龙门山地震带的地质背景与汶川地震的地表破裂

GEOLOGICAL BACKGROUND OF LONGMEN SHAN SEISMIC BELT AND SURFACE RUPTURES IN WENCHUAN EARTHQUAKE

龙门山位于青藏高原与扬子地台之间,系由一系列大致平行的叠瓦状冲断带构成,自西向东发育汶川—茂汶断裂、映秀—北川断裂和彭县—灌县断裂,并将龙门山划分为3个构造地层带,分别为变形变质构造地层带(主要由志留系—泥盆系浅变质岩和前寒武系杂岩构成)、变形变位构造地层带(主要由上古生界—三叠系沉积岩构成)、变形构造地层带(主要由侏罗系至第三系红层和第四纪松散堆积构成)。龙门山断裂带属地震危险区,3条主干断裂皆具备发生7级左右地震的能力,其中映秀—北川断裂是引发地震的最主要断层,据对彭县—灌县断裂青石坪探槽场地的研究结果表明,在该断裂带上最晚的一次强震发生在930±40a.B.P.左右,据此,可以初步判定,这3条主干断裂的单条断裂上的强震复发间隔至少应在1000a左右,表明龙门山构造带及其内部断裂属于地震活动频度低但具有发生超强地震的潜在危险的特殊断裂,以逆冲-右行走滑为其主要运动方式。汶川地震属于逆冲—走滑型的地震,地表破裂分布于映秀—北川断裂带和彭县—灌县断裂带上。根据近南北向的断裂(小鱼洞断层、擂鼓断层和邓家坝断层)和地表断距可将映秀—北川断层的地表破裂带划分为两个高值区和两个低值区,两个高值区分别位于南段的映秀-虹口一带和位于中北段的擂鼓—北川县城—邓家坝一带;两个低值区分别位于中南段的白水河—茶坪一带和北段的北川黄家坝至平武石坎子一带,两个高值区分别与小鱼洞断层和擂鼓断层相关。根据保存于破裂面上的擦痕,可将该地震破裂过程划分为两个阶段,早期为逆冲作用,晚期为斜向走滑作用,其与地壳增厚构造模式和侧向挤出摸式在青藏高原东缘的推论具有不吻合性。鉴于龙门山的表层运动速率与深部构造运动速率具有不一致性,初步探讨了龙门山地区的地表过程与下地壳流之间的地质动力模型,认为下地壳物质在龙门山近垂向挤出和垂向运动,从而造成导致龙门山向东的逆冲运动、龙门山构造带抬升和汶川特大地震。在此基础上,根据汶川地震所引发的地质灾害,对地震灾后重建提出了的几点建议。

Longmen Shan is located the special joint between the Qinghai - Tibetan Plateau in the west and the Yangtze craton in the east. Consisting of a series of parallel imbricated thrusts, it develops, from the west to the east, the Maoxian - Wenchuan, Yingxiu - Beichuan and Pengxian - Guanxian faults. The Longmen Shan fault belt is a dangerous earthquake belt and the three main faults have occurred Ms. 7 earthquakes. The Yingxiu - Beichuan fault would be one of the most important causes of the earthquake hazard. According to the Qingshiping trench in the Pengxian - Guanxian fault, we believe that in this fault, the latest strong earthquake took place in 930 ± 40 a. BP. Then we can preliminarily determine the recurrence interval of strong earthquakes should be at least about 1000 years. We think that the Longmen Shan fault zone and its inner faults belong not only to a low -frequency seismic activity region and also to a potential powerful earthquake region. Wenchuan Earthquake is a thrust with strike - slip type. The surface ruptures are located in Yingxiu - Beichuan fault zone and Pengxian -Guanxian fault zone. The surface ruptures of the Yingxiu - Beichuan fault can be divided into two high - value and two low - value zones. The two high - value zones are located in the southern part of Hongkou, Yingxiu and the northern part of the Leigu, Beichuan - Dengjiaba area. The two low - value zones are located in mid - south part of the Baishuihe - Chaping area and Huangjiaba - Shikanzi, Pingwu areas. These four sections are divided by Xiaoyudong fault, Leigu fault and Dengjiaba fault. The two high -value areas were caused by the Xiaoyudong and Leigu faults. The scratches of the surface ruptures reveal the thrust movement occurred early and the strike - slip movement occurred lately during the earthquake. The earthquake is characterized by thrust and dextral strike not agree with both the crustal thickening model and the lateral extrusion model in the - Tibetan Plateau. -slip movement, which does eastern margin of the Qinghai Based on the geological background, tectonic setting, stratigraphy and lithology, the active tectonics of Longmen Shan and surface ruptures of the Wenchuan Earthquake, a dynamical model to illustrate possible links between surface processes and upward extrusion of lower crustal flow channel at the eastern margin of the Tibetan plateau have been proposed. The result is the material in lower crust in the Longmen Shan moving as nearly -vertical extrusion and uplift, resulting in the surface rate of tectonic movement differing according to depth rate as well as the occur- rence of large shallow Wenchuan Earthquake. According to the hazards caused by Wenchuan Earthquake, this paper provides some suggestions on the reconstruction.