The Philippine Sea plate is subducting beneath the Boso peninsula, central Japan, with the Pacific plate underlying the Philippine Sea plate (1) (see Fig. 1). Because of the complicated tectonic setting, shown in Fig. 1, seismic events are numerous off the coast of the Boso peninsula (2). Permanent GPS observation sites set up by the Geographical Survey Institute ofJapan (GSI) have been in operation since 1994. Fig. 2A shows steady northwestward crustal deformation on the Boso peninsula, revealed by these GPS sites. Based on the crustal deformation rate in Fig. 2A, the coupling rate off the east coast of the Boso peninsula is estimated at between 1 and 2 cm/year with the southern part showing a higher coupling rate between the Philippine Sea plate and the overriding North American plate (3). Under this tectonic setting, ground motion deviating from the steady northwestward deformation in Fig. 2A occurred in 1996. Fig. 2B shows the detected transient crustal deformation at GPS stations relative to the Ohgata station, Fig. 1, for the period between April 8 and June 10, 1996 (Japan Time). As shown in this figure, the southeastward ground displacements are observed, in contrast with Fig. 2A. From this crustal deformation, aseismic slip was estimated off the east coast of the Boso peninsula with its rupture propagating to the south with a time duration of about 7 days (3, 4). Six years after the 1996 event, quite similar abnormal crustal movements started from around October 4 2002, indicating the occurrence of another interplate aseismic slip in this region. Employing a Kalman filter, we estimated the time evolution of the 2002 Boso aseismic slip and compared the 2002 event with the 1996 silent earthquake in order to investigate the nature of silent earthquakes.
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| Figure 1. Tectonic map in and around the Boso peninsula, central Japan |
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| Figure 2. (A) Velocity field in the Boso peninsula, estimated over the
period between 1997 and 1999. (B) Solid arrows represent ground displacements at GPS stations from April 8 toJune 10 1996 (Japan Time).White arrows indicate computed ground displacements from the estimated model. Open circles represent GPS sites whose time series data are shown in Figure 3. Small open circles represent hypocenter with depth<=50 km and Mw>=2 in May 1996 determined by the Japan Meteorological Agency (JMA). Data are from JMA. (C) Solid arrows show ground displacements at GPS stations from September 1 toDecember 2, 2002.White arrows indicate computed ground displacements at GPS site used in Kalman filtering analysis. Small open circles represent hypocenter with depth<=50 km and Mw>=2 in October 2002 determined tentatively by JMA. Data are from JMA. |
1. Ishida, M., J. Geophys. Res., 97, B1, 489-513, 1992.
2. Okada, Y. and K. Kasahara, Tectonophysics, 172, 351-361, 1990.
3. Sagiya T., EOS Trans. 78, F165, 1997.
4. Sagiya, T. and Inoue M., Western Pacific Geophysics Meeting, SE21A-03,2002.
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