中国南部典型地区地壳上地幔深部速度结构数据集（2019-2020）

（1）2019年6月17日，中国西南部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，长宁震源区周边发生了4起大于5.0 Ms的地震事件，其中3起发生在一周内。为了更好地理解这些中等规模地震的发生机制，我们利用双差层析成像技术，在震源区周围进行了地震重定位，同时建立了三维高分辨率速度模型。在本研究中，我们使用了39个地震台站记录的8818个地震事件的53487个P波和52527个S波到时间。结果表明，长宁主震和大部分余震震源深度约5～10km，形成一个陡倾角的断层面。大多数地震都在低Vp、低Vs和高Vp/Vs异常带的下方，反映了那里存在流体。这些结果表明，长宁主震和其他中等规模地震可能与流体降低了断层面有效正应力的影响有关。这些流体可能与印度板块向地幔过渡带的深俯冲导致大地幔楔中的热湿地幔上涌有关。在巩县群和兴文群下方约10km深处，观察到一条明显的高低速过渡带，与该区深部地震探测揭示的滑脱层吻合较好。这些结果表明，构造对比可以控制主震的产生和余震的扩展。
（2）郯庐断裂带是我国东部最重要的活动断裂带，发生过1668年郯城8.5级地震。目前尚不清楚大地震的发生与上地幔结构之间是否存在联系。为了解决这个问题，我们利用44047个远震P波到达时间研究了中国东部地区的P波上地幔层析成像。结果表明，在深度小于150km时，郯庐断裂带以西出现高波速异常，而断裂带以东出现低波速异常。沿断裂带显示出强烈的横向不均质性。在郯庐断裂带西北部230～470km深处，存在明显的低波速异常，可能反映了热湿地幔的上涌，而在东部，高波速异常是十分清晰的，可能反映了拆沉的欧亚岩石圈（下降流）。地幔转换带既有高波速异常，也有低波速异常，大范围的高波速异常可能反映了滞留的太平洋板块。在1668年郯城地震的震源下，上地幔至地幔转换带深度出现了间歇性的低波速异常，反映了热湿地幔上涌流。综合目前的研究结果和以往的研究结果，我们认为郯城地震是受岩石圈拆沉引起的热湿地幔上涌流体的影响。在郯庐断裂带下方，地幔转换带中滞留的太平洋板块上方的大地幔楔可能发生复杂的地幔对流，包括上升流和下降流。
（3）汶川地震后, 为认识发震机理, 在龙门山断裂带及周边地区开展了丰富的地震学和地球物理学等方面的 野外观测与研究工作, 获得了有意义研究成果. 近震成像获得的地壳结构显示, 以汶川主震震中为界, 龙门山断 裂带南北两侧波速存在明显差别, 以南地区为明显低波速异常, 而以北地区为显著横向不均匀性, 这也许可以解 释汶川地震的余震为何向东北方向延伸. 汶川主震发生在高低波速异常的边界, 且其下方存在低波速高泊松比异 常. 这一结果表明, 印度-欧亚板块的碰撞挤压在龙门山断裂带附近形成的高温高压导致了部分熔融或流体作用, 降低了断层面处的有效正应力, 因而诱发了汶川地震. 远震成像获得的上地幔结构显示, 龙门山断裂带处于松潘甘孜地块下方的低波速异常向四川盆地下方的高波速异常的过渡区, 且这种结构延伸至200~300 km深度, 而在地 幔转换带中的高波速异常与缅甸弧下方的上地幔高波速异常相连接, 说明汶川地震的发生与印度板块深俯冲而形 成的“大地幔楔”结构中的低波速异常所代表的热湿上涌等动力学过程密切相关. 因此, 本文认为汶川地震不 仅与地壳结构密切相关, 而且还受上地幔异常结构的影响. 另外, 汶川地震还可能与下地壳流、地壳缩短和紫坪铺 水库触发等动力学过程有关.

