2009 年 61 巻 3 号 p. 113-135
We investigated the transient crustal deformation in the Tokai district, central Japan, based on the GPS data in 2000-2008. Our modeling showed that observed transient deformation in the Tokai district can be explained by three different mechanisms, slow slip on the plate interface, viscoelastic response and an afterslip following the 2004 off southeast Kii peninsula earthquake. In our modeling, we first evaluated the effect of postseismic deformation due to the viscoelastic response of the 2004 off southeast Kii peninsula earthquake, and obtained the reasonable value of viscosity 1.0×1019 Pa·s. Then we estimated the afterslip model of the 2004 off southeast Kii peninsula earthquake, and obtained that the time constant of the postseismic deformation caused by the afterslip is 62.65 days. After removing the viscoelastic response and the effect of afterslip from original transient deformation, we modeled this transient deformation as a slow slip on the plate interface, which we call here as Tokai slow slip event. Our result revealed that the Tokai slow slip which occurred mainly under the Lake Hamana ended in July 2005. The maximum slip of this event was estimated at 25cm under the Lake Hamana. This maximum slip accounts for the backslip accumulated in the past 14 years, assuming that the backslip rate has been constant. The total moment release was equivalent to that of an Mw 7.1 earthquake. On the other hand, the aseismic slip still continues in the northeast region of the Lake Hamana at the same rate as that before July 2005. The maximum slip in this region since July 2005 reaches 5cm, and total moment release is equivalent to that of an Mw 6.5 earthquake.