汶川地震诱导的龙门山前陆盆地地腹构造形变机制研究

Ruptures and surface structure deformation caused by earthquakes are widely observed and well interpreted by both numerical and physical modeling. However, there are rare researches on the underground structure deformation away from the co-seismic rupture zone. After the Wenchuan earthquake, suspected detachment and fracture deformations are observed in western Sichuan basin. It is reported that an underground strata gliding of about 0.5 m was observed in a gas well in eastern Sichuan basin. These phenomena show that huge earthquake could cause underground structure deformation in near to middle fields, such as detachment and fracture. This project is planned to study the possible underground structure deformation induced by Wenchuan earthquake in Longmenshan foreland basin and its physical mechanism. The research methods to be adopted include mechanical analysis, numerical simulation and physical simulation. The key problems to solved include building a tectonic-stratigraphic framework model of Longmenshan-Western Sichuan basin system, calculating the 3D dynamic pressure system caused by Wenchuan earthquake’s shock wave in the Longmenshan foreland basin, and modeling the possible detachment, rupture and fracture deformations in Western Sichuan basin interior induced by Wenchuan earthquake. The study will provide the theoretical basis and practical methods for the mathematical analysis of the tectonic evolution of the Longmenshan-Sichuan basin system, and for the mathematical analysis of oil and gas accumulation in the Western Sichuan. It may also provide a safe design basis for the underground space development in Chengdu City.

迄今对地震破裂带和地震引起的地表构造变形有许多详尽的观测和数值与物理模拟研究，但对离开断层带的近场地腹构造变形鲜有研究报道。汶川地震后我们在川西地腹发现了疑似的滑脱和断裂构造变形；据报告在川东油气井观测到了幅度达0.5m的地层滑动。这表明大地震能在近场-中场地腹形成或触发滑脱、断裂等构造运动变形。本项目研究汶川地震在龙门山前陆盆地内部诱发的构造变形及其机制。拟依综合地质地球物理资料解释的龙门山-四川盆地盆山系统地层构造格架和岩石矿物物理性质构建该系统的力学结构模型，使用数值模拟方法计算汶川地震冲击波在龙门山前陆盆地内部形成的三维动压分布，使用数值仿真和物理模拟方法研究评价川西地腹地层在汶川地震冲击波作用下形成滑脱、破裂、断裂等构造变形的可能性及其机制。研究将为龙门山-四川盆地盆山系统构造演化和川西油气控藏构造形成的数理分析提供理论基础和方法技术，为成都地下空间开发的安全保障提供设计依据。

通过对比分析汶川地震前后在龙门山前陆盆地采集的勘探地震数据可以确认汶川地震后川西地腹发生了反射地震可分辨的断裂与褶皱构造变形。发现构造形变的地腹区域位于川西孝泉-新场构造带，距离汶川地震同震破裂带的直线距离约50-70km, 深度约为4-5km。同时有报道称汶川地震后在川东北普光气田多口井观测到了幅度约达0.2m的 错断。这表明大地震能在近场-中场地腹形成或触发滑脱、褶皱、断裂等构造运动变形。此前对大地震的这类构造形变效应及其机制甚少研究，主要原因是缺少资料证据支持。. 研究认为，大地震诱导的近场地腹构造变形主要是既有断层和褶皱的发展性构造形变。断层与褶皱的基本形变机制都是岩体沿破裂面的相对运动，只是断层的破裂面切过岩石层面，而褶皱的破裂面是岩层界面。研究触发地震问题的学者常用库仑破裂应力变化（ΔCFS）来预测断层的活动趋势。ΔCFS虽有多个表达式，但基本都未考虑地腹条件下断层错动时的高围压环境。项目在进行地腹岩石构造变形力学机制分析的基础上，建立了地腹高围压环境条件下断层错动的应力条件准则：地腹断层错动需要克服的阻力除断层面的摩擦阻力外，还应包括位移岩体向前侵位需克服的阻力、位移岩体后端拉张破裂应力、断层面相对侧斜拉张破裂应力以及提升位移岩体重力位的力，这些力可以依据岩石的体积模量、抗拉强度、抗剪强度、密度、岩体滑移距等参数估算。项目研究认为，汶川地震引起的川西地腹构造形变是地震波动应力与既存高强度静剪切应力共同作用于构造薄弱面的结果，既存高强度静剪切应力和构造薄弱面的存在是形变发生的前提条件，静、动应力作用方向与构造薄弱面走向的一致性是触发构造形变的有利条件。大地震触发的近场地腹构造形变是地质构造运动的重要机制，对油气运移等成矿作用有重要影响。. 项目同时研究了汶川地震引起的川西地腹岩石的微破裂、水力压裂诱发地震的机制与条件、依据InSAR观测资料反演地震引起的地下构造形变，以及基于油气井生产数据智能分析的地震前兆监测等相关问题。