利用密集台阵近震层析成像研究云南宾川上地壳速度结构

Upper crustal velocity structure of Binchuan,Yunnan revealed by dense array local seismic tomography

滇西北宾川盆地是发育于红河断裂和程海断裂交汇处的晚新生代张扭断陷盆地,该区活动断裂发育且历史地震比较活跃.对宾川盆地及邻区进行高精度浅层和上地壳精细结构研究,有助于深入认识该区主要发震构造的深浅部特征.基于2017年在宾川盆地及其附近开展的为期2个月的密集台阵观测数据,我们对该区96个小震共拾取了117221条初至P波和5475条初至S波震相,并利用simul2000开展了地震重定位和体波层析成像研究.结果表明:(1)小震活动主要集中在宾川盆地东缘断裂的弧形转折部位,并在洱海南侧呈现沿北东向断裂的条带状分布现象,反映了区域上近南北向至北东向断裂是主要控震构造,其次是北西向断裂带.(2)0 km的速度分布与区域地形有很好的对应关系.山地呈现高速异常,宾川盆地呈现低速异常.从3 km至9 km,高低速分界与断层有很好的对应,并且越往深部,近南北向至北北东向的宾川盆地东缘断裂在上地壳构造的控制作用越明显.(3)上地壳层析成像结果同时揭示了宾川盆地东缘断裂的三维形态变化在空间上呈现出南北部倾角大、中部倾角缓的变化特征,可能与区域地块的旋转变形过程有关.(4)综合高精度浅层速度结构和地震重定位结果可知,区域上的近南北向至北北东向断裂正逐步取代北西向构造,成为主要的区域分界断裂和控震构造.新的研究结果为深入理解该区的主要控震构造及其深部结构特征提供了重要依据.

The Binchuan basin,located in northwestern of Yunnan Province,is a large Late Cenozoic rift basin at the intersection of the Red River fault and Chenghai fault.The faults are well developed and the seismicity is very active in this area.Studying the fine structure of the upper crust and basin in Binchuan is important to understand the characteristics from deep to shallow of the main seismic structures in this area.Based on the dense array observation experiment around Binchuan basin for two months in 2017,we picked 117221 P-wave and 5475 S-wave first arrivals from 96 small local earthquakes,and performed earthquake relocation and body-wave tomography using the simul2000 program.The obtained results show that(1)the earthquakes in the Binchuan basin are concentrated at the turning area of the Binchuan basin eastern margin fault and show a banding distribution along the NE trending fault on the southern edge of the Erhai Lake,which indicate that NS-NE trending faults are the main seismic control structures and the NW trending faults are the secondary.(2)The velocity distribution at 0 km has a good correspondence with local topography,with high-velocity anomalies in the mountains and low-velocity anomalies in the Binchuan basin.From 3 km to 9 km,the high-low velocity boundaries correspond well to local faults.The controlling effect in the upper crustal structure of the Binchuan basin eastern margin fault(near NS-NNE trending)becomes more and more obvious with depth.(3)Our results also reveal that the dipping angle of Binchuan basin eastern margin fault is large in the northern and the southern portions,however,is gentle in the middle portion,reflecting the rotation characteristics of the regional tectonic block.(4)Relocation and tomographic results indicate that near NS and NNE trending faults are gradually replacing the NW trending faults as the main boundary faults and the main seismic control structures.Our new study results provide an important reference to better understand the main seismic-controlling structures and their deep structural features.