Revisiting the Tidal Activation of Seismicity with a Damage Mechanics and Friction Point of View

如果地震成核是一种严格的应力阈值效应，那么人们就会期望潮汐应力峰值时间与区域地震活动性之间存在相关性。统计上严格的检验未能发现这种关系，对成核过程造成了重要的限制。对这种未能观察到相关性的解释通常涉及加速的自驱动成核过程，该过程发生在比日潮更长的时间尺度上，从而破坏了预期的同步。虽然没有观察到潮汐和地震之间的一般相关性，但最近有报道说，在地震发生前的几年里，在一个有限大小的按震级划分的地区，观察到地震活动与潮汐之间的相关性。这种突现的相关性被解释为区域地震活动性的“临界点模型”的结果。根据临界点模型，地震一旦形成核，只有当地壳处于以长程应力相关为特征的“临界状态”时，才能跳过障碍变大。其理由是地壳在接近临界状态时对潮汐应力扰动更敏感。如果这是真的，地震活动性与潮汐之间的相关性提供了另一种工具（除了加速地震力矩之外）来监测断层网接近临界状态，以及随之而来的大地震的可能性。在大地震发生之前，对日潮汐和地震活动之间的紧急相关性的重现报道的初步尝试还没有成功。在回顾了支持紧急日相关的已发表的理论论据后，没有理由认为，即使在临界状态下，它也不应该被自驱动的延迟成核所破坏。然而，对该理论的回顾导致对地震活动性潮汐激活的新假设的形成：潮汐激活仅在构造和潮汐应力之和超过先前最大值时发生（类似于声发射中的凯撒效应）。在这种情况下，太阳和月球潮汐之间的15天周期预测了一个强烈的15天相关性。假设破坏日相关性的延迟成核可能对新的较长周期没有那么大的影响。该项目正在开发合适的观测技术，以优化对这种15天准周期信号的探测，并在各种构造环境下的一系列大地震之前寻找它。同时开发一个合适的零假设和检验的统计意义的任何信号，我们发现。即使没有观测到15天的信号，统计上显著的负结果也会对地震成核的物理学产生重要的限制。用元胞自动机研究了循环扰动对均匀驱动系统接近临界状态的影响。奥拉米-费德-克里斯滕森非保守自动机发展出导致临界状态的长期应力相关性。速率和状态相关的摩擦被添加到自动机的失效准则中，以产生自驱动的延迟形核。虽然这个自动机在模拟断层之间的远程弹性相互作用方面并不现实，但它为正在探索的两个假设提供了一个合适的测试平台：1)潮汐激活是否在临界状态附近变得更强，以及2)与速率和状态相关的摩擦相关的延迟成核是否预计会减弱15天相关性，其程度与日相关性相同。

If earthquake nucleation were strictly a stress-threshold effect, then one would expect a correlation between times of peak tidal stress and regional seismicity. Failure of statistically rigorous tests to find such a relationship places important constraints on the nucleation process. Explanations of this failure to observe correlation generally involve an accelerating self-driven nucleation process that occurs on a time scale longer than the diurnal tide thereby destroying the expected synchronization.Although a general correlation between tides and earthquakes is not observed, it has recently been reported that a correlation of seismicity with tides is observed to develop during the few years before an earthquake in a finite sized region that scales with the magnitude. This emergent correlation has been interpreted as a consequence of the "critical point model" for regional seismicity. According to the critical point model, an earthquake, once nucleated, can jump barriers to grow large only when the crust is in a "critical state" that is characterized by long-range stress correlation. The rational is that the crust is more sensitive to tidal stress perturbations when it is near the critical state. If true, the emergent correlation of seismicity with tides offers another tool (in addition to accelerating seismic moment) for monitoring the approach of a fault network to the critical state, and the attendant possibility of a large earthquake.A preliminary attempt to reproduce the reported observations of emergent correlation between diurnal tides and seismicity before large earthquakes has not been successful. After reviewing published theoretical arguments in support of emergent diurnal correlation, there is no reason why, even in the critical state, it should not be destroyed by self-driven delayed nucleation. However, review of the theory leads to formulation of a new hypothesis for the tidal activation of seismicity: tidal activation occurs only when the sum of tectonic plus tidal stress exceeds the prior maximum value (analogous to the Kaiser effect in acoustic emissions). In this case the 15-day beat between solar and lunar tides predicts a strong 15-day correlation. The delayed nucleation that is hypothesized to destroy the diurnal correlation may not have as large an effect on the new longer cycle.This project is developing suitable observational techniques to optimize detection of this 15-day quasiperiodic signal and searching for it before a suit of large earthquakes in a variety of tectonic settings. also developing a suitable null hypothesis and tests for the statistical significance of any signals we find. Even if the 15-day signal is not observed, a statistically significant negative result places important constraints on the physics of earthquake nucleation.The effects of cyclic perturbations on a uniformly driven system as it approaches a critical state using a cellular automaton are being investigated. The Olami-Feder-Christensen non-conservative automaton develops long-range stress correlations leading to a critical state. Rate- and state-dependent friction is added to the automaton's failure criterion to produce self-driven delayed nucleation. Although this automaton is not realistic in its simulation of long-range elastic interactions between faults, it provides a suitable testbed for the two hypotheses being explored: 1) whether tidal activation becomes stronger near the critical state, and 2) whether delayed nucleation associated with rate and state-dependent friction is expected to attenuate the 15-day correlation to the same extent as the diurnal correlation.