On the study of earthquake triggering:Solution to paradox that Coulomb stresses increase with frictional coefficients
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摘要: 近年来,通过计算库仑破裂应力变化研究地震触发及断层的相互作用,进而估计地震灾害已经成为国际上研究的热点.研究中,为考察库仑模型触发地震的效果,计算时往往要改变模型参数进行检验,特别是让有效摩擦系数从0.0到0.8之间变化.许多研究人员的计算结果表明,库仑破裂应力随着摩擦系数的增加而增大,即断层上摩擦系数的增大可以导致触发地震能力的提高.这显然与我们的常识相违背:摩擦总是阻碍断层滑动、抑制地震发生的,即断层面上的摩擦越大,地震越是难以被触发.文中通过对库仑破裂应力的计算公式进行详细分析后发现,之所以出现摩擦越大,地震越容易被触发的现象,其原因是研究者在计算中没有考虑在构造应力作用的环境里,摩擦系数本身的变化所带来的附加库仑应力变化.若某个地震使一个位于地下15 km的典型断层面上的正应力增加2 MPa,如果只考虑静岩压力,当摩擦系数从0.3增大到0.4后,传统库仑破裂应力变化为0.8 MPa;而综合库仑应力变化则大约为-39.2 MPa.所以,若从整体上来分析断层在地震位错及摩擦系数变化所造成的综合库仑应力改变,就不可能出现库仑应力随摩擦系数增加而增加的不正常现象.由此可见,今后在利用库仑模型研究地震触发问题时,应综合考虑构造应力场及摩擦系数本身变化所带来的库仑应力变化.Abstract: Coulomb stress change calculation has been playing an important part in investigating fault interactions and earthquake triggering. However, the results of most workers showed that Coulomb stress changes(or earthquake triggering effects) would become larger and larger with the increase of apparent frictional coefficients. This phenomenon is clearly in contradiction with our common knowledge in which frictional stress should resist fault slip and inhibit earthquakes under any circumstances. By analyzing the formula for calculating Coulomb stress changes(ÄCFS), we found that previous research did not take into account the additional ÄCFS which are only resulted from the variations of frictional coefficients. Suppose the depth of typical receiver fault is 15 km, the value of combined ÄCFS will be as large as about 39.2 MPa when the variation of apparent friction coefficient is 0.1(e.g., from 0.3 to 0.4), whereas traditional ÄCFS is only 0.8 MPa. If we incorporated the additional ÄCFS in calculation, the above contradiction will disappear completely. Therefore, it is suggested that we should consider changes of combined ÄCFS due to the variation of the friction coefficient, especially when we compare different Coulomb stress models with different apparent frictional coefficients.
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