2001 Volume 110 Issue 6 Pages 784-807
Extensive studies utilizing data from a high-density observation network deployed in the Tokai region have brought about remarkable progress in understanding the subducting process of the Philippine Sea plate. Configuration of the Philippine Sea slab has been modeled using precisely determined hypocentral data (Yamazaki and Oida, 1985; Noguchi, 1996; Harada et al., 1998), and the locked region between the Philippine Sea and the Eurasian plates has been surmised on the basis of spatial variation of the density and focal mechanisms of microerathquakes (Matsumura, 1997). The coupling region has been also estimated by inversion analysis using GPS data (Sagiya, 1998, 1999). Although the result that suggests a large back-slip in the sea region to the south off Omaezaki does not seem to be consistent with the estimation of the locked region that is surmised to exist beneath the western coast of Suruga Bay, we think it is a great achievement to be able to discuss the coupling state on the basis of the observational data. Besides, simulation studies on subduction process modeling the Tokai region have developed considerably in recent years, indicating that there will appear intermediate-term and short-term precursors that should be detectable before a great interplate earthquake (Kato and Hirasawa, 1999). Three-dimensional simulations supply more realistic information about the phenomena that occur in the preparatory process of a great earthquake (Kuroki et al., 2001). Recent detailed analyses on the fault motion of the 1944 Tonankai and the 1946 Nankai earthquakes as well as acoustic sounding investigations on the upper crustal structure in their focal regions have afforded us useful information to assess the focal area of the coming Tokai earthquake. In order to utilize those scientific attainments obtained since the proclamation of the Large-Scale Earthquake Countermeasures Act in 1978, the Central Disaster Prevention Council has commenced to re-consider the focal model of the Tokai earthquake and to propose a new one.
In spite of the remarkable progress in studies on tectonic processes in the Tokai region, the probability of a successful prediction of the Tokai earthquake seems not to have much increased in the last 20 years. At present we are not very optimistic concerning the possibility of the detection of immediate precursors compared to when the Earthquake Assessment Committee for the Areas under Intensified Measures against Earthquake Disasters (EAC) had been established. Experiments on rock fracture, analyses of seismograms on the commencement of fault motion, computer simulations on the repeating occurrence of an interplate large earthquake, and other relevant studies all suggest that immediate precursors would not be as large as we expected. Standing on such a recent viewpoint about the magnitude of precursors, the Japan Meteorological Agency has revised criteria to convene EAC in 1998. The new criteriasa was prescribed based upon the detection level of volumetric strainmeters. This revision is considered to express the intension that JMA will do its best to catch the immediate precursors.
Some researchers have been presenting the ideas that recently observed seismic quiescence and crustal deformation in the Tokai region exhibit warnings that the Tokai earthquake is actually impending. The data and their interpretations, however, seem insfficient for the majority of the researchers to have a common opinion about the forecasting. The divergence of opinions concerning various kinds of changes in observational data obtained by the densely deployed network itself shows the difficulty of the prediction of the Tokai earthquake. We have not yet achieved a unified method to interpret the data or a scenario to determine how precursory phenomena will appear in sequence. It is very important to acquire more knowledge and know-how concerning these problems.
