Earthquake Initiation From Laboratory Observations and Implications for Foreshocks
Earthquake Initiation From Laboratory Observations and Implications for Foreshocks
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DOI：
10.1029/2019jb018363
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发表时间：
2019-12-04
影响因子：
3.9
通讯作者：
McLaskey, Gregory C.
中科院分区：
文献类型：
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作者：
McLaskey, Gregory C.
本文回顾了地震起爆的实验室观测，并描述了在3米岩石样品上进行的新实验，其中详细成像了成核过程。许多实验室观测结果与先前的工作一致，表明地震成核过程缓慢而平稳地加速，扩展到临界成核长度尺度L-c，然后迅速加速到动态断层破裂。实验还突出了目前大多数理论和数值模型没有考虑到的复杂性。这包括加载速率依赖，其中“踢”高于稳态产生更小和更突然的启动。断层强度的非均匀性也会导致蠕变锋面在比周围断层稍强的粘滞块上合并时的突然起爆。综上所述，这两种机制为地震的发生提出了一个速率相关的“级联上升”模型。该模型同时考虑了前震是更大的成核过程的副产品，以及小地震和大地震的初始P波特征之间的相似性。地壳的成核条件多种多样，从L-c < 1 m的滑移限制环境到L-c bb0 ~ 10 km的点火限制环境。在后一种情况下，L-c不能完全描述起爆过程，因为地震成核不是因为滑动块达到临界长度，而是因为断层滑动率超过了引发动态破裂所需的临界功率密度。
This paper reviews laboratory observations of earthquake initiation and describes new experiments on a 3-m rock sample where the nucleation process is imaged in detail. Many of the laboratory observations are consistent with previous work that showed a slow and smoothly accelerating earthquake nucleation process that expands to a critical nucleation length scale L-c, before it rapidly accelerates to dynamic fault rupture. The experiments also highlight complexities not currently considered by most theoretical and numerical models. This includes a loading rate dependency where a "kick" above steady state produces smaller and more abrupt initiation. Heterogeneity of fault strength also causes abrupt initiation when creep fronts coalesce on a stuck patch that is somewhat stronger than the surrounding fault. Taken together, these two mechanisms suggest a rate-dependent "cascade up" model for earthquake initiation. This model simultaneously accounts for foreshocks that are a by-product of a larger nucleation process and similarities between initial P wave signatures of small and large earthquakes. A diversity of nucleation conditions are expected in the Earth's crust, ranging from slip limited environments with L-c < 1 m, to ignition-limited environments with L-c > 10 km. In the latter case, L-c fails to fully characterize the initiation process since earthquakes nucleate not because a slipping patch reaches a critical length but because fault slip rate exceeds a critical power density needed to ignite dynamic rupture.