Interplate coupling changes in the Tokai region, Japan, estimated from the vertical movements by leveling and tide gauge during 1960-2002

The large earthquakes of more than magnitude 7.8 have repeatedly occurred along the Nankai-Suruga trough with an interval of 100-150 years. Since the last events of the 1944 Tonankai and 1946 Nankai earthquakes, it has passed more than 50 years. Recent findings of the 2001 slow slip event in the Tokai region by GPS dense continuous network led us to re-examine the possibility of slow slip events from the 1960s to the present in the Tokai region. We discuss the spatial changes of interplate coupling to make clear. Tide gauge measurements have continuously recorded at four sites since the 1960s and the precise leveling has repeated every year or every two year since the 1970s in the Tokai region by GSI. We estimated the changes of interplate coupling from these vertical movements. Characteristics of the interplate coupling are; 1) interplate coupling area apparently reached just beneath the Mikawa bay, which is located 30km depth of plate boundary, and 2) main area of the interplate coupling does not limited to the shallow plate boundary of coast area, but it extends to the in-land area. Sagiya (1999) already discusses the interplate coupling model from GPS measurements, respectively. However there are some differences between the observation and model calculation. Especially Sagiya_fs model could not explain exactly the subsidence at Omaezaki and the uplift at Oodaka, which is 100km west of Omaezaki. Moreover the convergence rate of the Philippine Sea plate is estimated to be about 2 cm/yr from the GPS measurements at the Suruga trough whereas those are 4 to 5 cm/yr at the Nankai trough (Heki and Miyazaki, 2001). The interplate coupling in the 1960s shows the subsidence rate at Omaezaki was less than 5mm/yr and it was about a half in 197s-1990s. It is suggested that interplate coupling is not full beneath Omaezaki in 1960s. Moreover the uplift is not detected at the inland area (Ise Bay) in 1960s. It could also suggest that the interplate coupling might have been weak in the eastern part of the Tokai region, suggesting the effects of afterslip due to the 1944 Tonankai earthquake. We also conclude that the intraplate coupling along the Suruga-Nankai Trough has changed episodically.