERI, Univ. of Tokyo, 1-1-1 Yayoi, Bunkyo, 1130032 Japan
AU: Fujimoto, H
AF: Tohoku Univ., Tohoku Univ, Aobaku, 9808578 Japan
AU: Hino, R
AF: Tohoku Univ., Tohoku Univ, Aobaku, 9808578 Japan
AU: Mikada, H
AF: JAMSTEC, 2-15 Natsushimacho, Yokosuka, 2370061 Japan
AU: Matsumoto, H
AF: JAMSTEC, 2-15 Natsushimacho, Yokosuka, 2370061 Japan
AU: Sato, T
AF: JAMSTEC, 2-15 Natsushimacho, Yokosuka, 2370061 Japan
AU: Suyehiro, K
AF: JAMSTEC, 2-15 Natsushimacho, Yokosuka, 2370061 Japan
AB: In the forearc region of off-Sanriku area in the Northeast Japan Arc, subduction of the Pacific plate beneath the arc gives rise to seismogenesis of smallest to M7-class earthquakes. Seismicity in the area shows non-uniform distribution of earthquakes and its relation to the structural heterogeneity at the plate boundary has been suggested. Such a structural heterogeneity may relate to horst and graven structure of the subducting plate that is prominent in the oceanic crust of the Pacific plate before subduction in the area. The subducting plate motion is accommodated by deformations and displacements near the plate boundary. Condition of the plate boundary is subject to change under subduction. This, in turn, control the manner of seismogenesis and are observed as non-uniform seismicity, transition of earthquake mechanisms, or possible aseismic slip in the plate boundary. In order to understand such processes, observation of seismic activity to define precise location and mechanisms of earthquakes in relation to the subducting plate geometry is important. As the occurrence of large earthquakes account for only about 30 % of the plate motion of about 10 cm/y across the Japan Trench, we also need to search for possible slow slip in the area. Existing land observatories, which are apart from the area by more than 100km, are inappropriate for these targets. Therefore, we deployed network of seafloor instruments consisted of borehole seismo-geodetic observatories, seafloor pressure gauges, and arrays of ocean bottom seismometers (OBS) in the forearc region of off-Sanriku area. Two borehole observatories (JT1 and JT2) were installed in 1999. We installed 2 broadband seismographs (CMG1T and PMD2123), tiltmeter (AG510), and a strainmeter. Sensors are buried near the hole bottom approx. 1.1km below seafloor. With these state-of-art borehole sensors situated only 10km above the plate boundary, we expect earthquakes and slow crust deformation observed at the highest precision. We have conducted maintenance cruises to the borehole observatories, and recovered total of a half-year data from JT1 and two sets of 3-month data from JT2. The long period performances of the borehole observatories are good enough for tidal tilting of the boreholes to be observed. The seafloor pressure gauges (Paroscientific 8B7000) deployed adjacent to the JT1 and JT2 sites are useful to detect vertical crustal movement as small as a few millimeters. In June 2003, we recovered the first 8-month data from near JT1 site. We started 1-year pressure observation at the two sites from June 2003. Series of OBS array deployments in 2001, 2002 and 2003 of each several months enabled us to get detailed image of earthquakes with plate subduction in this area. In 2002, we deployed 17 OBS at 7-15km spacing around the JT2 site. During the deployment, we also shot airgun along 5 seismic lines on the OBS network. In 2003, we focused on the JT1 site deploying 5 OBS in 5 n.m. separations from the JT1 borehole, and we made simultaneous observation with the JT1 and JT2 borehole. From the year 2002 OBS dataset, clusters of microearthquakes are clearly imaged near the depth of plate boundary, and a small number of earthquakes are identified below the oceanic crust of the subducting plate. The airgun shot data along with the seismic structure obtained from previous airgun-OBS surveys enables us to correlate the seismicity with the crustal structure under the network.
DE: 1255 Tides--ocean (4560)
DE: 3025 Marine seismics (0935)
DE: 7230 Seismicity and seismotectonics
DE: 7294 Instruments and techniques
SC: Tectonophysics [T]
MN: 2003 Fall Meeting