The Earth's Surface Controls the Depth‐Dependent Seismic Radiation of Megathrust Earthquakes
The Earth's Surface Controls the Depth‐Dependent Seismic Radiation of Megathrust Earthquakes
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DOI：
10.1029/2021av000413
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发表时间：
2021-02
期刊：
影响因子：
8.4
通讯作者：
Jiuxun Yin;M. Denolle
中科院分区：
文献类型：
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作者：
Jiuxun Yin;M. Denolle
大逆冲地震表现出普遍存在的地震辐射类型：低频（LF）地震能量有效地从断层的最浅部分发射，而高频（HF）地震能量有效地从断层的最深处发射。尽管在许多特定案例的研究中观察到这一点，但我们表明，它在1995年至2021年之间的全球大逆冲地震中普遍存在。之前的研究将其解释为板块界面摩擦特性（Lay et al., 2012）或P波速度（salar<e:1> & Ranero, 2019）的系统深度变化的影响。这项工作提出了另一种假设：由于地球的自由表面，波浪和破裂之间的相互作用是产生这种行为的主要机制。俯冲带地震的二维动态破裂模拟支持这一假设。我们的模拟表明，在地球自由表面反射的地震波与上倾传播破裂之间的相互作用导致震源处的低频辐射。相反，断裂的下倾传播受自由表面的影响较小，因此主要受隐伏断层典型的高频辐射的影响。其次，通过P波速度层析成像得到的真实地球结构和钻孔中估计的真实VP/VS比进一步增强了源辐射的对比度。我们认为地球的自由表面对于解释观测到的大逆冲地震辐射类型是必要的，而俯冲带的真实结构对于更好地预测地震地面运动和海啸潜力是必要的。
Megathrust earthquakes exhibit a ubiquitous seismic radiation style: low‐frequency (LF) seismic energy is efficiently emitted from the shallowest portion of the fault, whereas high‐frequency (HF) seismic energy is efficiently emitted from the deepest part of the fault. Although this is observed in many case‐specific studies, we show that it is ubiquitous in global megathrust earthquakes between 1995 and 2021. Previous studies have interpreted this as an effect of systematic depth variation in either the plate interface frictional properties (Lay et al., 2012) or the P wavespeeds (Sallarès & Ranero, 2019). This work suggests an alternative hypothesis: the interaction between waves and ruptures due to the Earth's free surface is the leading mechanism that generates this behavior. Two‐dimensional dynamic rupture simulations of subduction zone earthquakes support this hypothesis. Our simulations show that the interaction between the seismic waves reflected at the Earth's free surface and the updip propagating rupture results in LF radiation at the source. In contrast, the downdip propagation of rupture is less affected by the free surface and is thus dominated by HF radiation typical of buried faults. To a second degree, the presence of a realistic Earth structure derived from P‐wave velocity (VP) tomographic images and realistic VP/VS ratio estimated in boreholes further enhances the contrast in source radiation. We conclude that the Earth's free surface is necessary to explain the observed megathrust earthquake radiation style, and the realistic structure of subduction zone is necessary to better predict earthquake ground motion and tsunami potential.