Abstract An Ms 6.0 earthquake struck Changning county, Sichuan basin, SW China on 17 June 2019, which caused huge casualties and economic losses. Four Ms greater than 5.0 events subsequently occurred around the Changning source area, three of which occurred within one week. In order to better understand the mechanism of these moderate-sized earthquakes, we determine 3-D high-resolution velocity models around the source area simultaneously relocating earthquakes using double-difference tomography. In the present study, we use a total of 53,487 P-wave and 52,527 S-wave arrival times from 8818 events recorded at 39 seismic stations. Our results show that focal depths of the Changning mainshock and most aftershocks are ~5–10 km, and they form a fault plane with a steep dip angle. Most earthquakes are underlain by the zone with low Vp, low Vs, and high Vp/Vs anomalies, reflecting the existence of fluids there. These results suggest that the Changning mainshock and other moderate-sized earthquakes might be associated with the influence of fluids that could decrease effective normal stress on the fault planes. These fluids might be related to the hot and wet mantle upwelling in the big mantle wedge due to the deep subduction of the Indian plate down to the mantle transition zone. A clear high-to-low velocity transition zone is observed at ~10 km depth beneath the Gongxian and Xingwen swarms, which matches well with the detachment layer revealed by deep seismic soundings in the area. All these results suggest that the structural contrast could control the mainshock generation and aftershock extension.

中文翻译：

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深部流体对2019年四川盆地长宁6.0级地震影响的地震证据

摘要 2019年6月17日，四川盆地长宁县发生6.0级地震，造成巨大人员伤亡和经济损失。随后，长宁源区周边发生4次Ms大于5.0的事件，其中3次发生在一周内。为了更好地理解这些中等规模地震的机制，我们确定了震源区周围的 3-D 高分辨率速度模型，同时使用双差断层扫描重新定位地震。在本研究中，我们使用了来自 39 个地震台站记录的 8818 个事件的总共 53,487 个 P 波和 52,527 个 S 波到达时间。我们的研究结果表明，长宁主震和大部分余震的震源深度为~5-10 km，它们形成了一个倾角陡峭的断层面。大多数地震都位于低 Vp、低 Vs、和高 Vp/Vs 异常，反映那里存在流体。这些结果表明，长宁主震和其他中等规模的地震可能与流体的影响有关，这些流体可以降低断层面上的有效正应力。这些流体可能与印度板块深俯冲到地幔过渡带导致大地幔楔中的湿热地幔上涌有关。在巩县群和兴文群下方约 10 km 深度处观察到清晰的高低速过渡带，与该地区深地震探测揭示的拆离层匹配良好。所有这些结果表明结构对比可以控制主震的产生和余震的扩展。这些结果表明，长宁主震和其他中等规模的地震可能与流体的影响有关，这些流体可以降低断层面上的有效正应力。这些流体可能与印度板块深俯冲到地幔过渡带导致大地幔楔中的湿热地幔上涌有关。在巩县群和兴文群下方约 10 km 深度处观察到清晰的高低速过渡带，与该地区深地震探测揭示的拆离层匹配良好。所有这些结果表明结构对比可以控制主震的产生和余震的扩展。这些结果表明，长宁主震和其他中等规模的地震可能与流体的影响有关，这些流体可以降低断层面上的有效正应力。这些流体可能与印度板块深俯冲到地幔过渡带导致大地幔楔中的湿热地幔上涌有关。在巩县群和兴文群下方约 10 km 深度处观察到清晰的高低速过渡带，与该地区深地震探测揭示的拆离层匹配良好。所有这些结果表明结构对比可以控制主震的产生和余震的扩展。