天水地震区地壳浅层速度结构

THE VELOCITY STRUCTURE OF SHALLOW CRUST IN TIANSHUI EARTHQUAKE AREA

摘要:

本文利用天水地震区成县-西吉和灵台-阿木去乎二条人工地震测深剖面取得的7个相遇系统Pg波资料,研究测线下方地壳浅层结构。在充分细致的震相分析前提下,采用经典的时间场法和算术平均法来反演,得到了天水地震区结晶基底界面以上层的浅层结构和速度分布。结果表明,该区基底界面上覆沉积层平均速度4.17公里/秒,基底界面速度平均为5.84公里/秒。武山炮点西支上覆沉积层速度偏低,平均速度只有3.88公里/秒,界面速度只有5.59公里/秒。秦安炮点的南支界面速度偏高,上覆沉积层平均速度为4.21公里/秒,界面速度达6.26公里/秒。天水地震区基底面的埋深平均只有1.9公里,埋深最浅的点在成县附近,出露地表,最深的点在灵台附近,基底界面的埋深达5公里左右。该区切割基底界面断裂较多,其中以灵台西,秦安附近和天水附近为最多,断距也大。

Abstract:

The shallow crust structure beneath Tianshui earthquake area has been studied by the Pg data of seven inversion groups aquired in Chengxian-Xiji and Lingtai-Amuquhu deep seismic sounding profile.On the premise of careful phase analysis,the shallow structure and velocity distribution of layer above basement interface is obtained by applying classical methods of time section and arithmetic mean.The result show that the average velocity of sedimentary cover above basement interface in this area is 4.17km/s.The speed of basement is 5.84km/s.The sedimentary cover at western of Wushan shot is lower with average velocity 3.88 km/s,interface velocity is 5.59 km/s.The interface velocity at south of Qinan shot is higher with velocity 6.26 km/s and average velocity of sedimenary cover is 4.21 km/s.The average buried depth of basement in Tianshui earthquake area is 1.9 km.The most shallow point is in the vicinity of Chengxian.The deepest point is in the vicinity of Lingtai,the buried depth of basement is about 5.0 km.There are many fractures which dissect basement in this area,especially in western of Lingtai and in the vicinity of Qinan and Tianshui are much many with large faults throw.