Natural earthquakes often have very few observable foreshocks which significantly complicates tracking potential preparatory processes. To better characterize expected preparatory processes before failures, we study stick-slip events in a series of triaxial compression tests on faulted Westerly granite samples. We focus on the influence of fault roughness on the duration and magnitude of recordable precursors before large stick–slip failure. Rupture preparation in the experiments is detectable over long time scales and involves acoustic emission (AE) and aseismic deformation events. Preparatory fault slip is found to be accelerating during the entire pre-failure loading period, and is accompanied by increasing AE rates punctuated by distinct activity spikes associated with large slip events. Damage evolution across the fault zones and surrounding wall rocks is manifested by precursory decrease of seismic b-values and spatial correlation dimensions. Peaks in spatial event correlation suggest that large slip initiation occurs by failure of multiple asperities. Shear strain estimated from AE data represents only a small fraction (< 1%) of total shear strain accumulated during the preparation phase, implying that most precursory deformation is aseismic. The relative contribution of aseismic deformation is amplified by larger fault roughness. Similarly, seismic coupling is larger for smooth saw-cut faults compared to rough faults. The laboratory observations point towards a long-lasting and continuous preparation process leading to failure and large seismic events. The strain partitioning between aseismic and observable seismic signatures depends on fault structure and instrument resolution.

中文翻译：

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大型粘滑失效前的抗震和抗震准备过程

自然地震通常很少有可观察到的前震，这使得追踪潜在的准备过程变得非常复杂。为了更好地表征失效前的预期准备过程，我们在一系列三轴压缩试验中研究了断层西风花岗岩样品中的粘滑事件。我们专注于断层粗糙度对大粘滑故障前可记录前兆的持续时间和大小的影响。实验中的破裂准备在长时间尺度上是可检测的，并且涉及声发射 (AE) 和抗震变形事件。发现预备断层滑动在整个故障前加载期间正在加速，并且伴随着 AE 率的增加，并被与大型滑动事件相关的不同活动尖峰所打断。断裂带和围岩的损伤演化表现为地震 b 值和空间相关维数的先兆减小。空间事件相关性的峰值表明，由于多个凹凸不平的破坏而发生大的滑移开始。根据 AE 数据估计的剪切应变仅占准备阶段积累的总剪切应变的一小部分 (< 1%)，这意味着大多数前兆变形是抗震的。较大的断层粗糙度放大了抗震变形的相对贡献。类似地，与粗糙断层相比，平滑锯切断层的地震耦合更大。实验室观察表明，长期持续的准备过程会导致失败和大地震事件。