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2005年8月,瑞士中部在12小时时段内出现了包含47次事件的异常地震序列。这些地震出现在降雨量超过300mm的持续3天暴雨结束的时候,最强的地震活动分别丛集在瑞士穆奥塔塔尔和里曼斯塔登地区,它们都是降雨量特别大的著名喀斯特地貌地区。与背景活动相比明显增加的地震活动,以及在突降暴雨与地震活动之间的短时延迟,有力地证明了地震是由降雨所触发。在我们的模型中,由大量降雨引起的地表流体压力的增加,导致深处局部孔隙压的增加。孔隙流体压力的增加会通过抵消正应力来减少渗水介质的剪应力,最终引发地震。瑞士中部的触发地震序列出现在以往地震活跃的地区,显示了相似的震中位置和震级。这表明这些地震发生在处于临界应力状态的已有断层上。我们将降雨模拟为地表流体压力逐步递增,且向深部传播满足均匀无限半空间一维扩散的过程。这使得我们可以依据触发地震震源深度的时间变化估计水压扩散率。结果表明,研究区域的水压扩散率数值在0.01～0.5m2s-1范围内。这些数值很好地吻合了前人流体触发地震的研究结果,并支持已观测到的地震序列由大量降雨所触发这一认识。
In August 2005, a series of unusual earthquakes with 47 events occurred in central Switzerland over a 12-hour period. These earthquakes occurred at the end of a three-day heavy rainfall event with a rainfall of over 300 mm. The strongest earthquakes were clustered in the regions of Muat Tatar and Remansstad, Switzerland, all of which are well-known areas of karst rainfall. The significant increase in seismicity compared to background activity and the short delay between sudden heavy rainfall and seismicity strongly suggest that the earthquake was triggered by rainfall. In our model, the increase in surface fluid pressure caused by heavy rainfall leads to an increase in the local pore pressure in the depths. The increase of pore fluid pressure will reduce the shear stress of seepage medium by counteracting the normal stress and finally trigger the earthquake. Triggered seismic sequences in central Switzerland appear in previously active seismic regions and show similar epicentral location and magnitude. This indicates that these earthquakes occurred on the existing faults in the state of critical stress. We simulate rainfall as a stepwise increase in surface fluid pressure and its propagation to the deep to satisfy one-dimensional diffusion in a uniform and infinite half-space. This allows us to estimate the hydraulic diffusivity based on the time variation that triggers the focal depth of the earthquake. The results show that the hydraulic diffusivities in the study area are in the range of 0.01-0.5 m 2 s -1. These values are in good agreement with the findings of previous fluid-triggered earthquakes and support the recognition that the observed seismic sequences were triggered by heavy rainfall.