(1) On June 17, 2019, a Ms 6.0 earthquake struck Changning County, Sichuan Basin, southwest China, causing massive casualties and economic losses. Subsequently, four seismic events with magnitude Ms > 5.0 occurred around the Changning source region, three of which took place within one week. To better understand the occurrence mechanism of these moderate-sized earthquakes, we employed double-difference tomography to conduct seismic relocation around the source region and established a 3D high-resolution velocity model. In this study, we utilized 53,487 P-wave and 52,527 S-wave arrival times from 8,818 seismic events recorded by 39 seismic stations. The results show that the mainshock and most aftershocks of the Changning earthquake have focal depths of approximately 5–10 km, forming a steeply dipping fault plane. Most earthquakes are located below the zones with low Vp, low Vs, and high Vp/Vs anomalies, reflecting the presence of fluids therein. These results suggest that the Changning mainshock and other moderate-sized earthquakes may be related to the effect of fluids reducing the effective normal stress on the fault plane. These fluids may be associated with the upwelling of hot and wet mantle in the big mantle wedge caused by the deep subduction of the Indian Plate into the mantle transition zone. A distinct high-low velocity transition zone was observed at a depth of ~10 km below the Gongxian and Xingwen Groups, which agrees well with the detachment layer revealed by deep seismic surveys in this region. These results indicate that structural contrasts can control the occurrence of the mainshock and the propagation of aftershocks.
(2) The Tan-Lu Fault Zone is the most important active fault zone in eastern China, which hosted the 1668 Tancheng Ms 8.5 earthquake. Currently, whether there is a link between the occurrence of large earthquakes and the upper mantle structure remains unclear. To address this issue, we conducted P-wave upper mantle tomography for eastern China using 44,047 teleseismic P-wave arrival times. The results show that at depths shallower than 150 km, high-velocity anomalies appear west of the Tan-Lu Fault Zone, while low-velocity anomalies appear east of the fault zone. Strong lateral heterogeneity is exhibited along the fault zone. At depths of 230–470 km in the northwestern part of the Tan-Lu Fault Zone, distinct low-velocity anomalies exist, which may reflect the upwelling of hot and wet mantle. In contrast, clear high-velocity anomalies are present in the east, which may correspond to delaminated Eurasian lithosphere (downwelling flow). Both high-velocity and low-velocity anomalies exist in the mantle transition zone; the widespread high-velocity anomalies may reflect the stalled Pacific plate. Below the source region of the 1668 Tancheng earthquake, intermittent low-velocity anomalies occur from the upper mantle to the mantle transition zone, reflecting the upwelling flow of hot and wet mantle. Combining the current and previous research results, we propose that the Tancheng earthquake was influenced by the fluids from the upwelling of hot and wet mantle caused by lithospheric delamination. Beneath the Tan-Lu Fault Zone, complex mantle convection, including upwelling and downwelling flows, may occur in the big mantle wedge above the stalled Pacific plate in the mantle transition zone.
(3) After the Wenchuan earthquake, extensive field observations and research in seismology and geophysics were carried out in the Longmenshan Fault Zone and its surrounding areas to understand the seismogenic mechanism, yielding meaningful research results. Crustal structure obtained from local earthquake imaging shows that, bounded by the epicenter of the Wenchuan mainshock, there are significant differences in wave velocities on the northern and southern sides of the Longmenshan Fault Zone: the southern region exhibits obvious low-velocity anomalies, while the northern region shows significant lateral heterogeneity, which may explain why the aftershocks of the Wenchuan earthquake extend northeastward. The Wenchuan mainshock occurred at the boundary between high and low velocity anomalies, with low-velocity and high Poisson’s ratio anomalies beneath it. This result indicates that the high temperature and pressure formed near the Longmenshan Fault Zone due to the collision and compression between the Indian and Eurasian plates led to partial melting or fluid activity, reducing the effective normal stress on the fault plane and thus triggering the Wenchuan earthquake. Upper mantle structure obtained from teleseismic imaging shows that the Longmenshan Fault Zone is located in the transition zone from low-velocity anomalies below the Songpan-Ganzi Block to high-velocity anomalies beneath the Sichuan Basin, and this structure extends to a depth of 200–300 km. The high-velocity anomalies in the mantle transition zone are connected to the upper mantle high-velocity anomalies below the Burma Arc, indicating that the occurrence of the Wenchuan earthquake is closely related to dynamic processes such as hot and wet upwelling represented by low-velocity anomalies in the "big mantle wedge" structure formed by the deep subduction of the Indian Plate. Therefore, this paper holds that the Wenchuan earthquake is not only closely related to the crustal structure but also affected by the anomalous structure of the upper mantle. In addition, the Wenchuan earthquake may also be related to dynamic processes such as lower crustal flow, crustal shortening, and triggering by the Zipingpu Reservoir.