A study on crustal structure in Japan by the use of seismic and gravity data

Abstract

Crustal structure in some regions of Japan has been discussed on the basis of the results of refraction measurements, correlation between computed and Bougucr gravity anomalies and between theoretical and observed phase velocities of Rayleigh waves. The already-derived single-layer crustal models do not seem to be consistent with observed gravity and surface wave data. Alternative structures with double layered or three-layer crust are presented for all the regions by the re-interpretation of travel time data. Comparison of theoretically-predicted values with the observations favors a three-layer solution for western and central Japan, suggesting the existence of a lower crustal layer and a thick intermediate layer over the mantle. A high Poisson's ratio would occur in the intermediate and the upper mantle under central Japan including the Chubu mountain region. In eastern Japan, it is not certain whether there is a thin intermediate layer or not, but the average crustal density might be slightly smaller than that in other regions. The depths to the base of the lower crust are estimated to be 32-35 km in the Tohoku, 27-31 km in the southeast Kwanto, 38-40 km in the northwest Kwanto, 36-40 km in the Chubu, 28-30 km in the Kinki, and 25-28 km in the Chugoku regions respectively. The average crustal structures in Japan and North America are compared.

日本の地殻構造については, すでに爆破地震動や表面波の観測, 重力異常の解析などによって多くの研究が行われているが, これら3種の情報を組合わせれば, いくつかの可能なモデルの取捨選択に役立つことが期待される.このような方法は日本では金森がすでに試みているが, こゝでは今迄に行われた爆破地震動の走時にもとづき, 種々の可能なモデルを推定し, これらのモデルから期待される重力異常とRayleigh波位相速度の理論値を計算してそれぞれの観測値と比較する方法を採った.重力異常の計算には, 地震波速度と岩石密度との関係, 比較の標準としてとるべき構造などの仮定が含まれるために, その絶対値を論ずることは難しいが, 相対的な変化をBouguer anomalyの傾向と比べることが出来る.一方, 表面波位相速度はS波の速度分布につよく支配されるために, Poisson ratioがとり得る値を検討しなければならない.これらの問題を考慮に入れて, 地殻構造の考察を行なった結果は次の通りである.