龙门山大地电磁深部结构及汶川地震(M_S 8.0)

Deep Structure of Magnetotelluric Profile on Longmen Mts.and Its Relation to the M_S 8.0 Wenchuan Earthquake

2008年5月12日汶川发生的MS 8.0地震使四川、甘肃和陕西等省遭受重大人员伤亡及财产损失,本文通过震前完成的穿过龙门山构造带中段的松潘-中江大地电磁测深剖面的反演解释,揭示了龙门山构造带及其两侧松潘-甘孜褶皱带、川西前陆盆地地壳内部30km深处电性结构。龙门山构造带东侧四川盆地为上部较厚低阻沉积盖层之下存在连续稳定高阻的扬子基底特征,而以西的松潘-甘孜褶皱带分上部和下部两部分,上部为高阻古生界夹低阻中新生界,下部(中下地壳)呈连续低阻层,推测可能存在一个连续稳定的壳内高导层。而龙门山恰好是青藏高原与扬子地台联合作用的结果,形成了上部高阻及下部基底高阻,中间夹西倾低阻带,低阻带最厚10km,其深度从地表10km连续向西延伸至20km深处,与松潘-甘孜褶皱带15～20km的低阻层相连。这个异常低阻带可能是松潘-甘孜地块向东向上移动的传输带,北川-映秀断层逆冲分量显然大于右行走滑分量,因此汶川地震属于右行平移-逆冲断裂型地震。

The Longmenshan Fault Zone is located in the eastern margin of the Qinghai-Tibet Plateau. By processing, interpreting and inversing the magnetotelluric observation data which were obtained in 2007 from the Songpan-Zhongjiang profile through the middle of Longmenshan tectonic zone, this paper reveals 30 km deep crust architecture of Songpan-Ganze foldbelt, Longmen Mts and Western Sichuan foreland basin, and finds the west-inclined the high-conductivity layer in the crust of 10-20 km of Longmen Mts. , it is estimated to deep conductivity characteristics of thrustbelt structure in Longmen Mts.. Sichuan basin, the east of Longmen Mts., which has the characteristics of under the upper thick low resistance sedimentary is the stable high resistance bedrock block. The west of it is Songpan-Ganze foldbelt, which in upper Paleozoic is high resistance rock mingled with low resistance fragments, the lower crust represent continuous low resistance, to speculate there may be a continuous and stable high-conductivity layer shell. Resulting from Songpan-Ganze foldbelt thrust in the Sichuan Basin stability block, Longmen Shan Fault Zone formed opportunely the unique geoelectrical structure which is a high resistance in the upper and lower part of high resistance basin basement, and mingling with low resistance zone in the middle. It is that the Wenchuan earthquake belongs to the dextial-reverse faulting type.