汶川Ms8.0地震近震源区震前形变特征的研究

发布时间：2018-03-14 17:23

本文选题：汶川地震　切入点：震前形变特征　出处：《中国地震局地震预测研究所》2015年硕士论文　论文类型：学位论文

【摘要】：汶川震前在震中区及其附近存在多种形变资料,比如GPS、地倾斜、跨断层水准、重力等。根据汶川震前的形变观测资料分析研究表明,和远震区变形相比,近震区变形趋势减弱,而且在龙门山断层附近呈现变异带;在汶川震前3 8年时间段的变形以及重力变化相对来说比较显著,而在震前1-3年的时间段内变化逐渐趋于平缓。呈现“越靠近震中变形越不显著;越靠近震前,变形越平缓”的这一特点。根据形变场、重力场以及密度场的资料及理论分析,表明三者存在较为紧密的关系。因为地壳物质密度的变化可以更加直观地反映汶川地震“孕育”、发生和发展的深层动力过程。为揭示震前变形现象,本文开展了利用重力反演地壳密度的研究。利用龙门山区域重力网提供的1996-2007年的13期的流动重力资料,借助重力位场三维反演技术,对近震区地下不同深度的物质密度的动态变化过程进行了反演。结果表明:空间分布上,整个地区的密度变化分布不均匀,个别区域密度变化显著,尤其在龙门山断裂带及其附近,布格重力异常图、地壳等厚图和密度变化图均表现为梯度陡变带,是物质变迁和构造变形差异运动强烈的地带,说明地表构造和深部构造都位于变异带上;在时间序列上,不同时间段内地壳介质密度的变化速率呈现横向不均匀性和阶段性,龙门山地区在震前10年密度变化就有增大的趋势,震前8-5年间变化最为剧烈,而临震前的变化则比较平缓;在垂向上,一定深度范围内,随深度的增加密度变化的程度越来越显著,说明深部的地壳活动比浅部活动大的多。此外,临震前短期内,重力变化、密度场变化空间分布上均呈现有序性和相对集中性,与形变场震前减熵的现象是吻合的。总之,密度场动态变化的趋势和重力场以及形变场的变化趋势比较一致。密度动态变化场的结果为选取适合汶川地震的孕育模型以及动力演化过程提供支持和佐证:为解释震前浅部小变形和深部密度显著变化的矛盾现象,提出汶川地震的“孕育”、发生和发展是上地壳和下地壳不处于同一个力源匹配系统,导致上、下地壳运动不统一的结果;结合密度变化的特征,选取出适合汶川地震的组合—硬化孕育模型,对震前短期以及近震源区的无显著变形这一现象进行了一定的揭示,加深了对震前形变前兆变化的机理的认识。本文的研究思路是通过地表形变和深部地球物理资料(地下密度)建立近震源区的孕震演化过程与动力学过程,用深部的地球物理资料为孕震模型提供支持,进而探讨震前小形变的机理。由于地下深部的变形涉及多方面的未明确因素,而且深部变形难以被精细探测,故本文的研究未能对地形变机理做出完善的解释,但为深部的密度变化及地壳形变研究提出了一种可行的研究方法。
[Abstract]:Before the Wenchuan earthquake, there are a variety of deformation data in and around the epicentre, such as GPS, ground tilt, cross-fault leveling, gravity, etc. According to the analysis of deformation observation data before Wenchuan earthquake, it is shown that the deformation trend in the near earthquake area is weaker than that in the far earthquake area. In addition, there is a variation zone near the Longmenshan fault, and the deformation and gravity changes are relatively significant in the period of 3 years before the Wenchuan earthquake. However, in the period of 1-3 years before the earthquake, the variation tends to be more and more gentle. It presents the characteristic of "the closer the epicentral deformation is, the more gentle the deformation is before the earthquake." according to the data and theoretical analysis of the deformation field, gravity field and density field, The results show that there is a close relationship among them, because the change of crustal material density can reflect more intuitively the deep dynamic process of Wenchuan earthquake's "gestation", occurrence and development, in order to reveal the phenomenon of deformation before the earthquake. In this paper, the study of inversion of crustal density by gravity is carried out. Using the 13 periods of fluid gravity data from 1996 to 2007 provided by the Longmen Mountain Regional Gravity Network, the three-dimensional inversion technique of gravity potential field is used. In this paper, the dynamic change process of mass density at different depths in the near seismic area is inversed. The results show that the density variation of the whole area is not uniform in spatial distribution, and the density change in individual regions is remarkable. Especially in the Longmenshan fault zone and its vicinity, the Bouguer gravity anomaly map, crustal isothickness map and density change map all show gradient steepness zone, which are the areas with strong difference between material change and tectonic deformation. It shows that both the surface structure and the deep structure are located in the variation zone, and in the time series, the change rate of the density of the crustal media in different time periods presents transversal inhomogeneity and phase. The density change in Longmen Mountain area has an increasing trend in 10 years before the earthquake, and the change is the most intense in the 8-5 years before the earthquake, but the change before the earthquake is relatively gentle, and in the vertical direction, within a certain depth, The change of density with depth is more and more obvious, which indicates that the crustal activity in depth is larger than that in shallow. In addition, the spatial distribution of gravity change and density field change is orderly and relatively concentrated in the short term before earthquake. It is consistent with the phenomenon of entropy reduction before the deformation field. In short, The dynamic change trend of density field is consistent with that of gravity field and deformation field. The result of density dynamic change field provides support and evidence for selecting the breeding model and dynamic evolution process suitable for Wenchuan earthquake. To explain the contradiction between the small deformation in shallow part before the earthquake and the significant change in deep density. It is proposed that the occurrence and development of Wenchuan earthquake is caused by the fact that the upper crust and the lower crust are not in the same force source matching system, which leads to the disunity of upper and lower crustal movements, and combines the characteristics of density variation. The combined hardening inoculation model suitable for Wenchuan earthquake is selected and the phenomenon of no significant deformation in the short term before the earthquake and near the focal area is revealed. The research idea of this paper is to establish the seismogenic evolution process and dynamic process of near-focal area through surface deformation and deep geophysical data (underground density). The deep geophysical data are used to support the seismogenic model and the mechanism of the small deformation before the earthquake is discussed. Because the deformation in the underground depth involves many undefined factors, and the deep deformation is difficult to be accurately detected, Therefore, the research in this paper can not make a perfect explanation for the mechanism of topographic deformation, but it provides a feasible method for the study of the density variation and crustal deformation in the deep part.
【学位授予单位】：中国地震局地震预测研究所
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：P315.72

【参考文献】