美国Cove Fort-Sulphurdale地热区地震震源机制和各向异性研究

Focal mechanisms and seismic anisotropy analysis of local earthquakes at the Cove Fort-Sulphurdale geothermal site,United States

Cove Fort-Sulphurdale位于美国犹他州中西部,是一个地质特征独特的地热资源富集区.为了有效开发地热资源,有必要了解该地区的应力分布和裂缝分布.本文在前人工作的基础上使用地震资料,利用波形匹配、P波极性和横纵波振幅比联合约束反演的方法,研究该地区地震的震源机制解;同时结合该地区的横波分裂分析研究,确定该地区的应力分布和断层/裂隙走向.波形反演结果显示,实际数据与模拟数据的波形匹配度非常高,且二者的P波初动极性和横纵波振幅比也都非常相近.由震源机制分析得出的断层面走向大部分趋于南北向,与美国区域应力图显示的最大水平主应力指向(南北方向),具有较好的一致性,符合目前研究对该地区的构造认知.横波分裂分析结果也表明如果各向异性主要是由该地区的断裂构造所引起,那么该地区的主要断层或裂缝的方向可能为南北向.

Cove Fort-Sulphurdale is located in central-western Utah,which is characterized by its unique geologic features and rich geothermal resources.For better development of geothermal reservoirs,it is necessary to have the knowledge of stress and fracture distribution of the region.Based on the previous work,we use the waveform matching method to invert the focal mechanisms of local earthquakes collected in Cove Fort-Sulphurdale,constrained by first arrival P wave polarities and S/P amplitude ratios.Furthermore,we also use the shear wave splitting analysis method to study the anisotropy in this region.Combining both focal mechanism and anisotropy results,we can determine the stress distribution and fault/fracture directions.The results show that the waveforms of synthetic data match real data very well,and the first arrival P wave polarities and S/P amplitude ratios are close.Fault plane solutions have strikes trending in approximately N--S direction,which is consistent with the maximum horizontal principle stress indicated by the stress map of America and our current understanding of the tectonics in this region.Shear wave-splitting analysis results also show that if the anisotropy is mainly caused by the fault structure in this region,the strikes of the main faults/fractures should be in the N--S direction.