岩石圈结构对大地震震后形变的影响——以1976年唐山大地震和2001年昆仑山大地震为例

大地震发生之后通常会诱发一系列的余震序列，对比1976年M_S7.8唐山大地震和2001年M_S8.1昆仑山大地震周边区域的地震事件可以看出，唐山大地震余震活动时间要明显长于昆仑山大地震余震活动时间.余震序列往往与震后形变密切相关，而影响震后形变的因素不仅与地震发震断层和震级有关，同时与岩石圈的结构有关.考虑到唐山大地震的发震区华北地块和昆仑山大地震的发震区青藏高原有着较大的岩石圈结构差异，本文采用PSGRN/PSCMP软件计算了岩石圈分层模型的大地震同震和震后形变，分析了地壳弹性模量、弹性厚度以及黏滞性系数对同震和震后形变的影响，进而讨论了影响唐山地震和昆仑山地震余震序列差异的原因.计算结果显示，震后形变会在黏弹性效应的作用下逐渐调整，震后形变的持续时间与地壳弹性模量、地壳弹性厚度和下地壳黏滞性系数有关.上地壳和下地壳弹性模量越大，震后形变达到稳定值的时间越短，弹性模量对震后形变稳定值影响很小.地壳弹性厚度越大，震后形变达到稳定值的时间越短，当断层面底端深度小于地壳弹性厚度时，地壳弹性厚度的增加会引起震后形变稳定值的减小；下地壳厚度对震后形变达到稳定值的时间和稳定值基本无影响.下地壳黏滞性系数越大，震后形变达到稳定值的时间越长，反之亦然.结合唐山地震区的华北地块和昆仑山地震的青藏高原深部结构发现，两者之间的上地壳弹性模型差别不大，唐山地震区地壳弹性厚度略大于昆仑山地震区，但昆仑山地震区下地壳黏滞性系数明显低于唐山地震区.这些因素均决定了昆仑山地震的震后形变持续时间短（余震时间序列短）而唐山地震的震后形变持续时间长（余震时间序列长）.由此可见，岩石圈结构差异可能是导致唐山地震和昆仑山地震余震序列差异的主要因素之一.

The effect of lithospheric structure on the seismic deformation-taking the 1976 Tangshan earthquake and 2001 Kunlunshan earthquake as an example

A large earthquake always induces a series of aftershocks. Tangshan earthquake occurred in 1976 with a magnitude of M_S7.8 and Kunlunshan earthquake occurred in 2001 with a magnitude of M_S8.1. Contrasting the seismic events occurred in these two earthquake regions, we find that the seismic sequence of Tangshan earthquake is clearly longer than that of Kunlunshan earthquake. The aftershock sequence is always associated with postseismic deformation. Factors affecting postseismic deformation include not only the seismogenic faults and the magnitude of earthquake, but also the lithospheric structure. The Tangshan earthquake and Kunlunshan earthquake occurred in the North China block and Tibetan Plateau, respectively. The lithospheric structure of these two regions is quite different. By changing the elastic modulus of crust, the elastic thickness of the crust and the viscosity of the lower crust, we calculated the coseismic and postseismic deformation by PSGRN/PSCMP software. The calculated results help us to analyze the effect of these factors on the postseismic deformation. The results showed that the postseismic deformation was adjusted gradually because of the viscoelastic relaxation. The relaxation time has relationship with the elastic modulus of crust, the elastic thickness of the crust and the viscosity of the lower crust. The larger elastic modulus of the upper crust and the lower crust corresponds to the shorter relaxation time. The elastic modulus of the crust has little influence on the steady value. The thicker the elastic thickness of the crust corresponds to the shorter relaxation time. When the bottom of fault is not deeper than the elastic thickness of the crust, increasing the elastic thickness of the crust will cause the steady value of the postseismic deformation decreased. The thickness of the lower crust has little influence on the relaxation time and steady value. The higher viscosity of the lower crust corresponds to the longer relaxation time. Combining the lithospheric structure of the North China block and Tibetan Plateau, there is little difference in the crustal elastic modulus between them. Comparing with the Kunlunshan region, thicker elastic thickness for upper crust and larger viscosity for the lower crust are found in the Tangshan earthquake region. These factors cause that the Kunlunshan earthquake has shorter relaxation time (corresponding to the shorter seismic sequence) and Tangshan earthquake has longer relaxation time (corresponding to the longer seismic sequence). Thus, the differences in lithospheric structure should be one of the main factors that cause huge differences in the aftershock sequence between Tangshan earthquake and Kunlunshan earthquake.