龙门山和安宁河断裂带周边地区的地壳三维P波速度结构

Three-dimensional crustal velocity structure around Longmenshan and Anninghe fault zones

利用2008—2021年区域地震台网及部分流动地震台阵的近震走时观测资料,采用双差层析成像方法获得了龙门山断裂带和安宁河断裂带周边地区的高分辨率地壳三维P波速度结构。结果显示:四川盆地浅部呈明显的低速异常,在前陆盆地低速异常可一直延伸至约15 km深,推测与该地区接收大量来自青藏高原东缘的沉积物有关;在龙门山断裂带附近,中上地壳存在一些与断裂带平行的高速异常体,它们较好地揭示了该地区穹隆体和逆冲推覆体深浅部的分布特征,例如,位于龙门山断裂带中部的彭灌穹隆体下延深度可达15 km左右,而与之相邻的雪隆山穹隆体下延深度不超过5 km。在安宁河断裂带与大凉山断裂带之间存在震源深度超过20 km的微震活动带,其分布形态与P波高速异常的分布形态基本一致,推测高速异常体的存在可能导致地壳内部脆韧性转换带深度的增加,并且受安宁河与大凉山交会部位强烈变形的影响,该地区出现深度较大的地震活动。上述结果为认识该地区的深部地质构造和地震活动机制提供了新的依据。

The Anninghe and Longmenshan fault zones are the eastern boundary of the Tibetan Plateau,and they are also important strong earthquake active zones.The study of the deep structure of the fault zones and surrounding areas is of great significance to understand the dynamic process of the eastward expansion of the Tibetan Plateau and the tectonic mechanism of strong earthquake.In this paper,based on the seismic travel time data from regional seismic network and temporary seismic arrays from 2008 to 2021,the high-resolution 3D crustal P-wave velocity structure around the Longmenshan and Anninghe fault zones is obtained by double difference tomography.There are obvious low-velocity anomalies in the shallow part of the Sichuan Basin,and the low-velocity anomalies can extend to about 15 km near the foreland basin,which is related to the foreland basin receiving sediments from the eastern margin of the Tibetan Plateau.Near the Longmenshan fault zone,there are some high-velocity anomalies in the upper to middle crust parallel to the fault zone,which reveals the spatial distribution of domal complexes and thrust complexes in this area.For example,the Pengguan domal complex located in the middle of the Longmenshan fault zone extends down to a depth of about 15 km,while the adjacent Xuelongshan domal complex extends down to a depth not deeper than 5 km.There is an active microseismic zone with a focal depth deeper than 20 km between the Anninghe and Daliangshan fault zones,which is consistent with the distribution of P-wave high-velocity anomaly,therefore we speculate that the existence of high-velocity anomalies may increase the depth of the crustal brittle-ductile transition zone,and deep seismicity appeared in this region due to the strong deformation at the intersection of the Anninghe and the Daliangshan fault zones.The research results of this paper provide new information for further understanding of the deep geological structure and seismic activity mechanism in this area.