Scale Dependence of Earthquake Rupture Prestress in Models With Enhanced Weakening: Implications for Event Statistics and Inferences of Fault Stress
Scale Dependence of Earthquake Rupture Prestress in Models With Enhanced Weakening: Implications for Event Statistics and Inferences of Fault Stress
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DOI：
10.1029/2021jb021886
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发表时间：
2021-02
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影响因子：
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通讯作者：
Valère Lambert;N. Lapusta;D. Faulkner
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作者：
Valère Lambert;N. Lapusta;D. Faulkner
确定断层上发生地震滑动的条件是断层力学与地震灾害密切相关的一个重要目标。以往的研究表明，增强动力弱化（EDW）可以导致动态破裂的断层与低得多的剪切应力比破裂成核所需的。我们研究了不同规模的地震破裂前的应力条件，这些地震破裂是由于孔隙流体的热加压而在EDW速率和状态断层上的地震序列的数值模拟中自发演化的。我们发现，动态破裂前的平均剪应力（又名剪切预应力）系统地随破裂大小而变化。最小的破裂具有与成核所需的局部剪切应力相当的预应力。较大的破裂会使断层更加弱化，由于动应力集中，在应力不足的区域传播，导致整个破裂的平均预应力逐渐降低。在具有更有效EDW的故障模型中，效果更显著。我们发现，因此，更有效的削弱故障模型产生更少的小事件，并导致系统较低的B-值的频率-幅度事件分布。研究结果（a）表明，与滑移成核所需的应力相比，大地震可能发生在似乎没有临界应力的断层上;（B）强调了有限断层建模在将实验室确定的局部摩擦行为与现场规模相关联方面的重要性;以及（c）这表明，缺乏小事件或地震平静可能是成熟断层在低剪切应力下运行的观测指标，EDW。
Determining conditions for earthquake slip on faults is a key goal of fault mechanics highly relevant to seismic hazard. Previous studies have demonstrated that enhanced dynamic weakening (EDW) can lead to dynamic rupture of faults with much lower shear stress than required for rupture nucleation. We study the stress conditions before earthquake ruptures of different sizes that spontaneously evolve in numerical simulations of earthquake sequences on rate‐and‐state faults with EDW due to thermal pressurization of pore fluids. We find that average shear stress right before dynamic rupture (aka shear prestress) systematically varies with the rupture size. The smallest ruptures have prestress comparable to the local shear stress required for nucleation. Larger ruptures weaken the fault more, propagate over increasingly under‐stressed areas due to dynamic stress concentration, and result in progressively lower average prestress over the entire rupture. The effect is more significant in fault models with more efficient EDW. We find that, as a result, fault models with more efficient weakening produce fewer small events and result in systematically lower b‐values of the frequency‐magnitude event distributions. The findings (a) illustrate that large earthquakes can occur on faults that appear not to be critically stressed compared to stresses required for slip nucleation; (b) highlight the importance of finite‐fault modeling in relating the local friction behavior determined in the lab to the field scale; and (c) suggest that paucity of small events or seismic quiescence may be the observational indication of mature faults that operate under low shear stress due to EDW.