龙门山断裂带深部构造变形的粘弹性模拟与分析
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摘要
2008年5月12日四川龙门山断裂带上发生Ms8.0汶川大地震,造成了巨大的人员伤亡和经济损失。而由于龙门山断裂带地区具有复杂的地质结构以及该地区的地表滑动速率较小,被认为是地震的“平静区”,此次汶川地震的突发性及其强度出乎地学家们意料,带来了很大的疑问：在如此小的地表滑动速率的条件下,何以造成如此巨大的地震的能量又是如何积累的?由此,使得我们对建立和研究龙门山力学理论模型有了新的思考。
龙门山断裂带地表的低滑动速率并不意味着地震活动性就是较弱的,考虑到地质结构的复杂性,其深浅构造变形可能存在明显的差异。或许该地区深处的滑动速率才是造成此次地震的主要因素,值得我们进行深入的研究。因此,本文根据龙门山断裂带地区的构造特征,在前人研究的基础上,建立二维的地质模型,采用粘弹性接触分析方法,对龙门山断裂带应力分布和深层构造变形进行研究,进行了以下工作：
(1)采用有限元分析程序ANSYS对研究对象进行建模。在有限元建模中,考虑到龙门山断裂带地质结构的特殊性及复杂性,模型将涉及尽量大的区域,包括长度和深度。
(2)模型所涉及到的上地壳定义为弹性体区域,下地壳和上地幔则定义为粘弹性体区域,而且模型的断层处、上地壳与下地壳之间以及下地壳与上地幔之间施加接触约束。(3)通过有限元程序对青藏高原物质东流过程中龙门山断裂带及其周边地区的应力分布进行模拟分析。
(4)依据模型的数值模拟,考察龙门山断裂带断层位移滑动情况。在此基础上,对模型深浅滑动速率进行分析并探讨浅表变形速率较小情况下发生汶川大地震的可能潜在因素。
通过对龙门山断裂带地区有限元研究,得到的理论数据结果具有理论和应用分析意义。结合龙门山深部构造特征、断层的活动性及青藏高原物质运移的作用机理,对汶川地震成因及其动力学机制进行了探讨。
At 2:28p.m on May 12th 2008, the great earthquake took on the Longmen Shan fault in Sichuan province. The earthquake caused huge casualties and economic loss. The Wenchuan earthquake occurred on the Longmen Shan fault belt which is between the stabile Sichuan Basin and the Qinghai-Tibet plateau having a complex geological structure. And the region's surface slip rate is lesser. So the region has been considered "calm" area of the earthquake. Therefore, sudden and strength of the great earthquake gave a surprise to scientists and brought a huge doubt that how did the huge energy accumulate to trigger the great Wenchuan earthquake while the rate of superficial tectonic deformation was very lower? Therefore, the great earthquake made us the new thinking of establishment of dynamics theory model and study.
We got an inspiration from the Wenchuan earthquake that low slip rate on surface of the fault zone doesn't mean weak seismicity. Although the surface deformation of Longmen region is lesser, but the area of geological structure is complex and its deformation situation of depth has not been studied, maybe slip rate of the region deep is the main factors which cause the earthquake. According to the Longmen mountain fault's characteristic, we developed a geological model in the Tibet plateau and took the viscoelastic contact analysis method to make an analysis research on the stress distribution and tectonic deformation of the Longmen Shan fault zone, and got the following research:
(1) Develop a geological model on research objects with the finite element analysis program ANSYS. Considering the geological structure of particularity and complexity in the Longmen mountain fault zone, the model would involve as far as possible big regional in the length and depth.
(2) Define the upper crust and of the model as elastic area, the lower crust and upper mantle as viscoelastic materials area and exert contact constraint between the crust and the crust and t upper mantle.
(3) Use the finite element program to simulate and analyze the stress distribution of the Longmen mountain fault zone and its surrounding areas in the process of flow east of the Tibet plateau substances.
(4) Based on numerical simulation, inspect slip displacement conditions of the Longmen Shan fault and make analysis on slip rate of the model to explore potential factors of the Wenchuan Earthquake in the case of superficial deformation rate lesser.
