基于地震探测的盐源盆地构造特征及动力学意义

Structural characteristics of Yanyuan basin deduced from seismic survey and its dynamic implication

基于覆盖盐源盆地的短周期天然地震台阵和布设的一条人工地震测线所获得的地震数据,从中提取地震能量属性,并通过地震层析成像获得该地区的浅部地震速度结构,继而对短周期地震台阵一个月的噪声数据进行互相关得到经验格林函数,再通过时频分析获得相速度频散曲线,反演获得不同深度的S波速度分布。研究结果显示,盐源盆地地震特征主要分为三层:浅部为低速新生代沉积地层,P波速度为1.0—2.1 km/s,反射轴明显、连续性好,反射能量强,S波速度为中高南低,南部低速体与盐源断裂走向一致,新生代盆地整体呈南深北浅,沉积发育和构造形态受盐源断裂控制;中部为中低速三叠系地层,P波速度为3.5—4.5 km/s,反射轴不连续,反射能量较弱,S波速度逐渐变大,整体变化变小;深部为高速古生代地层,地震反射较为杂乱,反射能量弱;上地壳存在滑脱面,该界面为沉积盖层与结晶基底的分界面,且向浅部发育一系列断层,其中金河—箐河断裂为盐源盆地与康滇地块的分界线,这些断裂带也是盐源盆地地震频发的部位。

In this paper,an artificial seismic line and a short period natural seismic array covering Yanyuan basin are set up.The seismic energy attribute is extracted from the artificial seismic data,and the seismic velocity structure is obtained by seismic tomography.Meanwhile the S-wave velocity distribution at different depths is obtained by inverting the empirical Green’s functions retrieved from cross-correlation of the ambient noise data recorded by the short period seismic array.The results show that Yanyuan basin is mainly divided into three layers by seis-mic characteristics.The shallow layer is the Cenozoic sedimentary stratum with low P wave velocity ranging from 1.0 km/s to 2.1 km/s,and it is characterized by good continuity and strong reflection.The S wave velocity is low in the south and high in the middle of Yanyuan basin,and the trend of the low-velocity anomaly in the south is in agreement with the strike of Yanyuan fault.Cenozoic basin is thick in the south and thin in the north,where the sedimentary and structure are controlled by Yanyuan fault.The middle layer is medium-low P wave velocity ranging from 3.5 km/s to 4.5 km/s,and it is characterized by discontinuous reflection axis and weak reflection,which can be interpreted as Triassic strata.The S-velocity becomes higher gradually and the change becomes smaller in general.The deep layer is high velocity,charac-terized by chaotic seismic reflection and weak reflection energy,interpreted as Paleozoic strata.Furthermone,there is a detachment surface in the upper crust,which is the interface between sedimentary and basement.A series of faults are developed from the detachment to the surface,and these faults are also an earthquake-prone area in Yanyuan basin.One of the faults is Jinhe-Qinghe fault,which is the boundary fault between Yanyuan basin and Kangdian block.