基于区域宽频带和近震波形数据的多点源反演——以2016 <i>M</i><sub>W</sub>7.1日本熊本地震为例

doi:10.6038/cjg2021P0228

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摘要中强度（M6-7）地震可以对人类社会基础设施和生命财产造成威胁，并且其发震频率要远高于M8+地震.从震源物理的角度来说，此类地震的发震断层的几何形态和破裂过程往往较为复杂.因此，准确地确定此类地震的震源参数对地震防治和了解震源物理都具有重大意义.作为承接地震单点源解和有限破裂模型的桥梁，地震多点源解可以为有限破裂模型反演提供可靠的断层几何，并有效地帮助约束破裂的时空演化过程.本文基于马尔可夫链蒙特卡洛（MCMC）非线性反演算法，将一种多点源波形反演方法的适用范围从近震距离推广到了区域距离，并验证了利用余/前震（或历史地震）对地震波传播路径进行校正的有效性.我们将改进后的方法应用于具有代表性的2016 M_W7.1熊本地震的研究中.结果发现，利用一个M_W6.0和一个M_W5.4的前震，可以对半径100 km范围内强震记录和1000 km内的宽频带记录进行有效的路径校正，筛选出现有速度模型所适用的地震台和震相，并且可以确定波形拟合中高质量Pnl波和面波所能利用的频率上限，即高达0.3 Hz（Pnl）和0.2 Hz（面波）.研究结果还显示区域宽频带数据反演的结果和强震+高频GPS数据结果有高度的一致性，验证了将该方法推广到区域数据反演的可行性.对熊本M_W7.1主震的反演结果显示，在反演所用的频段，我们需要至少四个子事件来表示该地震的运动学过程.第一个M_W6.7子事件发生在地下12 km深的Hinagu断层上，并具有高倾角的纯走滑位错解；第二（M_W6.7）和第三个（M_W6.7）子事件的深度均为7 km，其走向和Futagawa断层一致并有一定的正断滑移成分；最后一个子事件（M_W6.6）最浅（1~2 km），也具有最强（滑动角=-137°）的正断滑移成分.这些子事件揭示了发震层的厚度随着主震向东北方向的破裂传播距离增加而变薄，并且其深度随着破裂位置靠近Aso火山而变得越浅.当使用离断层最近的0701高频GPS台站时，五点源反演可以分辨出在破裂最后阶段沿一个次级断层的传播，我们认为该断层分叉现象有可能对地震的停止起到了关键性的作用.

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	施其斌
	韦生吉

关键词 ：地震破裂过程, 非线性反演, 多点源, 非对称震源时间函数, 熊本地震

Abstract：Moderate-size (M6-7) earthquakes possess great threats to human society, as they occur much more frequent than M8+ earthquakes and many take place inland. The rupture processes and fault geometry of this kind of earthquake are usually very complex. Precise determination of their kinematic rupture parameters is a fundamental task in seismology, which is critical to understanding the earthquake physics as well as to better prepare for seismic hazard. As an earthquake source model with a precision between point source and finite fault model, Multiple Point Source (MPS) solution of large earthquakes provide reliable fault geometry and first order rupture evolution of earthquakes, which is extremely helpful for more detailed finite fault inversion and modeling. Here we generalize a Markov-Chain-Monte-Carlo (MCMC) based MPS inversion algorithm to the regional broadband waveform inversion and apply to the 2016 M_W7.1 Kumamoto mainshock. We first calibrate the path and select the frequency ranges for the strong motion (<100 km) and regional (<1000 km) broadband stations through the point source modeling of the M_W5.4 and M_W6.0 foreshocks, which results in up to 0.3 Hz for Pnl waves and 0.2 Hz for surface waves. The inversion results from regional, strong motion and high-rate GPS datasets show highly consistent solutions, validating the generalization of our MPS inversion method. Our preferred model show that the earthquake is composed of at least four sub-events. The first (M_W6.7) sub-event is a high-dip-angle purely strike-slip event, taking place near the hypocenter at the depth of 12 km. The strike of the first sub-event is well consistent with the strike of the Hinagu fault. The second (M_W6.7) and the third (M_W6.7) sub-events all took place at the depth of 7 km, with strike more consistent with the Futagawa fault and with slight normal slip component. The last sub-event (M_W6.6) is the shallowest (1~2 km) and its focal mechanism is composed of almost equal percentage of strike-slip and normal-slip components. Our five point source inversion with the closest high-rate GPS station (0701) shows that the last stage of the rupture bifurcated onto a secondly fault, probably playing an important role in stopping the rupture.

Key words： Earthquake rupture process Non-linear inversion Multiple point source Asymmetric source time function Kumamoto earthquake

收稿日期: 2021-04-09

PACS:

P315

基金资助:新加坡教育部项目（MOE2019-T2-1-182）资助.

通讯作者: 韦生吉,男,1982年生,助理教授,博士,2009年毕业于中国科学技术大学,主要从事地震震源研究.E-mail:shjwei@gmail.com;shjwei@ntu.edu.sg E-mail: shjwei@gmail.com;shjwei@ntu.edu.sg

作者简介: 施其斌,男,1994年生,博士研究生,主要从事地震破裂过程研究.E-mail:qibin.nan@gmail.com;qshi003@e.ntu.edu.sg

链接本文:

http://www.geophy.cn//CN/10.6038/cjg2021P0228 或 http://www.geophy.cn//CN/Y2021/V64/I12/4378

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