Through the finite element research on Longmen Shan fault region, the model results have theoretical and practical analysis significance. Considering the deep tectonic characteristics of the Longmen Shan and the activity of the fault and physical mechanism of the Tibet plateau migration, discuss dynamics mechanism of the Wenchuan Earthquake.
龙门山断裂带地表的低滑动速率并不意味着地震活动性就是较弱的,考虑到地质结构的复杂性,其深浅构造变形可能存在明显的差异。或许该地区深处的滑动速率才是造成此次地震的主要因素,值得我们进行深入的研究。因此,本文根据龙门山断裂带地区的构造特征,在前人研究的基础上,建立二维的地质模型,采用粘弹性接触分析方法,对龙门山断裂带应力分布和深层构造变形进行研究,进行了以下工作：
(1)采用有限元分析程序ANSYS对研究对象进行建模。在有限元建模中,考虑到龙门山断裂带地质结构的特殊性及复杂性,模型将涉及尽量大的区域,包括长度和深度。
(2)模型所涉及到的上地壳定义为弹性体区域,下地壳和上地幔则定义为粘弹性体区域,而且模型的断层处、上地壳与下地壳之间以及下地壳与上地幔之间施加接触约束。(3)通过有限元程序对青藏高原物质东流过程中龙门山断裂带及其周边地区的应力分布进行模拟分析。
(4)依据模型的数值模拟,考察龙门山断裂带断层位移滑动情况。在此基础上,对模型深浅滑动速率进行分析并探讨浅表变形速率较小情况下发生汶川大地震的可能潜在因素。
通过对龙门山断裂带地区有限元研究,得到的理论数据结果具有理论和应用分析意义。结合龙门山深部构造特征、断层的活动性及青藏高原物质运移的作用机理,对汶川地震成因及其动力学机制进行了探讨。
At 2:28p.m on May 12th 2008, the great earthquake took on the Longmen Shan fault in Sichuan province. The earthquake caused huge casualties and economic loss. The Wenchuan earthquake occurred on the Longmen Shan fault belt which is between the stabile Sichuan Basin and the Qinghai-Tibet plateau having a complex geological structure. And the region's surface slip rate is lesser. So the region has been considered "calm" area of the earthquake. Therefore, sudden and strength of the great earthquake gave a surprise to scientists and brought a huge doubt that how did the huge energy accumulate to trigger the great Wenchuan earthquake while the rate of superficial tectonic deformation was very lower? Therefore, the great earthquake made us the new thinking of establishment of dynamics theory model and study.
We got an inspiration from the Wenchuan earthquake that low slip rate on surface of the fault zone doesn't mean weak seismicity. Although the surface deformation of Longmen region is lesser, but the area of geological structure is complex and its deformation situation of depth has not been studied, maybe slip rate of the region deep is the main factors which cause the earthquake. According to the Longmen mountain fault's characteristic, we developed a geological model in the Tibet plateau and took the viscoelastic contact analysis method to make an analysis research on the stress distribution and tectonic deformation of the Longmen Shan fault zone, and got the following research:
(1) Develop a geological model on research objects with the finite element analysis program ANSYS. Considering the geological structure of particularity and complexity in the Longmen mountain fault zone, the model would involve as far as possible big regional in the length and depth.
(2) Define the upper crust and of the model as elastic area, the lower crust and upper mantle as viscoelastic materials area and exert contact constraint between the crust and the crust and t upper mantle.
(3) Use the finite element program to simulate and analyze the stress distribution of the Longmen mountain fault zone and its surrounding areas in the process of flow east of the Tibet plateau substances.
(4) Based on numerical simulation, inspect slip displacement conditions of the Longmen Shan fault and make analysis on slip rate of the model to explore potential factors of the Wenchuan Earthquake in the case of superficial deformation rate lesser.
Through the finite element research on Longmen Shan fault region, the model results have theoretical and practical analysis significance. Considering the deep tectonic characteristics of the Longmen Shan and the activity of the fault and physical mechanism of the Tibet plateau migration, discuss dynamics mechanism of the Wenchuan Earthquake.
