腾冲地区近震S波分裂研究

Shear wave splitting analysis of local earthquakes at Tengchong

本文利用布设在云南腾冲地区的15个固定和流动地震台站记录的近震波形数据,采用剪切波分裂分析方法得到了593对高质量的各向异性分裂参数.结果显示,腾冲火山区地震台站下方的近震各向异性的慢波延迟时间为0.02~0.37 s,平均延迟时间0.2 s.结合已有接收函数地壳各向异性研究结果,推测研究区地壳各向异性的主要贡献源自中上地壳.研究区不同台站的快波偏振方向变化很大,似乎反映了构造和区域应力场的共同作用.其中腾冲火山断裂西侧多数台站的快波偏振方向呈近N-S向,而东部多数台站的快速偏振方向呈NE-SW向,与区域主压应力方向一致,暗示研究区中上地壳各向异性主要是受主压应力引起定向排列的裂隙所致.基于近震走时得到的研究区平均VP/VS为1.68,推测腾冲火山区地壳应力场的局部变化可能与上地壳中富含气体的中酸性岩浆膨胀活动有关.另一方面,在腾冲火山区外围个别台站(MIZ、MZT)观测到了快波偏振方向与主压应力、已知断层等构造走向不一致的现象,暗示其各向异性是构造或构造和区域应力场共同作用的结果.

We have presented 593 pairs of shear wave splitting parameters and crustal V P/V S ration using local seismic data set recorded at 15 permanent and temporary stations deployed at the Tengchong volcano,Yunnan.The results show that the delay time of local seismic anisotrophy in the Tengchong volcanic area vary from 0.02 s to 0.37 s,with an average time of 0.2 s.The comparison of our result and existing crustal anisotropy from receiver functions analysis imply that the main contribution of crustal anisotropy in the study area originates from the upper crust.The significant lateral variations of fast wave polarization direction reflect the combined effect of tectonic and regional stress fields.Among them,the fast polarization direction for most stations on the west side of the Tengchong volcanic fault is near N-S,while the fast polarization direction of most stations in the east is NE-SW.These polarization directions are consistent with the maximum main compressive stress direction,suggesting that the upper crust anisotropy is mainly associated with alignment cracks resulted from the regional stress.The low value of VP/VS rations(1.68)imply that the local variation of the crustal stress field in the Tengchong volcano may be related to the expansion of the gas-rich Intermediate-acid magma in the upper crust.While the fast polarization directions for some stations(e.g.MIZ,MZT)located away from the Tengchong volcanic area is inconsistent with both the direction of fault nearly and the main compressive stress,implying the combination effect of both fault and regional stress field.