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尝试用剪切波分裂分析的方法探测地壳内部的线性构造。从地震图上可以看出日本西南部地区存在一些线性构造。在该地区没有发现明显的活断层能证明这些线性构造的存在。我们用两种后续震相,即PpPms和PpSms震相对日本西南部地区的剪切波分裂进行了研究。PpPms是一种后续S波震相,它是P波在地表反射后又入射到地壳中经莫霍面反射转换成的S波。PpSms也是一种后续S波震相,它是P波在地表反射转换成S波后经莫霍面再次反射入射到台站的S波。我们用这两种震相检测了地壳中的线性构造。观测到的快波偏振方向呈现北东东-南西西、北东-南西和南北的横向变化。得到的快波偏振方向与该地区最大主应力方向不一致。最大主应力作用下产生的裂隙引起的各向异性解释不了日本西南部地区的剪切波分裂结果。北东-南西的快波偏振方向与该地区的地质线性构造方向一致。该地区的剪切波分裂有可能是地壳内的线性构造引起的。
Attempt to detect the linear structure inside the crust by means of shear-wave splitting analysis. From the seismogram, we can see that there are some linear structures in southwestern Japan. No obvious active faults were found in this area to prove the existence of these linear structures. We studied the shear-wave splitting of two subsequent phases, PpPms and PpSms, relative to the southwestern Japan. PpPms is a follow-up of S-wave phase, which is the S-wave converted by P wave into the earth’s crust after the P wave is reflected by the surface and then reflected by the Moho. PpSms is also a follow-up of the S-wave phase, which is the S-wave of the P-wave that is reflected by the Moho back into the station after the surface reflection is converted into an S-wave. We use these two phases to examine the linear structure in the crust. The observed fast wave polarization direction shows the horizontal variation of NE-SWS, NE-SWN and NS. The polarization direction of the fast wave obtained is not consistent with the direction of the maximum principal stress in the area. The anisotropy due to the fracture caused by the maximum principal stress can not explain the shear-wave splitting result in southwestern Japan. The fast-polariza- tion directions of NE-SW are consistent with the geologic linear tectonics in this area. Shear wave splitting in this area may be caused by the linear structure in the crust.