引文
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[5]董树文,张岳桥,龙长兴等.四川汶川MS8.0地震地表破裂构造初步调查与发震背景分析[J].地球学报,2008,29(3)：392-396
[6]马博琳.青藏高原东缘上地壳的拱起冲断作用与汶川地震的动力学机制[J].2地震学报,2009,14(3)：123-129
[7]徐锡伟,闻学泽,冉勇康,等.5.12汶川8.0级地震地表破裂图集[M].北京：地震出版社,2009
[8]张致伟.汶川8.0级地震前龙门山断裂带的地震活动性和构造应力场特征[J].2009,地震学报,31(2)：117-127[9]朱介寿.汶川地震的岩石圈深部结构与动力学背景[J].成都理工大学学报(自然科学版)2008,35(4)：348-356.
[10]卢华复,贾东,王良书,等.关于汶川地震发震机制[J].高校地质学报,2008,14(2)：133-138.
[11]张培震,徐锡伟,闻学泽,冉勇康.2008年汶川8.0级地震发震断裂的滑动速率、复发周期和构造成因[J].地球物理学报,2008,52(4)：1066-1073
[12]韦伟.汶川地震孕震机理的研究及其对地震预报的启示[J].防灾科技学院学报,2009,11(1)：10-19
[13]万永革.汶川地震产生的应变场和位移场[J].防灾科技学院学报,2009,11(1)：5-9
[14]陈学忠,赵晓燕,李艳娥,陆小华.从2008年5月12日四川汶川地震看地震的成冈[J].防灾科技学院学报,2009,11(1)：7-12
[15]马文涛,徐锡伟,于贵华,徐长朋.汶川地震发震构造应力场分析[JJ.国际地震动态, 2010
[16]孙竹凤,范天佑.汶川地震主震断层构造应力场数值分析[J].2010,30(3)：253-257
[17]Qifu Chen,Le Li,Fenglin Niu.Jinrong Su. Large slip rate detected at the seismogenic zone of 2008 MW 7.9 Wenchuan earthquake[J].Earthq Sci.2011,24:101-106
[18]Alexandra Witze, The sleeping dragon,Nature,2009, Vol(459):153-157
[19]Burchfiel B C,Chen Z,Liu Y,et al.Tectonics of the Longmen Shan and adjacent regions,central China.Int Geol Rev,1995,37:661-735
[20]Royden L H,Burchfiel B C,King R W,et al.Surface deformation and lower crustal flow in eastern Tibet.Science,1997,276:788-790
[21]Royden L H,Burchfiel B C,Robert D van der Hilst,The geological evolution of Tibetan plateau,Science,2008,Vol(321):1054-1058
[22]Densmore A L, Ellis M A, Li Yong, R. Zhou, Hancock G S,Richardson N J. Active tectonics of the Beichuan and Pengguan faults at theeastern margin of the Tibetan Plateau[J].Tectonics,2007,80(8):113-127
[23]England P C, Molnar P. Right-lateral shear and rotation as the explanation for strike-slip faulting in eastern Tibet[J]. Nature,1990, (344):140-142
[24]Avouac J P, Tapponnier P. Kinematic model of active deformation in central Asia[J]. Geophysical Research Letters,1993, (20):895-898
[25]Godard, V., R. Pik, J. Lave, R. Cattin, B. Tibari, J. de Sigoyer, M. Pubellier, and J. Zhu (2009), Late Cenozoic evolution of the central Longmen Shan, eastern Tibet:Insight from (U-Th)/He thermochronometry,Tectonics,28, TC5009, doi:10.1029/2008TC002407
[26]朱守彪,张培震.2008年汶川地震Ms8.0地震发生过程的动力学机制研究[J].地球物理学报,2009,52(2)：418-427
[27]李勇,黄润秋,周荣军,Alexander L等.龙门山地震带的地质背景与汶川地震的地表破裂[J].工程地质学报,2009,17(1)：3-16
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