Kinematic Source Model of 2018 Hokkaido Eastern Iburi Mw6.6 Japan Earthquake
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摘要: 通过观测的地表波形反演震源机制以理解地震震源破裂过程是研究强震动特征非常有效的途径之一.主要针对强震动的产生机制，采用中小震作为经验格林函数，选取0.2~2.0 Hz频段的强震动速度波形进行波形反演2018年日本北海道$ {M}_{w}6.6 $地震的破裂过程，提出了该地震的震源模型.结果表明：该地震的主要最大滑移量区域集中在沿断层面西南部-东北部6 km范围、距离震源~12.0 km的浅层区域内，该区域内最大滑动量约3.5 m；识别出两个最大滑移速度分布区，分别位于断层西南6.0 km、东北4.0 km，距离震源~15.0 km的浅层区域内，最大滑动速度约2.0 m/s，破裂速度为2.0 km/s，该震源模型对应地震震级$ {M}_{W}7.0 $.此外，通过多种组合的中小震记录作为经验的格林函数及近断层强震观测台站探讨了该震源模型的鲁棒性，进一步通过合成未参与反演的台站强震动波形，结果显示合成波形与观测波形的匹配度较高，表明模型的时空特征描述合理；最后，通过与其他已公开发表的震源模型的综合对比发现最大滑移分布相似，该系列对比充分验证了该震源模型是稳定可靠的，可为未来强震动模拟提供重要参考.Abstract: Conducting waveform inversions to estimate the rupture process of media to large size of earthquakes is one of the effective methods to better understand the characteristics of strong ground motions. To investigate the generation mechanism of strong ground motions, this study evaluates the rupture process of the 2018 Hokkaido, Japan, earthquake through waveform inversion based on the corrected empirical Green's functions. It is found that, large slip regions are concentrated along the shallow southwestern- to northeastern-section of the fault around 6.0 km in length and within 12.0 km from the hypocenter. Within this region, the maximum final slip approximates to 3.5 m; two peak slip velocity regions are identified, with the primary one located 6.0 km southwestern, and the secondary one located 4.0 km northeastern, and both within shallow areas 15.0 km from the hypocenter. The maximum peak slip velocity is about 2.0 m/s. A rupture velocity of 2.0 km/s is identified, and the inverted source model corresponds to a magnitude $ {M}_{W}7.0 $. Furthermore, another 3 waveform inversions using different combinations of empirical Green's functions and additional 7 different combinations of near-fault strong motion stations are operated to investigate the robustness and reliability of the source model. Based on the evaluated source model, additional strong motions at stations, which are not used in the waveform inversion, are synthesized and the synthesized and observed velocities could match well. Similarities of the final slip distribution among different source models also could be obtained. Results demonstrate that the major spatiotemporal characteristics of slip are robust and reliable, which could offer useful information for future strong motion simulation and analysis.
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图 1 2018年日本北海道$ {M}_{w}6.6 $地震及波形反演中选用的中小震震中位置、发震断层和所选取的强震观测台站
Fig. 1. Location of hypocenters of the 2018 Hokkaido earthquake and the small events used in the inversion analysis
图 2 所选取的典型台站的中小震记录与主震速度波形的相位特性相关性比较示例
每个子图中黑色实线为主震波形，红色实线为中小震相位+主震振幅
Fig. 2. Phase characteristics among mainshock and the selected small events for typical strong-motion stations
图 3 反演的震源模型
a.最终滑移分布；b. 峰值滑移速度分布；c. 基于反演获得的整个地震震源时间函数；d. VR随地震波破裂速度（Vr）的敏感性分析
Fig. 3. Inverted source models
图 4 断层面上滑动量分布的时间增量为1.5 s的快照，图中五角星为JMA发布的破裂起始点
Fig. 4. Snapshot of the rupture slip values for the preferred earthquake source model in every 1.5 s, the star is the JMA rupture start point
图 5 基于反演的震源模型模拟的强震动速度波形与观测速度波形的比较
a.时域（蓝色虚线段10 s为应用于波形反演的波形数据）；b.频域，频率范围为0.2~2.0 Hz
Fig. 5. Comparisons between the simulated strong ground motion based on the preferred source model and the observed velocity waveforms
图 6 基于不同EGF组合反演获得的震源模型
Fig. 6. Inverted source models based on different combinations of EGF events
图 7 基于4组不同中小震组合(工况1~工况4)反演获得的(a)VR值、(b)$ {M}_{0} $、(c)最大滑移量及(d)峰值滑移速度的比较
Fig. 7. Comparisons of (a) VR values, (b) $ {M}_{0} $, (c) maximum slip value and (d) maximum peak slip velocity through different waveform inversions on different combinations of EGF events (case 1-case 4)
图 8 基于提出的震源模型合成的未参与反演的强震动台站速度波形与观测速度波形(频率范围0.2~2.0 Hz)的比较
Fig. 8. Comparisons of velocity waveforms (target frequency range: 0.2-2.0 Hz) between the simulated strong ground motions based on kinematic source for the sites which are not adopted in the inversion and the observed ones
图 9 基于不同频率范围进行反演所提出的不同运动学震源模型间的比较
a.本文提出的震源模型；b. Kobayashi组模型（Kobayashi et al.，2019）；c. NIED模型（NIED，2019）；d. Asano and Iwata模型（Asano and Iwata, 2019）
Fig. 9. Comparisons among different source models (which are focused on different frequency ranges proposed from different researchers)
表 1 主震及选取的中小震发震时刻、震源位置及震源机制解参数
Table 1. Focal mechanics of the mainshock and EGF events adopted in this calculation
地震 发震时间(JST)a
年‒月‒日
时: 分: 秒震源位置a $ {M}_{j} $a 震源机制b $ {M}_{o} $b
(N∙m)经度
(oE)纬度
(oN)深度
(km)走向
(o)倾角
(o)滑移角
(o)主震 2018‒09‒06 03:07:59.33 142.006 7 42.690 8 37.04 6.7 349 65 107 1.00×1019 EGF1 2018‒09‒06 16:53:24.31 141.972 8 42.687 8 34.78 4.4 179 79 ‒169 2.76×1015 EGF2 2018‒09‒08 18:21:07.13 141.968 5 42.696 0 33.78 4.2 215 78 ‒146 1.45×1015 EGF3 2018‒09‒09 22:55:13.75 141.983 7 42.781 2 34.87 4.9 341 70 100 2.66×1016 EGF4 2018‒09‒17 02:51:31.63 141.862 8 42.718 0 27.59 4.6 357 39 ‒155 7.43×1015 EGF5 2018‒09‒21 07:56:08.20 141.994 8 42.641 3 36.55 4.2 133 74 50 1.25×1015 EGF6 2018‒10‒08 21:53:58.77 141.963 2 42.627 8 31.79 4.3 21 72 110 3.46×1015 EGF7 2018‒11‒14 19:07:30.10 141.965 3 42.698 0 31.50 4.7 305 51 70 1.44×1015 注：a据JMA，b据NIED F-net（ Fukuyama et al.，1996 ）.
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