Focal mechanism determination of Nima MS6.6 and MS4.8 earthquakes in Tibet in 2020
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摘要: 北京时间2020年7月23日04时07分，西藏自治区那曲市尼玛县发生MS6.6地震，震源深度10 km，震中位置为（33.19°N，86.81°E）。主震发生当日18时50分，发生一次MS4.8强余震，震源深度为10 km。本文基于西藏、青海、新疆区域波形资料，采用ISOLA近震全波形方法对这两次地震进行震源机制反演。结果显示，尼玛MS6.6主震的最佳断层面解为：节面Ⅰ走向8°/倾角46°/滑动角−93°，节面Ⅱ走向191°/倾角44°/滑动角−87°；矩震级MW6.4，最佳矩心深度7 km。震源区应力主轴的空间取向为：主压力轴P的方位角220°、倾伏角88°，主张力轴T方位角99°、倾伏角1°。MS4.8强余震的最佳断层面解为：节面Ⅰ走向12°/倾角47°/滑动角−106°，节面Ⅱ走向214°/倾角45°/滑动角−74°；矩震级MW5.0，最佳矩心深度6 km。震源区应力主轴的空间取向为：主压力轴P的方位角207°、倾伏角78°，主张力轴T方位角113°、倾伏角1°。震源机制反演结果表明，这两次地震均为以正断型为主的地震事件，与震源区附近先前地震的震源机制有较好的一致性。结合周边地质构造和余震分布，我们认为尼玛MS6.6地震可能是由位于日干配错断裂和依布茶卡盆地西缘断裂之间的一条正断层活动所引发的。Abstract: A MS6.6 earthquake occurred at 4:07 am on July 23, 2020 in Nima County, Nagqu, Tibet Autonomous Region. The location of the main shock is 33.19°N, 86.81°E, and the focal depth is 10 km. At 18:50 on the same day, a strong aftershock of MS4.8 occurred with a focal depth of 10 km. Based on the regional waveform data of Tibet, Qinghai and Xinjiang, we use the ISOLA full waveform method for local earthquakes to retrieve the focal mechanism of these two events. The results show that the best fault plane solution of Nima MS6.6 main earthquake is as follows: strike 8°/dip 46°/rake −93° for nodal plane Ⅰ, strike 191°/dip 44°/rake −87° for nodal plane Ⅱ; the moment magnitude is MW6.4, and the centroid depth is 7 km. The spatial orientation of the principal stress axis in the focal area is defined where the azimuth of the main pressure axis P is 220° and the plunge angle is 88°, and the azimuth of the main tension axis T is 99° and the plunge angle is 1°. The results show the best fault plane solution of the MS4.8 aftershock with strike 12°/dip 47°/rake −106° for nodal plane Ⅰ and strike 214°/dip 45°/rake −74° for nodal plane Ⅱ; Moment magnitude is MW5.0, and the best depth of moment center is 6 km. The spatial orientation of the main stress axis in the focal area is as follows: the azimuth angle of the main pressure axis P is 207° and the plunge angle is 78°, and the main tension axis T is 113° and the plunge angle is 1°. Focal mechanism inversion results show that these two earthquakes are mainly normal fault events, which are in good agreement with the previous focal mechanisms near the focal area. Combined with the surrounding geological structure and aftershock distribution, we believe the Nima MS6.6 earthquake may be caused by the activity of a normal fault between the Riganpei Co fault and the fault on the western margin of the Yibug Caka Basin.
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图 1 研究区域构造背景图及自1970年以来3.0级以上历史地震分布图
Figure 1. Tectonic background map of the study area and historical earthquake distribution map with magnitude 3.0 or above since 1970
图 2 西藏尼玛MS6.6地震震中位置和本研究使用的台站分布图
Figure 2. The epicenter location of the Nima MS6.6 earthquake in Tibet and the distribution of stations used in this study
图 4 尼玛MS6.6主震及MS4.8余震的震源机制解
Figure 4. Focal mechanism solutions of the Nima MS6.6 main earthquake and MS4.8 aftershock
图 5 西藏尼玛MS6.6地震矩心深度与波形互相关系数
Figure 5. The centroid depth and the correlation coefficient of the waveform of the Nima MS6.6 earthquake in Tibet
图 6 西藏尼玛MS6.6地震震源机制解对应的三分量理论（红色）和实际（黑色）波形对比图
波形下方数字表示波形拟合方差；左侧大写字母表示台站名
Figure 6. Comparison of the three-component theoretical（red）and observational（black）waveforms corresponding to focal mechanism solutions of the Nima MS6.6 earthquake in Tibet
The figure below the waveform represents the waveform fitting variance；Capital letters on the left indicate station names
图 7 断层面参数不确定度的估计（依次减少1个台站）
Figure 7. Estimation of parameter uncertainty on fault plane（decreased by 1 station successively）
表 1 不同研究机构给出的2020年7月23日西藏尼玛主震的震源机制解参数
Table 1. Focal mechanism solution parameters of the main Nima earthquake in Tibet on July 23，2020 given by different research institutions
结果来源 节面Ⅰ 节面Ⅱ 矩心深度
/kmMW 沙滩球 最小空间
旋转角/°走向/° 倾角/° 滑动角/° 走向/° 倾角/° 滑动角/° GFZ 353 52 −109 204 41 −65 13 6.4 14.16 GCMT 10 48 −88 187 42 −92 16.8 6.4 4.39 USGS 20 61 −91 203 29 −88 11.5 6.3 18.53 IPGP 17 47 −74 174 45 −106 11 6.3 14.39 本研究 8 46 −93 191 44 −87 11 6.4 
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