摘要
2008年5月12日我国四川省汶川地区发生了震惊世界的M_S8.0地震.历史上,同类地震在大陆内部极为罕见.该地震深部构造背景的研究对理解其成因极为重要.本文利用中国地震局地质研究所地震动力学国家重点实验室在川西地区布设的大规模密集流动宽频带地震台阵记录的远震P波波形数据和接收函数非线性反演方法,得到了沿北纬31°线的19个台站下方120 km深度范围内的S波速度结构及台站下方地壳的平均泊松比.该观测剖面穿越了主震区,总长度约为420 km. 我们的结果揭示了川滇地块、松潘—甘孜地块和四川盆地三个不同地块构造差异.上述三个地块的地壳结构特征可以概括为:(1)四川盆地前陆壳幔界面向西侧倾斜并有较为明显的横向变形.地壳厚度存在46-52 km的横向变化,中下地壳S波速度存在横向变化,地壳平均泊松比值较高(0.28-0.31),但在龙门山断裂带附近,显示了坚硬地壳的特征,地壳平均泊松比仅为0.2;(2)松潘—甘孜地块地壳厚度由西侧靠近鲜水河断裂的60 km,向东减薄为52 km,在14-50 km深度范围内存在S波速度2.75-3.15 km/s的楔状低速区,其厚度由西侧的-30 km向东逐渐减薄为-15 km,相应区域的地壳平均泊松比高达。0.29-0.31;(3)鲜水河断裂西侧,川滇地块地壳结构相对简单,地壳厚度为58 km,并在26 km深度存在约10 km厚度的高速层,地壳内平均泊松比约为0.25; (4)汶川大震区在12-23 km深度上具有近乎4.0 km/的S波高速结构.而其下方的地壳为低速结构.地壳平均泊松比0.31-0.32,汶川大震的余震序列主要分布在高速介质区域内. 本文的结果表明松潘—甘孜地块的地壳相对软弱;而且并不存在四川盆地向西侧的俯冲.我们认为在青藏高原东向挤压的长期作用下,四川盆地强硬地壳的阻挡作用可导致松潘—甘孜地块内部蓄积很大的应变能量以及上、下地壳在壳内低速层顶部边界的解耦,在龙门山断裂带附近形成上地壳的铲形逆冲推覆.汶川大地震及其邻近区域所具有的坚硬上地壳和四川盆地的阻挡作用为低应变率下的高强度应力积累创造了必要条件,而松潘—甘孜地块长期变形积累的高应变能构成了孕育汶川大地震的动力来源.
On 12th of May, 2008, a devastating Ms8. 0 event shocking the global world occurred in the Wenchuan region of Sichuan province. Historically, the same kind of earthquakes is very rare inside of the continent. The study on the tectonic environment of this event is crucial for understanding its genesis. From the teleseismic waveform data recorded by the dense large-scale movable seismic array in the western Sichuan by the State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration （CEA）, we investigate the Swave velocity structure of the crust and upper mantle within the depth range of 120 km and averaged Poisson＇s ratio over the crust beneath 19 stations distributed along the profile of 31°N by using the non-linear receiver function inversion technique. This profile is about 420 km long and crosses the main shock area. Our results show the differences of the crustal structure among the blocks of Chuandian, Songpan-Garze and Sichuan basin. Their main features can be summarized as follows： （1）the crust-mantle boundary in the foreland of the Sichuan basin declines westward and has been deformed obviously; The crustal thickness has lateral variations of 46-52 km; the velocity of middle and lower crust has lateral variations; the Poissonrs ratio averaged over the crust reaches up to 0. 28-0. 31; however, the crust is hard nearby the Longmen Shan faults, where the Poisson＇s ratio averaged over the crust is only 0.2; （2）in the Songpan-Garze block, from the west to the east, the crustal thickness becomes thinner from 60 km to 52 km; in the depth range of 14-50 km exists a wedged low-velocity zone with the S-wave velocity of 2.75-3. 15 km/s, whose thickness decreases from -30 km in the western side to -15 km in the eastern side; the Poisson＇s ratio averaged over the crust containing the low-velocity zone reaches up to 0. 29- 0.31; （3）the crustal structure of the Chuandian block looks simple to the west of the Xianshuihe faults and the crustal thickness reaches to 58 km; but a high-velocity layer with thickness of -10 km exists in the depth of 26 km; the Poisson＇s ratio averaged over the crust is -0.25; （4）within the crust beneath the Wenchuan earthquake region exists a high-velocity structure of 44.0 km/s at the depth of 12-23 km, and below it exists a low-velocity structure, where the Poisson＇s ratio averaged over the crust is 0.31-0.32; the aftershocks of the Wenchuan earthquake are mainly distributed in the region of the upper crust with high-velocity structure. Our results manifest that the Songpan-Garze block has a soft and weakened crust, and the westward subduction of the Sichuan basin does not exist. It could be inferred that under the longterm pressure eastward from the Tibetan plateau, the Songpan-Garze block obstructed by the Sichuan basin has been deformed greatly and a large energy of deformation has been accumulated inside before the Wenchuan earthquake and that the upper crust decoupled with the lower crust at the top boundary of the low-velocity zone leads to the listric thrust of the upper crust. The rigid upper crust with high-velocity underneath the Wenchuan earthquake and its adjacent region as well as the obstruction of the Sichuan basin make it possible to accumulate high stress with low strain rate. The vast strain energy accumulated in the long-term deformation within the Songpan- Garze block should be the dynamic source of the Wenchuan great earthquake.
出处
《地球物理学报》
SCIE
EI
CAS
CSCD
北大核心
2009年第2期309-319,共11页
Chinese Journal of Geophysics
基金
国家重点基础研究项目(973) (2004CB418402)资助
关键词
汶川地震
接收函数反演
泊松比
流动地震台阵
地壳上地幔结构
Wenchuan earthquake, Receiver function inversion, Poisson＇s ratio, Movable seismic array, Crust and upper mantle structure
