Toward automated directivity estimates in earthquake moment tensor inversion
Toward automated directivity estimates in earthquake moment tensor inversion
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DOI：
10.1093/gji/ggx354
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发表时间：
2017-11
影响因子：
2.8
通讯作者：
Hsin‐Hua Huang;N. Aso;V. Tsai
中科院分区：
文献类型：
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作者：
Hsin‐Hua Huang;N. Aso;V. Tsai
对地震破裂性质的快速估计对于地震的科学表征和对地震灾害的应急反应都是有用的。破裂方向性是一个需要限制的特别重要的属性，因为沿破裂方向辐射的地震波可能会被极大地放大，即使是中等震级的地震有时也可能造成严重的破坏。了解地震的方向性对于地震动预测和减灾很重要，也有助于区分哪个节面对应于实际的断裂面，特别是当地震没有余震或露头断层痕迹时。在这里，我们提出了一种直接波形拟合的三维多时间窗方向性反演方法，将震源时间函数按给定的方向性拉伸到每个台站。通过在三维空间中对方向性矢量进行网格搜索，该方法不仅确定水平方向分量，而且确定垂直方向分量，提供不确定性估计，并且具有实时自动化的潜力。综合测试表明，该方法对噪声、拾取误差和场地放大都是稳定的，对站点覆盖的敏感度比其他方法要低。水平方向性可以适当地恢复，最小方位站覆盖率为180°，而垂直方向性需要更好的覆盖率才能分辨。将新方法应用于台湾南投6.0级地震、日本熊本7.0级地震和南加州圣哈辛托4.7级断层三分叉地震。对于南投地震，我们证实了以前的发现，即地震发生在一个东倾的浅层断层面上，而不是西倾的浅层断层面上。对于熊本地震和SJFT地震，方向性计算结果与前人的研究结果吻合较好，表明该方法反映了包含多条断裂的大地震的一般破裂特征，适用于小至Mw4.7级的地震。
Rapid estimates of earthquake rupture properties are useful for both scientific characterization of earthquakes and emergency response to earthquake hazards. Rupture directivity is a particularly important property to constrain since seismic waves radiated in the direction of rupture can be greatly amplified, and even moderate magnitude earthquakes can sometimes cause serious damage. Knowing the directivity of earthquakes is important for ground shaking prediction and hazard mitigation, and is also useful for discriminating which nodal plane corresponds to the actual fault plane particularly when the event lacks aftershocks or outcropped fault traces. Here, we propose a 3-D multiple-time-window directivity inversion method through direct waveform fitting, with source time functions stretched for each station according to a given directivity. By grid searching for the directivity vector in 3-D space, this method determines not only horizontal but vertical directivity components, provides uncertainty estimates, and has the potential to be automated in real time. Synthetic tests show that the method is stable with respect to noise, picking errors, and site amplification, and is less sensitive to station coverage than other methods. Horizontal directivity can be properly recovered with a minimum azimuthal station coverage of 180°, whereas vertical directivity requires better coverage to resolve. We apply the new method to the M_w 6.0 Nantou, Taiwan earthquake, M_w 7.0 Kumamoto, Japan earthquake, and M_w 4.7 San Jacinto fault trifurcation (SJFT) earthquake in southern California. For the Nantou earthquake, we corroborate previous findings that the earthquake occurred on a shallow east-dipping fault plane rather than a west-dipping one. For the Kumamoto and SJFT earthquakes, the directivity results show good agreement with previous studies and demonstrate that the method captures the general rupture characteristics of large earthquakes involving multiple fault ruptures and applies to earthquakes with magnitudes as small as M_w 4.7.