大森-宇津定律的一种可能机制——以唐山大地震为例

A possible mechanism of Omori-Utsu's law through an example of the great Tangshan earthquake

为了探讨大森-宇津定律的物理机制,本文在余震区等效黏度远低于其外部,且构造应力场在整个余震活动时间间隔内基本保持不变的假设条件下,提出了一个开尔文黏弹性地震震源体概念模型.该模型可用于模拟主震后断层蠕变和震源区应力调整触发的余震序列以及蠕变停止后余震终结、介质恢复到弹性状态、断层重新闭锁和积累下一次地震的整个过程.有限元方法可用来计算非均匀黏弹性地震震源体模型中主震和每次余震所引起的应力场及其随时间的演化过程.在此基础上,采用开尔文黏弹性地震震源体概念模型和有限元方法模拟了1976年唐山M_S7.8地震余震序列.结果表明:经验的大森-宇津定律可以用开尔文黏弹性震源体模型来解释,这意味着余震衰减的频度取决于蠕变的速率;余震序列持续时间受控于震源体的黏度,即黏度越大,蠕变时间越长,余震持续的时间也就越长.

This paper proposes a conceptual model of earthquake source body with Kelvin viscoelastic property to investigate the physical mechanism of OmoriUtsu＇s law,supposing that tectonic stress field after main shock does not change with time and equivalent viscosity in the aftershock region is much lower than that of its outside in the period of total aftershock activity.This model can simulate aftershock sequence induced by post-seismic creep and stress readjustment,and the whole process including creep stopping,materials recovering to its elastic state,and faulting turning to stick state for next earthquake.Finite element method is used to calculate stress field evolution caused by a main shock and its aftershocks in the model with heterogeneous material properties.Furthermore,the model and the method are used to simulate decay of the aftershock frequency of the 1976 MS7.8Tangshan earthquake.The results show that the mechanism of Omori-Utsu＇s law may be attributed to the stress changes caused by the creep of the fault and earthquake source body,which implies that aftershock frequency depends on the creep rate and decay time of the aftershocks is controlled by the equivalent viscosity.The lager the viscosity is,the longer the creep time or the aftershocks last.