Regional Variation of Pn Residuals and Its Application to the Location of Earthquakes in and around the Kanto District
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概要
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Regional variation of the crust and uppermost mantle structure in Japan was estimated from the analysis of Pn arrivals. Travel-time curves represented by the cubic equation were separately fitted to 83 well-observed shallow earthquakes which occurred in and around the Kanto district by taking into account the velocity gradients with depths. Pn velocities vary from 7.4 to 8.1 km/sec and velocity gradients with depths have a mean of 0.007 km/sec per km. Residuals from the fitted travel-time curves show some systematic geographical features, i. e. early arrivals on the Pacific Ocean side of Northeast Japan and late arrivals on Southwest Japan and on the Japan Sea side of Northeast Japan. Such features are closely correlated with the regional variation of the crustal thickness derived from the Bouguer gravity anomaly and the surface dispersion. Hypocenters of 4865 earthquakes occurring in and around the Kanto district during the 16 years from 1963 to 1978 have been relocated by correcting the mean Pn residuals at each station. Focal depths have been determined to a tenth of a kilometer instead of the 10 or 20 km step employed by JMA. Relocated epicenters show a tendency of clustering in smaller regions. A low Pn velocity of about 7.4 km/sec was observed for the Middle Gifu Prefecture earthquake of September 9,1969. The travel-time curve for this earthquake requires lower velocities by about 0.4 km/sec over the entire depth down to about 100 km than those for the Ichikawa and Mochizuki (1971) model. An exceptionally low Pn velocity of 7.1 km/sec was observed for an earthquake occurring near Sado Island, Niigata Prefecture. Its velocity profile suggests either the absence of the normal velocity jump around the Moho discontinuity or the existence of the unusually thick "intermediate layer".Pn走時を用いて地殻最下部・最上部マントル構造の地域性が調べられた.関東地方周辺に発生した浅い地震のうち,良く観測されている83個の地震の観測走時(気象庁)のデータから,3次項を加えた走時曲線を求めた.平均的な結果として,Pn速度は7.4から8.1km/sec,最上部マントルの速度勾配は0.007km/sec/kmが得られた.走時曲線からの残差(Pn残差)は観測点下の地殻構造の違いを表わす.一般的傾向としては,東北日本の太平洋側では負のPn残差(相対的にうすい地殻),東北日本の日本海側および西南日本では正のPn残差(厚い地殻)がみられる.Pn残差が,深発地震に対する残差と大体等しいことから,いわゆる"走時異常"の大部分は地殻構造の地域性によると考えられる.求められたPn走時残差を観測走時から補正して関東地方と周辺におこった地震(1963年から1978年,n=4865個)が再決定された.震源の深さが良く決められた.又震央は従来よりせまい範囲に集中する傾向がみられる.いくつかの走時曲線に対応する速度構造がしらべられた.最も低いPn速度7.1km/secが佐渡附近に起こった地震(1964年11月27日,深さ27km)にみられた.速度構造は,通常のモホ面がないこと,或いは異常に厚い"中間層"が考えられる.このPn速度は7.4km/sec(岐阜県中部地震,1969年9月9日,深さ0.5km)の走時曲線に対しても,市川,望月(1971)のものより深さ100kmまで0.4km/sec低い.
- 1981-09-30
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