日本で観測される遠地地震のP波走時残差

Anomalies in teleseismic P-wave travel-times observed at stations in Japan have been studied by using 8920 arrival time data of 223 distant earthquakes. Earlier arrivals are observed at stations on the Pacific side of Northeast Japan, and later arrivals are observed at stations in Southwest Japan and on the Japan Sea side of Northeast Japan. Exceptionally early arrivals are observed at stations in the Chugoku region, Southwest Japan. The geographical distribution of the station anomalies in the teleseismic travel times is consistent with the features of seismic activity, Bouger gravity anomaly, heat-flow and attenuation of seismic waves. Sinusoidal variations of the teleseismic travel-time residuals with azimuth are observed. Especially at high-sensitivity stations in the Kanto region the time differences of up to 2 seconds are observed between the earliest and latest arrivals. These azimuthal variations can not be explained only by the regional variation in the crustal structure directly beneath the stations. Mean residuals of the teleseismic travel times at each station are more in conformity with the station corrections of travel-times determined by the joint hypocenter determination (JHD) method for the mantle earthquakes rather than for the crustal earthquakes. Regional and lateral variations of the travel-time anomaly are mainly interpreted by the presence of a high-velocity zone in the upper mantle, associated with the descending slab beneath the Japanese Islands from the Pacific side. A negative correlation is observed between the mean residuals of teleseismic travel times and the Bouguer gravity anomalies at each station. Lateral variation of the velocity and density is suggested from the negative correlation of azimuthal variation of teleseismic residuals and the Bouguer gravity anomaly around some stations. A tendency of separation of stations into two groups with earlier and later arrivals is observed in the relation of the station anomalies with the station corrections of travel times and earthquake magnitude, and the Bouguer gravity anomaly. Regional differences of the attenuation of seismic waves seem to be consistent with the station anomalies of teleseismic travel times.223個の遠地地震(震央距離30°～100°)に対して観測された8920個の到着時データ(気象庁・大学等)を用いて走時異常が調べられた.Jeffreys-Bullenの標準走時に対する系統的傾向として,東北日本の太平洋側で速い走時,日本海側で遅い走時が観測され,又西南日本では一般に遅い走時が観測される.中国・四国地方の一部では例外的に速い走時が観測される.遠地地震に対する走時異常の地理的分布は,地震活動・重力異常・熱流量・地震波減衰係数の分布とも調和する.観測点のまわりの方位に対し,遠地地震の走時残差はsin変化を示す.特に関東地方の高感度観測点では,速い走時と遅い走時の間に2秒の差がみられる.この大きな差は,観測点直下の地殻構造の地域的変化だけでは説明できない.各観測点の平均残差は,複合震源決定法,(JHD)で求められた観測点補正値の内で地殻内地震に対するよりもマントル地震に対するものと良く対応する.遠地地震に対する走時異常の地域性や方位変化は主として,日本列島下に沈み込む太平洋プレートの構造によって説明される.遠地地震に対する平均残差と重力異常値,及び残差と重力異常の方位変化には負の関係がみられる.又遠地地震の平均残差の,マントル地震に対する観測点補正値・マグニチュード補正値・重力異常値に対する関係では,直線関係と共に,2つのグループに分かれる傾向がみられる