抄録
A method is shown to study the crustal and upper mantle structure from the amplitude and phase spectra obtained from the analysis of long-period P-waves. Taking the amplitude ratio of the vertical component to horizontal one and the phase difference between them, we obtain two observational curves which depend only on the structure beneath a recording station. Comparing these curves with the corresponding theoretical ones derived from an assumption of the horizontal parallel layering of homogeneous media, we can select a most probable model in which the theoretical and observational curves fit well each other.
In order to study the structure under the central mountain area, the records registered at Matsushiro have been analyzed. Among four sets of observational curves corresponding to four regions classified by the incident direction, there appear fairly remarkable differences, implying the possible lateral variations of layer interfaces. Confining, therefore, the validity of the above assumption to each of the four regions respectively, and varying layer parameters of the models so far obtained from gravity, travel time and surface wave studies, we have obtained a few probable models for each region. Taking into account all available information on the structure in this area, we have selected one model for each region. The models obtained reveal that the crustal thickness (the thickness of the intermediate layer is not included) is about 40km over this area, and that there is a thick 7.4km/sec intermediate layer whose thickness is considerably different among the four regions. Although Matsushiro is situated in the tectonic region with a thick underlying intermediate layer, surrounded by highly mountain ranges, travel-time residuals from Jeffreys-Bullen table show a remarkable negative value, suggesting the possibility of rather higher velocity in the upper mantle below the intermediate layer or/and the lack of a low-velocity layer, compared with other tectonic regions.
