Johnson Paul A, Savage Heather, Knuth Matt, Gomberg Joan, Marone Chris
Geophysics Group EES-11, Los Alamos National Laboratory of the University of California, MS D443, Los Alamos, New Mexico 87545, USA.
Nature. 2008 Jan 3;451(7174):57-60. doi: 10.1038/nature06440.
It remains unknown how the small strains induced by seismic waves can trigger earthquakes at large distances, in some cases thousands of kilometres from the triggering earthquake, with failure often occurring long after the waves have passed. Earthquake nucleation is usually observed to take place at depths of 10-20 km, and so static overburden should be large enough to inhibit triggering by seismic-wave stress perturbations. To understand the physics of dynamic triggering better, as well as the influence of dynamic stressing on earthquake recurrence, we have conducted laboratory studies of stick-slip in granular media with and without applied acoustic vibration. Glass beads were used to simulate granular fault zone material, sheared under constant normal stress, and subject to transient or continuous perturbation by acoustic waves. Here we show that small-magnitude failure events, corresponding to triggered aftershocks, occur when applied sound-wave amplitudes exceed several microstrain. These events are frequently delayed or occur as part of a cascade of small events. Vibrations also cause large slip events to be disrupted in time relative to those without wave perturbation. The effects are observed for many large-event cycles after vibrations cease, indicating a strain memory in the granular material. Dynamic stressing of tectonic faults may play a similar role in determining the complexity of earthquake recurrence.
地震波引发的小应变如何在远距离触发地震，在某些情况下距离触发地震数千公里，且破坏往往在地震波通过很久之后才发生，这一点仍然未知。通常观察到地震成核发生在10 - 20公里的深度，因此静态上覆压力应该足够大，以抑制地震波应力扰动引发地震。为了更好地理解动态触发的物理过程，以及动态应力对地震复发的影响，我们对有和没有施加声振动的颗粒介质中的粘滑进行了实验室研究。使用玻璃珠模拟颗粒状断层带材料，在恒定法向应力下剪切，并受到声波的瞬态或连续扰动。在此我们表明，当施加的声波振幅超过几个微应变时，会发生对应于触发余震的小震级破坏事件。这些事件经常延迟发生或作为一系列小事件的一部分发生。振动还会导致大滑动事件相对于没有波扰动的情况在时间上被打乱。在振动停止后的许多大事件周期中都观察到了这种效应，表明颗粒材料中存在应变记忆。构造断层的动态应力在决定地震复发的复杂性方面可能起类似作用。
