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天然地震后续PmP震相的射线路径两次穿越地壳内部,能够极大地提高射线的交叉和覆盖程度,对改善地壳内部结构的成像精度具有重要作用.本文首先对不同震源深度的天然地震后续PmP震相走时特征进行了理论研究,发现随着偏移距的增加,不同震源深度的天然地震后续PmP震相的理论走时差逐渐地变小;并且将PmP震相与初至P波震相的到时差进行了对比分析,其结果显示当偏移距小于约120km时初至震相为来自地壳内部的折射震相(Pg震相),震源深度越浅,两震相的走时差越大;而当偏移距大于约150km时初至震相为来自上地幔的折射震相(Pn震相),震源深度越浅,两震相的走时差却越小.然后根据不同震源深度的PmP震相走时特征、波形振幅特征和粒子运动轨迹特征,以日本东北活火山地区为例,从地震波形中拾取到了394个PmP震相数据和3356个初至P波数据.使用这些走时数据,分析了PmP数据对日本东北活火山地区地壳结构成像精度的改善程度,发现加入PmP震相后,地壳内部的射线交叉程度得到很大的改善,地壳尤其是下地壳结构成像的精度得到了有效提高,获得了清晰的火山岩浆上升通道,且岩浆上升过程中产生的低频微震均分布在该低速通道的周边,具有很好的耦合关系.这说明了天然地震后续PmP震相在地壳精细结构成像中能发挥极其重要的作用.
After the natural earthquake, the ray path of the subsequent PmP seismic phase traverses the interior of the crust twice, which can greatly enhance the cross and coverage of the ray, which plays an important role in improving the imaging accuracy of the crustal structure.In this paper, We found that with the increase of offset distance, the theoretical travel time difference of PmP phase of natural earthquake with different depths of source gradually decreases. The results show that the first strike phase is the refractile phase (Pg phase) from the crust when the offset is less than about 120 km. The shallower the focal depth, the greater the time lag between two phases. However, When the offset is greater than about 150km, the first strike phase is the refractile phase (Pn phase) from the upper mantle, and the shallower the focal depth, the smaller the travel time difference between the two phases. Then according to the PmP phase Travel time characteristics, amplitude characteristics of the waveform and trajectories of the particles, taking the active volcanic area in northeastern Japan as an example, 394 PmP phase data and 3356 first arrival P-wave data were picked up from the seismic waveforms. Using these travel time data, PmP data to improve the imaging accuracy of the crustal structure in the active volcanic area of northeastern Japan, found that the degree of ray intersection inside the crust was greatly improved after the PmP seismic phase was added, and the accuracy of crustal crust imaging, especially the imaging of the lower crust, was effectively enhanced. The clear volcanic magma ascending channel, and the low frequency microseism generated during the magma rising are all distributed around the low-speed channel, which shows that the natural seismic follow-up PmP phase can play a role in crustal fine structure imaging Extremely important role.