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

如果地震成核严格地说是一种应力门槛效应，那么人们会认为潮汐应力峰值时间与区域地震活动之间存在相关性。统计上严格的测试未能找到这样的关系，这对成核过程造成了重要的限制。未能观测到相关性的原因通常是加速的自驱动成核过程，该过程发生的时间尺度比日潮汐更长，从而破坏了预期的同步性。尽管没有观察到潮汐和地震之间的一般相关性，但最近有报道称，在地震发生前的几年里，观察到地震活动与潮汐的相关性是在地震前几年发生的，其规模随震级而变化。这种新出现的相关性被解释为区域地震活动的“临界点模型”的结果。根据临界点模型，地震一旦成核，只有当地壳处于以长期应力相关为特征的“临界状态”时，才能跳过障碍变大。合理的解释是，地壳在接近临界状态时对潮汐应力扰动更敏感。如果是真的，地震活动与潮汐的紧急关联提供了另一种工具(除了加速地震矩)来监测断层网络接近临界状态的过程，以及随之而来的大地震的可能性。复制大地震前报道的日潮和地震活动之间的紧急关联的初步尝试没有成功。在回顾了已发表的支持新出现的昼夜关联的理论论点后，没有理由说，即使在临界状态下，它也没有理由不被自驱动的延迟成核摧毁。然而，对这一理论的回顾导致了一个关于地震活动潮汐激活的新假说的提出：只有当构造和潮汐应力之和超过先前的最大值(类似于声发射中的Kaiser效应)时，才会发生潮汐激活。在这种情况下，太阳潮汐和月球潮汐之间的15天节拍预示着15天的强烈相关性。被假设为破坏日相关性的延迟成核可能不会对新的较长周期产生那么大的影响。该项目正在开发合适的观测技术，以优化对这种15天准周期信号的检测，并在各种构造环境下的一系列大地震之前寻找它。还开发了一个合适的零假设，并对我们发现的任何信号的统计意义进行了检验。即使没有观测到15天的信号，统计上显著的负结果也对地震成核的物理学产生了重要的限制。人们正在使用元胞自动机研究循环扰动对接近临界状态的均匀驱动系统的影响。Olami-Feder-Christensen非保守自动机发展了导致临界状态的长程应力关联。在自动机的失效准则中加入了与速率和状态相关的摩擦，以产生自驱动延迟成核。尽管这个自动机在模拟断层间的长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试验台：1)潮汐活动是否在临界态附近变得更强，2)与速率和状态相关的摩擦力相关的延迟成核是否会像日相关那样衰减15天的相关性。

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.