利用重力资料研究汶川地震震源区及周边介质密度分布

发布时间：2018-03-08 03:07

本文选题：龙门山断裂带　切入点：汶川地震　出处：《长安大学》2017年硕士论文　论文类型：学位论文

【摘要】：2008年5月12日在龙门山断裂带上发生了汶川Ms8.0地震,龙门山断裂带北邻秦岭造山带,南接则木河断裂带、安宁河断裂带、大凉山断裂带,西部是青藏高原,东部是四川盆地,同时也是我国南北地震带的重要组成部分,本文基于震前重力资料分析了重力变化特征,利用PSGRN/PSCMP软件模拟计算了汶川Ms8.0地震引起的地表同震水平位移、垂直位移和重力变化,然后基于变密度长方体模型和粒子群算法,利用区域剩余重力异常反演了汶川地震震源区地下介质密度结构,得到主要研究成果有:(1)震前流动重力观测资料较好的记录了重力场由稳定→上升→下降→平缓的变化过程,重力点值时序变化显示映秀附近率先出现重力极值区,然后北川附近也出现重力极值,龙门山断裂带两侧的相对重力变化表明在地震前四川盆地重力场相对稳定,川西高原重力变化比较显著。(2)模拟结果表明龙门山断裂带逆冲兼具右旋走滑的特征;同震垂直位移和同震重力具有较好的一致性,近震区隆升,重力值较小,远震区位移和重力基本没有变化,介质密度变化对同震重力也有显著影响,同震水平位移和重力变化模拟结果与实测值相比较为一致,垂直位移相差较大。(3)自由空气重力异常与地形有密切联系;布格重力异常与地壳厚度有密切联系;区域范围内大部分表现为不均衡状态,特别是龙门山断裂带不仅没有下降,反而处于隆升状态,因此在龙门山断裂带地壳内或上地幔必然存在大于均衡作用力的上升力推动龙门山断裂带隆升,这可能是地下深部剧烈的构造运动引起的;莫霍面起伏引起重力异常与区域地壳厚度联系较为密切;区域剩余重力异常基本反映了地壳各层密度横向分布特征。(4)各个深度的密度分布特征与区域剩余重力异常存在密切关系;不同深度层的密度均表现为西北区域较低,东南区域较高,自西向东逐渐增加的特征,反演结果与CRUST1.0地壳密度模型基本一致;川西高原地壳物质软弱,容易发生“东流”而“积压”在龙门山断裂带上,龙门山断裂带处于区域密度变化的陡变带,东西两侧密度差异较大,是密度“交换”的强烈地带,四川盆地是克拉通地壳,地壳物质稳定,有效的阻挡了川西高原的向东扩展。
[Abstract]:In May 12th 2008, the Wenchuan Ms8.0 earthquake occurred in the Longmenshan fault zone. The Longmenshan fault zone is adjacent to the Qinling orogenic belt in the north, the Zimuhe fault zone to the south, the Anning River fault zone, the Daliangshan fault zone, the Qinghai-Tibet Plateau to the west, and the Sichuan basin to the east. At the same time, it is also an important part of the north and south seismic belt in China. Based on the gravity data before the earthquake, the variation characteristics of gravity are analyzed in this paper, and the horizontal displacement, vertical displacement and gravity change caused by the Wenchuan Ms8.0 earthquake are simulated and calculated by using PSGRN/PSCMP software. Then, based on the variable density cuboid model and particle swarm optimization algorithm, the density structure of underground media in Wenchuan earthquake focal area is inversed by using regional residual gravity anomaly. The main research results are as follows: 1) the gravity observation data of flow before the earthquake recorded the gravity field from stable to stable. 鈫扲ise. 鈫扗rop. 鈫扞n the gentle process of change, the time series of gravity point values change shows that the gravity extremum region appears first near Yingxiu, and then the gravity extreme value also appears near Beichuan. The relative gravity variation on both sides of the Longmenshan fault zone indicates that the gravity field of Sichuan basin was relatively stable before the earthquake, and the gravity variation of the western Sichuan Plateau was significant. The simulation results show that the thrust of the Longmenshan fault zone has the characteristics of dextral strike-slip. The vertical displacement of coearthquake is consistent with the gravity of coseismic region, the gravity value is small, the displacement and gravity of the distant seismic area are not changed, and the change of the density of the medium has a significant effect on the coseismic gravity. The simulated results of horizontal displacement and gravity variation of coearthquake are consistent with the measured values, and the vertical displacement is larger than that of the measured values. The vertical displacement is larger than that of the measured values. The free air gravity anomaly is closely related to the topography, and the Bouguer gravity anomaly is closely related to the crustal thickness. Most of the faults in the region are unbalanced, especially in the Longmenshan fault zone, which is not only in the uplift state, but also in the uplift state. Therefore, in the crust or upper mantle of the Longmenshan fault zone, there must be an upward force greater than the equilibrium force to promote the uplift of the Longmenshan fault zone, which may be caused by the intense tectonic movement in the deep underground. The gravity anomaly caused by Moho surface fluctuation is closely related to the thickness of the regional crust. The regional residual gravity anomaly basically reflects the characteristics of the transverse distribution of the density of each layer of the crust. (4) the density distribution of each depth is closely related to the regional residual gravity anomaly, and the density of the different depth layers is lower than that of the northwest region. The inversion results are consistent with the CRUST1.0 crustal density model, and the crustal material in the western Sichuan Plateau is weak and prone to "eastward flow" and "overstock" in the Longmenshan fault zone. The Longmenshan fault zone is located in the abrupt change zone of regional density change, and the density difference between east and west is great, which is a strong zone of density "exchange". The Sichuan basin is craton crust, and the crustal material is stable, which effectively blocks the eastward expansion of the western Sichuan Plateau.
【学位授予单位】：长安大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P315

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