Focal zone of a future Tokai earthquake inferred from the seismicity pattern around the plate interface

A future large Tokai earthquake has been considered to be the most urgent problem of impending disasters in the central part of the Japanese islands since the first warning was announced in 1976. Recent progress in observations has provided a great amount of data, thus making a detailed investigation possible. In this study, based on the seismicity and the focal mechanism patterns of microearthquake data, the mechanical situation that governs plate interactions around the Tokai district was investigated. It was found that the seismicity in the subducting Philippine Sea slab is composed of three characteristic groups of activities; intense activity beneath Lake Hamana, a linear cluster extending from Lake Hamana toward the northwest, and a broad cluster expanding over the inland area in the west of Suruga Bay. We propose that these activities occur inside the subducting oceanic crust and reflect the present stress state acting there. The mechanism inducing these activities is assumed to come from two stress sources: the lateral stretching of the slab, and the partial locking between both plates. This interpretation leads to the hypothesis that the broad cluster in the west of Suruga Bay represents a stress concentration due to locking. The activity beneath Lake Hamana is proposed to represent shearing motion induced there, and is interpreted to form the boundary between the rupture zones of the 1944 Tonankai earthquake and a future large Tokai earthquake. These analyses resulted in identifying the wide inland area extending from the west coast of Suruga Bay to Lake Hamana to be a focal zone of a future Tokai earthquake. The zone proposed in this study indicates a significant deviation in its strike from the original model proposed by Ishibashi, in that the former parallels the Nankai trough, while the latter parallels the Suruga trough.