反向传播破裂：性质、激发及其意义
Back-Propagating Rupture: Nature, Excitation, and Implications
AI总结：
本研究结合理论、数值模拟与观测，揭示反向传播破裂（BPR）的固有属性、可观测激发条件及两类机制，并阐明其对地震物理和灾害评估的意义。
摘要：
近期观测表明，在单次地震事件中，某一破裂相位相对于更早的相位可能向后传播。这种反向传播破裂（BPR）在传统地震震源研究中未得到充分重视，对地震学界而言仍是一个谜。在此，我们结合理论分析、数值模拟和观测证据，对BPR进行了全面分析。首先，我们论证了以反向传播应力波形式出现的BPR是动态破裂过程中的固有特征；然而，其信号很容易被主破裂锋后方的相消干涉所掩盖。随后，我们提出一种观点：对原本平滑的破裂过程施加扰动，可能使某些BPR相位变得可观测。我们通过数值模拟在多种扰动下的破裂传播来检验并验证这一观点，这些扰动包括沿破裂传播路径上应力、体相或界面性质以及断层几何形态的突然变化。我们进一步利用实验室和天然地震的现有观测对数值结果进行交叉验证，并确认自由表面处的破裂“反射”、破裂合并以及显著凹凸体的破坏，对于激发可观测的BPR非常有效。基于模拟和观测结果，我们分别依据BPR到达前后的应力恢复和应力降，将BPR分为两种一般类型：界面波型和高阶再破裂型。我们的工作阐明了BPR的性质和激发机制，并有助于增进对地震物理、断层性质分布与演化推断以及地震灾害评估的理解。
英文摘要：
Recent observations show that certain rupture phase can propagate backward relative to the earlier one during a single earthquake event. Such back-propagating rupture (BPR) was not well considered by the conventional earthquake source studies and remains a mystery to the seismological community. Here we present a comprehensive analysis of BPR, by combining theoretical considerations, numerical simulations, and observational evidences. First, we argue that BPR in terms of back-propagating stress wave is an intrinsic feature during dynamic ruptures; however, its signature can be easily masked by the destructive interference behind the primary rupture front. Then, we propose an idea that perturbation to an otherwise smooth rupture process may make some phases of BPR observable. We test and verify this idea by numerically simulating rupture propagation under a variety of perturbations, including a sudden change of stress, bulk or interfacial property and fault geometry along rupture propagation path. We further cross-validate the numerical results by available observations from laboratory and natural earthquakes, and confirm that rupture "reflection" at free surface, rupture coalescence and breakage of prominent asperity are very efficient for exciting observable BPR. Based on the simulated and observed results, we classify BPR into two general types: interface wave and high-order re-rupture, depending on the stress recovery and drop before and after the arrival of BPR, respectively. Our work clarifies the nature and excitation of BPR, and can help improve the understanding of earthquake physics, the inference of fault property distribution and evolution, and the assessment of earthquake hazard.
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