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摘要: 利用四川地震台网的观测资料和体波地震层析成像方法反演了龙门山地区的S波速度结构，据此分析了龙门山断裂带的地壳结构和汶川震源区的深部构造特征.反演结果表明，地震破裂与龙门山断裂及其两侧的地壳结构差异存在明显的对应关系，汶川以北的龙门山上地壳具备较高的强度且明显抬升，灌县至江油是龙门山西侧应力积累的主要地区，汶川8.0级地震位于其南部边缘；四川盆地的刚性地壳向西俯冲于龙门山之下，其凸出部与造山带古老基底在汶川附近发生碰撞是汶川成为8.0级地震破裂起始点的主要原因.汶川以南的龙门山地区地壳上层具有较大的韧性，岩石强度相对减弱，与龙门山北部相比不易于应力积累和产生破裂，因而汶川以南的龙门山断裂缺少余震活动.龙门山地区地壳厚度明显增加，其原因与中下地壳具备较大的柔韧性有关.由于青藏东部向东挤出时受到四川盆地刚性岩石层的阻挡，龙门山中下地壳的塑性变形和垂向物质的增加导致地壳厚度加大和莫霍面下沉，以此方式吸收了龙门山地区的大部分地壳缩短量，地表则强烈褶皱抬升形成数千米的龙门山脉.Abstract: Using earthquake data from the Sichuan seismic network, the S-wave velocity structure of the Longmen Shan area was reconstructed by a body wave seismic tomography. It is followed by an analysis of the crustal structure along the Longmen Shan fault zone and in the Wenchuan earthquake area. Seismic rupture is well correlated with the Longmen Shan fault zone and the variation of crustal structure on the east and west of the fault zone. On the north of Wenchuan, the upper crust of the Longmen Shan has relatively high strength and is uplifted apparently. The Pengguan massif between Guanxian and Jiangyou is the main area for stress accumulation on the west of the Longmen Shan. The Wenchuan earthquake is located on the southern edge of this area. The rigid basement of the crust of the Sichuan basin underthrusts beneath the Longmen Shan, and the Wenchuan earthquake can be attributed to the collision of its western bulge with the base of the Pengguan massif along the Longmen Shan fault zone near Wenchuan. On the contrary, the Longmen Shan south of Wenchuan has a weak upper crust with lower mechanical strength, where stress is not easy to accumulate, compared with the northern Longmen Shan. It partly explains a lack of aftershocks along the Longmen Shan fault zone south of Wenchuan. Crustal thickening is observed beneath the Longmen Shan and this is because the strong lithosphere of the Sichuan basin obstructs the eastward flow of the weak crust of the eastern Tibetan. A large part of shortening is accommodated in a manner of ductile deformation, which causes the thickening of the deep crust and the downward flex of the Moho, while the upper crust is uplifted and folded, forming high mountains in the Longmen Shan.
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Key words:
- Longmen Shan /
- Wenchuan earthquake /
- Seismic tomography /
- S-wave velocity
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