宜宾-金川剖面地壳上部速度结构及构造含义

Upper Crustal Velocity Structure of the Yibin-Jinchuan Profile and Its Implications

利用有限差分和时间项两种方法对宜宾-金川人工地震剖面沿线8炮Pg波走时进行反演,得到沿测线结晶基底起伏变化特征和上地壳顶部覆盖层速度分布变化特征。结果表明,剖面东南段四川盆地内基底埋深约3～5km,覆盖层较厚,速度较低;剖面西北段川西高原内结晶基底埋藏深度逐渐减浅至0～3km,覆盖层变薄,局部出露地表,速度大部分区段明显增大;沿剖面多个部位基底的埋深出现明显的隆起或下凹,上部地壳速度结构横向出现剧烈的上下起伏变化,结晶基底界面起伏变化强烈的位置分别与该地区分布的龙泉山断裂、名山-成都断裂以及龙门山地区的断裂构造有较好的对应关系,结晶基底速度结构和界面形态变化特征为该区活动断裂构造深浅耦合关系和强震孕育机制研究提供参考。

This paper applies the finite difference travel time inversion and time-term method to model the upper crustal velocity structure and the crystalline basement structure beneath Yibin-Jinchuan profile.The tomographic and time-term method results show that:the basement depth of the Sichuan basin in the southeastern section is about 3~5 km,the unconsolidated sedimentation is very thick,and the shallow velocity is very low.In the northwest section the burial depth of the crystalline basement is gradually reduced by 0~3 km,partly exposed on the surface,and the shallow velocity increases obviously relative to other segments.The crystalline basement depth beneath the profile is obviously heterogeneous.The velocity structure gradual variations obviously correlate with the faults of Longquanshan mountain,Mingshan-Chengdu and the area of Longmenshan mountain.The velocity structure and the interfacial morphological characteristics of the crystalline basement provide seismic evidence for the studies of seismogenic mechanism and earthquake risk assessment in this area.