Institute of Earthquake Prediction Theory and Mathematical Geophysics, 84/32 Profsouznaya, Moscow, 117997, Russia.
Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Univ Paris Diderot, UMR 7154 CNRS, 1 rue Jussieu, 75238, Paris Cedex 05, France.
Nat Commun. 2017 Nov 6;8(1):1317. doi: 10.1038/s41467-017-01446-y.
Characterising the state of stress in the brittle upper-crust is essential in mechanics of faulting, industrial production processes, and operational earthquake forecasting. Nevertheless, unresolved questions concern the variation of pore-fluid with depth and the absolute strength on tectonically active faults. Here we show that, along the San Andreas fault system, the time-delay before the onset of the power-law aftershock decay rate (the c-value) varies by three orders of magnitude in the first 20 km below the surface. Despite the influence of the lithostatic stress, there is no continuous change in c-value with depth. Instead, two decay phases are separated by an abrupt increase at an intermediate depth range of 2-5 km. This transitional regime is the only one observed in fluid-injection-induced seismic areas. This provides strong evidence for the role of fluid and a porosity reduction mechanism at depth of few kilometres in active fault zones. Aftershock statistics can then be used to predict changes in differential shear stress with depth until the brittle-ductile transition is reached.
研究脆性上地壳的受力状态对于断层力学、工业生产过程和地震预测都至关重要。然而，目前仍有一些悬而未决的问题，如孔隙流体随深度的变化以及构造活跃断层的绝对强度。在这里，我们发现，沿着圣安德烈亚斯断层系统，在地表以下 20 公里范围内，幂律余震衰减率（c 值）开始之前的时间延迟变化幅度高达三个数量级。尽管受到岩石静压力的影响，但 c 值并没有随深度连续变化。相反，两个衰减阶段之间在 2-5 公里的中间深度范围内突然增加，被一个过渡阶段隔开。这种过渡状态是仅在流体注入诱发地震区观察到的。这为流体和几公里深处的孔隙率降低机制在活动断层带中的作用提供了有力证据。然后，可以利用余震统计数据来预测随深度变化的剪切应力差，直到达到脆性-韧性转变。
