HR: 1340h
AN: T23B-1419    [Abstracts]
TI: Ocean bottom seismographic study on the 2005 off-Sanriku intralate earthquake (M7.1) occurred in the outer rise of the Japan Trench
AU: * Hino, R
EM: hino@aob.geophys.tohoku.ac.jp
AF: Tohoku University, Research Center for Prediction of Earthquake, 6-6 Aramaki-Aza-Aoba, Aoba-ku, Sendai, 9808578, Japan
AU: Ito, Y
AF: Tohoku University, Research Center for Prediction of Earthquake, 6-6 Aramaki-Aza-Aoba, Aoba-ku, Sendai, 9808578, Japan
AU: Yamamoto, Y
AF: Tohoku University, Research Center for Prediction of Earthquake, 6-6 Aramaki-Aza-Aoba, Aoba-ku, Sendai, 9808578, Japan
AU: Suzuki, K
AF: Tohoku University, Research Center for Prediction of Earthquake, 6-6 Aramaki-Aza-Aoba, Aoba-ku, Sendai, 9808578, Japan
AU: Tsushima, H
AF: Tohoku University, Research Center for Prediction of Earthquake, 6-6 Aramaki-Aza-Aoba, Aoba-ku, Sendai, 9808578, Japan
AU: Suzuki, S
AF: Tohoku University, Research Center for Prediction of Earthquake, 6-6 Aramaki-Aza-Aoba, Aoba-ku, Sendai, 9808578, Japan
AU: Miyashita, M
AF: Japan Metrological Agency, 1-3-4 Otemachi, Chiyoda-ku, Tokyo, 1008122, Japan
AU: Tomori, T
AF: Japan Metrological Agency, 1-3-4 Otemachi, Chiyoda-ku, Tokyo, 1008122, Japan
AU: Arizono, M
AF: Sendai Metrological Observatory, 1-3-15 Gorin, Miyagino-ku, Sendai, 9830842,
AU: Tange, G
AF: Sendai Metrological Observatory, 1-3-15 Gorin, Miyagino-ku, Sendai, 9830842,
AB: An earthquake with magnitude of 7.1 occurred on November 15, 2005 in the outer rise region of the middle part of the Japan Trench subudction system. This earthquake was the largest earthquake in this area since the occurrence of the 1933 Sanriku earthquake (M 8.1), which was accompanied with devastating tsunami along the Pacific coast of northeastern Japan. The normal fault type focal mechanism solutions estimated from global seismic network data (Harvard CMT, F-net) of the 2005 earthquake indicate that this earthquake is related to the bending of the pre-subducted Pacific plate as well as the 1933 earthquake (Kanamori, 1971). We deployed six ocean bottom seismographs (OBS) to observe the aftershocks of the 2005 earthquake for two months, from April to June, 2007. Although it has passed one and a half year after the mainshock occurrence, our OBS observed intense micro-seismicity in the aftershock area presumed from the land-based seismic data. Source regions of the outer rise earthquake are usually quite far from land area and analyses based on teleseismic data have been only the way to understand the faulting process of earthquakes of this kind. The present OBS observation provides us an invaluable opportunity to study outer rise intraplate earthquakes in anatomical way. By using P- and S-waves arrival time data, we estimated the hypocenter locations of more than 300 earthquakes. We also sought optimum Vp and Vp/Vs in the aftershock area to minimize travel time residuals in the hypocenter determination. Obtained Vp is about 6 km/s and 7 km/s at the top and the bottom of the crust, respectively, well within the range of the standard Vp value of the oceanic crust (e.g. White et al., 1992). The Vp/Vs estimated from slope of the Wadachi diagram is 1.73 or less, smaller than that estimated in the NE Pacific Basin (Fukano et al., 2006). The relocated epicenters are constrained within a rectangular area with size of 40 x 20 km, in along-strike and across-strike directions of the trench strike. This size of the aftershock area is consistent with the fault dimension approximated from the magnitude of the mainshock. Estimated focal depths of the aftershocks are ranging from about 1 to 15 km measured from the sea-bottom, indicating that the aftershocks occurred both within the oceanic crust and the uppermost part of the mantle. The hypocenters form two planes perpendicular to each other; one of them dips towards the trench by about 60 degree and the other dips to opposite direction by about 30 degree. It is notable that the strike and dip of these planes almost agree with those of the nodal planes of the CMT solutions estimated by the F-net data. This suggests that the aftershocks distribute not only along the fault plane of the mainshock rupture but also its conjugate plane. The two planes are connected at the bottom to form 'V' shape geometry. The steeper plane seems to cut through the oceanic crust but does not reach the sea bottom. On the other hand, the shallow dipping plane remains in the deeper part of the crust and in the mantle. The combination of a steep trenchward dipping fault and an oceanward dipping shallow fault system is concordant with the asymmetric fault system of outer rise earthquakes found by Jiao et al. (2000). Present result may help to understand how the asymmetric fault system in the trench outer rise.
DE: 7215 Earthquake source observations (1240)
DE: 7220 Oceanic crust
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting