Crustal and Upper Mantle Structure in Japan from Amplitude and Phase Spectra of Long-period P-waves

Part 3. Chugoku Region

Abstract

Crustal and upper mantle structure in the Chugoku region has been studied by the same method as described in Part 1 of this paper, by analyzing the seismograms of long-period P-waves recorded at Shiraki. Further theoretical evaluation of the method for the dissipative and anisotropic media is made, in addition to Part 1. The observational curves of the amplitude ratio and phase difference for 25 shocks are classified into six sets by the direction of wave approach to Shiraki. The first high peaks of the curves for waves approaching from the near south to southeast direction appear at frequencies higher than those from the other directions. This suggests that the crust becomes thinner towards this direction. Comparing these observational curves with the theoretical curves, which are calculated by varying layer parameters of the models so far proposed for the structure near this region, four combinations of probable models are finally obtained and the average layer thicknesses are estimated. Among these models, model SHK70-4 is considered as most probable, whose thicknesses of the crust and 7.4km/sec intermediate layer are about 41km and 26km, respectively. However, models SHK70-2 and 3 cannot be excluded. The former, which has a crustal thickness of about 26km and the thickness of the intermediate layer of about 17km, is more probable than the latter. From travel-time studies late arrivals of P-waves at Shiraki as much as about 0.6 seconds are observed, compared with those at Matsushiro and Tsukuba. This implies that the upper mantle with a lower velocity is fairly thicker around this area than in the central mountain area and the Kanto plain.