PL波の群速度に基づく地殻速度構造の推定について

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Group velocities of PL waves recorded at Osakayama and Amagase Observatories of Kyoto University for earthquakes occurring in the east and northeast regions of Japan show a considerable regional difference with respect to propagation paths. The group velocities for propagation paths through the northern region (Tohoku and northern Chubu Districts) are much lower than those for propagation paths through the southern region of Honshu Island (Kanto and Southern Chubu Districts).Comparison between the observed and theoretical group velocities shows that the low group velocities for the propagation paths through the northern region should be attributed to a significant decrease in the velocity in the lower-crust. The theoretical group velocities were calculated by using OLIVER and MAJOR'S (1960) method, since our examination showed that this method was more appropriate than that of SU and DORMAN (1965).It is shown that a surface layer with low velocity, such as a sedimentary layer, could create a condition favorable for generation of large-amplitude PL waves. It is also shown that, at least in the range of period between 15 and 30 sec., group velocities of fundamental PL waves are more sensitive to shear velocity than to compressional velocity. There remains a problem, however, that it is difficult to explain dispersions both of Rayleigh waves and PL waves by one and the same structural model.