[SCG57-P14] フィリピン海スラブの変形速度から見積もった中部日本の短縮速度
キーワード：フィリピン海スラブ、中部日本、短縮速度
It has been widely recognized that the main part of Japanese islands are currently situated under the stress field of east-west compression. However, it is not easy to quantitatively estimate the contraction rate of Japanese islands in a long term, though this is a fundamental quantity to understand the tectonics of Japan. For example, GNSS data provide accurate estimates of the east-west contraction rate, but the observation period is less than a few decades (e.g., Sagiya 2004). Traditional triangulation and trilateration data covers more than a century. However, it is still unclear that the observed deformation rates can be extrapolated to a geological time scale. From moment release rate due to historically recorded large inland earthquakes and long-term slip rates on active faults, Wesnowsky (1982) estimated the E-W contraction rates of Honshu island. However, the recurrence interval of large inland earthquakes is much longer than several hundred years large, and inland earthquakes have often happened on faults that have not been identified as active faults before the earthquakes.
By the way, a slab usually descends into the mantle with little deformation as indicated by the iso-depth contours, which are nearly parallel to the trench in most subduction zones. Little deformation of slabs is reasonable from the view point of elastic energy. But the Philippine Sea slab is an exception; large deformation beneath central Japan has been estimated (e.g., Hashimoto et al. 2004; Shiomi et al. 2008; Hirose et al. 2008). The reason for the horizontal deformation of the Philippine Sea slab is not known, but it is likely that E-W compression applied to Japanese islands is the cause of the horizontal deformation. The Philippine Sea slab severely deforms beneath central Japan, while the deformation is much smaller beneath western Japan. Such characteristics well correspond to geological, seismological and geodetic data.
If the deformation of crust is well correlated with the deformation of the slab beneath the crust, we can use the deformation rate of the slab as a proxy to estimate the contraction rate of the crust, which is not easy to constrain well. The advantage of this method consists in the simplicity of the initial configuration of a slab. Continental crusts generally have very long history, and so, it is a difficult task to extract deformation that occurred only recent a few million years. On the other hand, an oceanic plate is subject to almost no horizontal deformation before the subduction. Therefore, we can consider that the currently observed deformation of the Philippine Sea slab has been formed only after the subduction along the Nankai trough.
This study is the first attempt to estimate the horizontal contraction rate of the Philippine Sea slab. Through the deformation of the Philippine Sea slab, we will show the estimate of the contraction rate of central Japan.
By the way, a slab usually descends into the mantle with little deformation as indicated by the iso-depth contours, which are nearly parallel to the trench in most subduction zones. Little deformation of slabs is reasonable from the view point of elastic energy. But the Philippine Sea slab is an exception; large deformation beneath central Japan has been estimated (e.g., Hashimoto et al. 2004; Shiomi et al. 2008; Hirose et al. 2008). The reason for the horizontal deformation of the Philippine Sea slab is not known, but it is likely that E-W compression applied to Japanese islands is the cause of the horizontal deformation. The Philippine Sea slab severely deforms beneath central Japan, while the deformation is much smaller beneath western Japan. Such characteristics well correspond to geological, seismological and geodetic data.
If the deformation of crust is well correlated with the deformation of the slab beneath the crust, we can use the deformation rate of the slab as a proxy to estimate the contraction rate of the crust, which is not easy to constrain well. The advantage of this method consists in the simplicity of the initial configuration of a slab. Continental crusts generally have very long history, and so, it is a difficult task to extract deformation that occurred only recent a few million years. On the other hand, an oceanic plate is subject to almost no horizontal deformation before the subduction. Therefore, we can consider that the currently observed deformation of the Philippine Sea slab has been formed only after the subduction along the Nankai trough.
This study is the first attempt to estimate the horizontal contraction rate of the Philippine Sea slab. Through the deformation of the Philippine Sea slab, we will show the estimate of the contraction rate of central Japan.