大地震对地球自转的影响

Changes in the Earth's rotation induced by large earthquakes

地震会改变地球的形状,引起内部物质的重新分布,从而导致地球的惯性张量产生变化.依据角动量守恒定律,地球的自转将会发生变化.为了探究大地震对地球自转的影响,本文基于简正模和点源位错两种不同的理论,应用PREM地球模型,以2000年至今的10个8.3级以上的大地震为例,分别计算了地震发生后地球重力场的斯托克斯系数、地球的总惯性张量、地球扁率等的变化.讨论了简正模方法叠加的收敛性问题,并对震级的影响进行了简单的分析,着重探究了深度变化对地球极移和日长变化的影响.结果表明:在相同的地球模型下,利用简正模和位错两种理论计算的日长和极移的结果是一致的;简正模方法的收敛速度比较快,只需叠加3个左右的低频简正模就已经收敛;对于震级大于4.5级的地震,极移激发的方向出现在120°E^160°E的概率大一些;通常情况下,越深源的和震级较大的地震,极移激发的幅度和日长变化量越大.此外,在0~700 km深度区间内,随着深度的增加,极移激发的幅度和日长变化总体表现为增加的趋势.

An earthquake changes the shape of the Earth,causing the redistribution of internal material,which alters the Earth ＇s inertial tensor. According to the law of conservation of angular momentum,the rotation of the Earth will change correspondingly.To study the effects of large earthquakes on the Earth＇s rotation,in this paper we used the PREM earth model to calculate changes in the Stokes coefficients,the total inertial tensor and in the Earth＇s oblateness caused by ten large earthquakes with MW≧ 8. 3 since 2000 based on the theory of normal mode and the point dislocation methods. Besides,we discussed the convergence problem of normal mode superposition,also the effect of the earthquake magnitude and the influence of the focal depth change on the polar motion excitation and the length of day. The results show that the changes of the Earth＇s rotation calculated by the normal mode and the point dislocation methods are in good agreement. The convergence of the normal mode method is very fast. Summation of about 3 lower frequency modes is sufficient to make the result convergent. Polar motion excitation due to earthquakes with magnitude greater than 4. 5 is mostly in the direction between 120° E and 160° E,which is consistent with the previous results. In general,the earthquakes with deeper focal depth and larger magnitude cause greater impact on the polar motion excitation and length of day. In addition,with the increase of focal depth,the amplitude of polar motion excitation and the length of day increases totally within the depth range of 700 km.