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In recent years, some anomalous crustal activities have been detected in the Tokai area, central Japan. First, a seismicity change was detected and was recognized to be the quiescence of microearthquake activities. Anoma- lous ground deformation was then detected in high-density GPS observations conducted by GSI, and also in tilt observations by NIED at the MKB station. East-southeast upward tilting of about 2 microradians from the mid- dle of 2000 to the present is consistent with the result of the GPS observation. An analysis using the GPS data attributed the ground deformation to a slow slip occurring on the plate boundary beneath Lake Hamana, where distinct seismic quiescence has also been detected. Tilt observation at MKB began in 1981, so we have carefully examined past records to determine whether similar events have ever taken place. An anomalous tilt change of nearly the same direction as the current one and seismic quiescence evidently occurred simultaneously from 1988 to 1990. Apparently, a similar slow slip took place in almost the same place during this epoch. We speculate that slow-slip events beneath Lake Hamana have been repeatedly occurring under the steady subduction of the Philippine Sea plate. However, the magnitude and duration of the current slip now in progress are nearly twice those of the last slip (1988 to 1990). Therefore, a slow slip in this area may not repeat in exactly the same way. Though it is unclear how the ongoing event will proceed, redistribution of stress due to the slow slip may promote the anticipated Tokai earthquake. Careful monitoring and intense observation are necessary.

A slow slip event in the Tokai area detected by tilt and seismic observation and its possible recurrence