龙门山断裂带深部结构与2008年汶川地震发震机理

Deep structure of the Longmenshan fault zone and mechanismof the 2008 Wenchuan earthquake

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物质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.

Since the occurrence of the 2008 Wenchuan earthquake（Ms8.0）, many researchers have conducted extensive seismological and geophysical observations and investigations and obtained important results about the Longmenshan fault zone. Crustal structure inferred from local tomography shows that seismic velocity exhibits significant changes across the Wenchuan earthquake hypocenter from the south to the north. To the south, obvious low-velocity（low-V） anomalies exist, whereas strong lateral heterogeneities are revealed to the north, which may explain why the aftershocks extend northeastward. The Wenchuan earthquake occurred at the boundary between high-velocity（high-V） and low-V anomalies and a significant low-V zone is revealed below the mainshock hypocenter, suggesting that the nucleation of the Wenchuan earthquake was related to partial melts and/or fluid effects and associated with the reduction of effective normal stress on the fault plane, due to high temperature and high pressure in the Longmenshan fault zone caused by the India-Asia collision. The upper-mantle structure inferred from teleseismic tomography shows that the Longmenshan fault zone is located in the transition zone from low-V anomalies beneath the Songpan-Ganzi block to high-V anomalies beneath the Sichuan basin. This structural feature extends down to 200-300 km depths. High-V anomalies in the mantle transition zone are connected with those in the upper mantle beneath the Burma arc, indicating that the Wenchuan earthquake could be associated with upwelling of hot and wet materials in the big mantle wedge formed by the deep subduction of the Indian plate. These results suggest that the generation of the Wenchuan earthquake was related to structural heterogeneities in not only the crust but also the upper mantle. In addition, high-poisson＇s ratio and high-conductivity anomalies are revealed beneath the Wenchuan earthquake source area, which may also reflect lower crustal flow that is compressed and moving eastward. When the eastward flow encountered the strong Sichuan basin block around the Longmenshan fault zone, the flow is further compressed upward along the margin of the Sichuan basin, which could have caused the fault rupture and generated the Wenchuan earthquake. Some geologists found a vertical co-seismic displacement of 6.5 m in the Longmenshan fault zone, suggesting that there exists an imbricate structure due to the crustal shortening. Such a shortening mechanism could rupture the seismogenic fault leading to the large earthquake. The Zipingpu reservoir is located very close to the Wenchuan earthquake epicenter. It is still debated whether the reservoir triggered the 2008 Wenchuan earthquake or not. Although some simulation results suggest that the Wenchuan earthquake could be triggered by the reservoir, there are large differences in the estimated stress in the hypocentral area due to large uncertainties of the physical parameters adopted in the numerical simulations. Hence, the physical parameters should be determined precisely in future investigations so as to clarify the effect of the reservoir on the seismogenesis. We also discuss the causal mechanism of the 2013 Lushan earthquake（Ms7.0） and future seismic risk in the gap between the aftershock zones of the Wenchuan and Lushan earthquakes.