東海地域推定固着域内に発生する地震活動とその変化

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The Tokai district is one of the most intensely monitored places in Japan with respect to seismic risk due to the anticipated Tokai earthquake. We have been performing continuous observations of microerathquakes in this region for more than two decades. The GPS observations to monitor ground surface deformation have been performed by the Geographical Survey Institute since 1997. Recently, unusual signal changes have been found nearly simultaneously by both monitoring operations. First, a small but significant quiescence was detected in seismicity within the subducted Philippine Sea slab, which commenced in August 1999. A similar quiescence was also detected in other regions, such as seismicity within the overriding plate and seismic clusters beneath Lake Hamana. All of these activities are considered to be caused by the interaction of two plates, i.e., due to the locked subduction of the Philippine Sea slab. The seismicity change was represented by a spatial pattern with a decrease/increase of earthquakes compared to the standard one. We recognized that the characteristic pattern appears at the present. Following this seismic event, GPS monitoring found a gradual movement of the ground surface, which deviates from the previous trend, and was analyzed to be due to a slow-slip occurring on the plate interface beneath Lake Hamana. All of these anomalous events happened in the period from the late 1990s to 2000, and have continued to date. Such slowand small changes in crustal activity seem not to be directly bound up with the forthcoming megathrust, that is, the next Tokai earthquake, but lead us to imagine a progress of change in the locked state as follows. In the early or middle stage of locked subduction, the plate coupling should be sustained in the entire locked region. In the later or critical stage, the inhomogeneity in the strength of locking may emerge explicitly. The latter will result in a situation in which the stress sustained at a relatively weakly locked zone is released, and be concentrated. The pattern change in seismicity may agree with this interpretation. If this is the case, it means there is steady progress being made toward the final failure. It is indispensable not only to carefully monitor crustal activities, but also to build a consistent model to interpret all of the unusual evidence detected to forecast the next Tokai earthquake.

東海地域推定固着域内に発生する地震活動とその変化

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