Time‐dependent inversion study of the slow thrust event in the Nankai trough subduction zone, southwestern Japan

Abstract

We estimate time variation of interplate coupling between the subducting Philippine Sea and the overriding continental plates along the western part of the Nankai trough, Japan, during the period between 1996 and 1999. After subtracting annual components and average annual velocities, deviation from steady crustal deformation in the Shikoku and Kyushu islands is examined. Around 15 GPS stations facing the Bungo channel detected the following three‐stage ground displacements of up to 3 cm in 1997, with a duration of ∼1 year: (1) increased displacement rate associated with the 1996 Hyuga‐nada earthquakes; (2) an increasingly rapid anomalous period of deformation from June 1997 to the end of 1997; and (3) a slowdown period and return to normal rates of deformation after the end of 1997. Linearized least squares locate the slip area of this slow event near the plate boundary beneath the Bungo channel. The estimated slip is opposite the motion of the Philippine Sea plate. These results support the hypothesis that the observed anomalous caistal deformation is due to a rebound process of the Philippine Sea plate. Time‐dependent inversion analysis shows the following slip history. During the initial period of increased displacement rate, a visible slip area is resolved 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 postseismic slip of the 1996 Hyuga‐nada earthquakes almost ended. After this accelerated period, the estimated thrust slip ends with time.