Abstract: Longmenshan fault zone is a famous orogenic belt and seismic zone in the southeastern Tibetan plateau of China. The Wenchuan M_S8.0 earthquake on May 12, 2008 and the Ya'an M_S7.0 earthquake on April 20, 2013 occurred in the central-southern part of Longmenshan fault zone. Because of its complex geological structures, frequent earthquakes and special geographical locations, it has attracted the attention of many scholars around the world. Satellite gravity field has advantages in studying gravity field and gravity anomaly changes before and after earthquake. It covers wide range, can be updated regularly, without difficulty in terms of geographical restrictions, and is not affected by environmental factors such as weather, terrain and traffic. Therefore, the use of high-precision Earth satellite gravity field data inversion and interpretation of seismic phenomena has become a hot topic in earth science research. In order to understand satellite gravity field characteristics of the Longmenshan earthquake zone in the southeastern Tibetan plateau and its seismogenic mechanism of earthquake disasters, the satellite gravity data was used to present the terrain information of the study area. Then, by solving the regional gravity anomaly of the Moho surface, the crustal thickness of the study area was inverted, and the GPS velocity field data was used to detect the crustal deformation rate and direction of the study area. Combining the tectonic setting of the Longmenshan fault zone and the existing deep seismic sounding results of the previous researchers, the dynamic characteristics of the gravity time-varying field after the earthquake in the Longmenshan earthquake zone was analyzed and the mechanism of the earthquake was explored. The results show that the eastward flow of deep materials in the eastern Tibetan plateau is strongly blocked at the Longmenshan fault zone. The continuous collision and extrusion process result in a "deep drop zone" in the Moho surface, and the long-term stress effect is conducive to the formation of thrust-nappe and strike-slip structures. The Longmenshan earthquake zone was in the large-scale gradient zone of gravity change before the earthquake, the deep plastic fluid material transport velocity differed greatly, the fluid pressure was enhanced, and the rock mechanical strength in the seismic source region was weakened, which contributed to the intrusion of crustal fluid and the upwelling of the asthenosphere. As a result, the continuous accumulation of material and energy eventually led to continuous stress imbalance in the deep part and shear rupture of the deep weak structure, causing the occurrence of the thrust-nappe and strike-slip earthquake.

Key words: Longmenshan fault zone, satellite gravity field, Moho surface, GPS velocity field, gravity time-varying field, earthquake

摘要： 为了整体了解青藏高原东南缘龙门山断裂带的卫星重力场特征及其发震机制，文中基于重力卫星数据展现出研究区域的地形信息，并通过解算反映莫霍面起伏的区域重力异常，反演出研究区域的莫霍面深度，同时运用GPS速度场数据探测了研究区域地壳的形变速率及方向；最后，结合龙门山断裂带的构造背景及前人已有的深部探测结果，分析其发震前后重力时变场的动态特征并探讨了发震机理。研究结果表明：青藏高原东缘深部物质逐年向E流展，在龙门山断裂带处强势受阻，持续的挤压、碰撞过程形成了深部莫霍面的"陡降带"，长期的应力作用利于逆冲推覆和走滑构造的形成；龙门山地震带震前处于明显的重力变化梯度带，深部物质运移的流速差异大，流体压力增强，有助于地壳流体入侵和软流圈物质底侵上涌；物质和能量的持续积累最终将导致深部应力严重失衡、深部薄弱构造剪切破裂，从而发生逆冲、推覆构造运动的大地震。

关键词: 龙门山断裂带, 卫星重力场, 莫霍面, GPS速度场, 重力时变场, 地震