Abstract
An earthquake of M 4.3 occurred in the central area of Shizuoka prefecture on Oct. 5, 1996. The event, including the main shock and its aftershocks, presented strange characteristics in the hypocenter distribution pattern, and also in the focal mechanism pattern. The hypocenters occupy a particular position where seismic activity was rarely observed, the place supposed to lie in the zone of the plate boundary between the subducting Philippine Sea plate and the overriding one. It is considered that both plates are strongly coupled and that their relative motion is locked at that point, and consequently such locking may be the mechanical source of a future Tokai earthquake. The focal mechanism of the main shock is low angle thrust type in accordance with the assumption of the locking. By contrast, those of the aftershocks presenting normal type faultings imply that there is no longer any strong locking. This difference in the focal mechanism between the main shock and aftershocks leads to the supposition that a weakening process of locking progresses at that site. Similar phenomena were found in an investigation of focal mechanisms in a wider area ranging over the focal zone inferred for a future Tokai earthquake. Since the beginning of the 1990s, normal type mechanisms appear to prevail in the shallower side of the focal zone. In this case, there is a concern about the possibility of pre-slipping, that is, a gradual slip starting from the deeper side of the focal zone of a forthcoming earthquake.
