Potency density tensor inversion of complex body waveforms with time-adaptive smoothing constraint
Potency density tensor inversion of complex body waveforms with time-adaptive smoothing constraint
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DOI：
10.1093/gji/ggac181
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发表时间：
2022-06-09
影响因子：
2.8
通讯作者：
Fukahata, Yukitoshi
中科院分区：
文献类型：
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作者：
Yamashita, Shinji;Yagi, Yuji;Fukahata, Yukitoshi
大地震往往伴随着复杂的断层破裂，但由于断层几何精度有限而产生的建模误差，用传统的波形分析工具很难可靠地估计这种复杂的破裂过程。为了解决这个问题，最近发展了一种势密度张量反演方法，该方法允许在假设的模型平面上有任何类型的断层机制，用断层定位的模拟误差取代了在低空间分辨率的远震波形中不敏感的断层定位模拟误差。该方法成功地揭示了具有几何复杂断层破裂的震源过程。然而，该方法对所有的基滑分量施加了相同强度的平滑约束，而不考虑在破裂期间可能发生的滑移方向的变化。这导致了对幅度较大的滑动分量的过度平滑，从而导致了破裂过程的模糊。在这项研究中，我们提出了一种时间自适应的平滑约束，该约束动态地调整每个基滑移函数的平滑强度与幅度成反比。通过一个涉及破裂过程中震源机制(反向、走滑和正断层)剧烈变化的输入模型的数值实验，我们发现时间自适应平滑约束解决了对主要滑动分量过度平滑的问题，成功地再现了具有不同震源机制的时空不均匀破裂幕。为了评价时间自适应平滑约束的可行性和有效性，我们将该方法应用于2002年德纳利断裂和2008年汶川地震的远震体波，这些地震涉及具有变化震源机制的复杂断层破裂。我们发现，该方法很好地捕捉了2002年德纳利断裂破裂和2008年汶川地震期间从反向断裂到走滑断裂的震源机制在时空上的转变。尽管这些震源模型仅使用由水平矩形平面表示的简单模型断层几何的远震P波来建立，但它们很好地解释了复杂的观测波形，并与以前利用地震和大地测量数据以及野外调查获得的震源过程特征相一致。具有时间自适应平滑约束的能量密度张量反演法是分析复杂断裂几何、不同断裂类型地震破裂过程的有力工具。
Large earthquakes are often accompanied by complex fault rupture, but it has been difficult to reliably estimate such a complex rupture process with conventional waveform analysis tools due to modelling errors originating from limited accuracy of the fault geometry. Recently, a potency density tensor inversion method has been developed to solve this problem; allowing any types of faulting mechanism on an assumed model plane, the method replaces the modelling error of fault orientation with that of fault location, which is insensitive in teleseismic waveforms with low spatial resolution. The method has successfully unveiled earthquake source processes with geometrically complex fault rupture. However, the method imposes the same intensity of smoothing constraint on all the basis slip components irrespective of possible changes of slip direction during the rupture. This leads to excessive smoothing to a slip component with large amplitude, which results in obscuring the rupture process. In this study, we propose a time-adaptive smoothing constraint that dynamically adjusts the smoothness intensity inversely proportional to the amplitude for each basis slip function. Through a numerical experiment assigning an input model involving a drastic change in the focal mechanism (reverse, strike-slip and normal faulting) during the rupture, we find that the time-adaptive smoothing constraint solves the problem of excessive smoothing to the dominant slip component, and the spatiotemporally non-uniform rupture episodes with different focal mechanisms are successfully reproduced. To evaluate the feasibility and effectiveness of the time-adaptive smoothing constraint, we apply the method to the teleseismic body waves of the 2002 Denali fault and the 2008 Wenchuan earthquakes, which involve complex fault ruptures with changing focal mechanisms. We find that the developed method well captures the focal mechanism transition in space and time from reverse to strike-slip faulting during the ruptures of the 2002 Denali fault and the 2008 Wenchuan earthquakes. Even though these source models are built using only the teleseismic P waveforms with simple model fault geometry that is represented by a horizontal rectangular plane, they well explain the complex observed waveforms and agree with characteristics of source processes obtained in previous studies using seismic and geodetic data as well as field surveys. The potency density tensor inversion method with time-adaptive smoothing constraint is a powerful tool to analyse earthquake rupture processes with complex fault geometries involving different faulting types.