浜名湖直下の地震活動とその変化

東海地域の地震活動変化: その3

抄録

Peculiar seismic activities are occurring beneath Lake Hamana inside the subducted Philippine Sea slab at depths of about 30km. They consist of three spindle-shaped earthquake clusters with NW-SE axes, in a left-stepping alignment in the EW direction, with several kilometers between them. Illustrations of the focal mechanisms indicate that they act like an open crack under NW-SE compression and NE-SW tension and are caused by a right-lateral shear force acting on the Philippine Sea slab. A stress pattern simulation model suggests the following explanation of the situation. A localized locked zone is positioned on the plate boundary just north of the clusters. Separated from the main locked zone, it is considered to be one of the satellite asperities surrounding the main one. The main locked zone is located eastward from Lake Hamana and is expected to become the seismogenic zone of the forthcoming Tokai earthquake. These clusters have demonstrated a remarkable decrease in activity since the second half of the year 2000. An anomalous tectonic movement detected by GPS measurements occurred almost simultaneously. This indicates that a slow slip event progressed on the plate boundary beneath Lake Hamana; that is, the locking must be released there. Since the current change in seismic activity corresponds with this movement, it can be attributed to tectonic stress change due to the slow slip. We estimate that at least three similar periods of quiescence have occurred during the last quarter century. Tidal gauge findings at Maisaka and the crustal tilt at Mikkabi, both of which were observed near Lake Hamana, have indicated almost simultaneous occurrences of similar anomalies. As a result, three episodes of slow slip were identified: the first occurred before 1980, the second around 1990, and the last has been ongoing since late 2000. This implies that the slow slip repeats quasi-periodically with an interval of about one decade. We consider that the locked zone beneath Lake Hamana is a small asperity with a potential of slowly and intermittently slipping due to a weak coupling condition in an area sandwiched between two seismogenic zones of the Tokai and Tonankai megathrust earthquakes.