次の南海トラフ巨大地震に備えて １９９７年豊後水道ゆっくり地震の時間発展インバージョン解析

Abstract

We investigated deviative components from average crustal deformation in southwestern Japan with the annual component and the average velocity removed from the raw-time series data of the GSI's GPS network (GEONET), during the period between April 1996 and October 1999. The result shows anomalous crustal deformation at GPS sites around the Bungo channel after the 1996 Hyuganada earthquakes with the following three characteristic periods : (1) increased displacement rate associated with the Mw=6.8 Hyuga-nada earthquake of October 19, 1996 and with the Mw=6.7 Hyuga-nada earthquake of December 3, 1996 ; (2) an increa-singly rapid anomalous period of deformation from June 1997 to around the end of 1997 ; and, (3) a slowdown period and return to normal rates of deformation after the end of 1997. During these periods, no earthquakes larger than Mw=5 occurred in the Bungo channel area. Linearized least squares, using a rectangular fault, locate the slip area of this slow event beneath the Bungo channel, Kyushu, southwestern Japan; an area dipping west with the lower edge at a depth of 50 km, very close to the plate boundary between the Philippine sea and the overriding continental plates in this region. In addition to its proximity to the plate boundary, this estimated slip of the hanging side is opposite to the motion of the Philippine Sea plate. These results support the hypothesis that the observed anomalous crustal deformation is due to a rebound process of the overriding continental plate against the Philippine Sea plate, as is pointed out by Hirose et al. (1999). In order to estimate the rupture propagation of this slow earthquake in detail over time, we apply a time-dependent inversion scheme for the data at GPS sites on the Shikoku and Kyushu islands. The result shows that a reverse slip occurred in an area beneath the Bungo channel between the Shikoku and Kyushu islands with a time span ranging from several months to a year, depending on the location estimated. During the initial period of slowly rising displacement rates, the visible slip area is resolved by our inversions beneath the coastal area of southwestern Shikoku. In the next stage of accelerated displacements, the slip area expanded southwestward to the Bungo channel beginning around June 1997, when a postseismic slip almost ended in the area near the epicentral region of the 1996 Hyuga-nada earthquake. After this accelerated period, the estimated slip area remained almost the same in size and location over time, with the time transient anomalous deformation signals ending.