汶川M_S8.0地震发震断裂大地震原地重复现象初析

PRIMARY ANALYSES OF IN-SITU RECURRENCE OF LARGE EARTHQUAKE ALONG SEISMOGENIC FAULT OF THE M_S8.0 WENCHUAN EARTHQUAKE

在历史记录中,成都和龙门山地区没有发生过类似汶川MS8.0地震强度的地震。那么,在地质记录中是否会存在类似震级的古地震遗迹?作者分别在中央和前山断裂中段的地表破裂带上4个地点开挖了探槽4个和剖面1个,并进行了断错地貌面的实测。文中从几个地点新老地貌面累计变形量、探槽揭露的古地震遗迹等方面讨论汶川地震发震断裂大地震原地重复现象存在的基本事实。结果表明:无论在中央断裂的小鱼洞、擂鼓镇还是前山断裂的白鹿镇、汉旺等地,汶川5.12地震之后Ⅱ级阶地断层陡坎与Ⅰ级阶地陡坎高度基本呈倍数关系,探槽揭露Ⅱ级阶地标志地层(黄砂土层)在断裂两盘的位差也是5.12地震的约2倍,显示在龙门山地区区域Ⅱ级阶地形成之后,汶川5.12地震发生之前,存在一次与汶川MS8.0地震地表变形规模相当的地震事件。

In the historical rocords,there have been no comparable earthquakes with the May 12 Wenchuan earthquake in Chengdu and Longmenshan mountain region. Then, whether the ancient earthquake traces with comparable magnitude in the geological records can be found has become an important scientific issue. The authors and other members of Wenchuan earthquake geological investigation team did fieldwork in the earthquake region for more than one month. Four trenches and one geologic section were excavated along the middle segment of surface faulting of both the central fault and the mountain front fault. And geomorphologic surfaces of deformation were measured. In this paper,we discuss the fact that there is prehistory large earthquake along the seismogenic fault of Wenchuan earthquake from analyzing the accumulated deformation of old and young fault scarps or geomorphologic surfaces,trenching,and comparing the activities of related faults and so on. The result shows that whether at Xiaoyudong, Leigu located on the central fault or Bailu, Hanwang on the mountain front fault,or at other places along the surface faulting of the the Wenchuan earthquake, the height of the fault scarp on the second terrace has a multiple relation with that on the first terrace after the Wen- chuan earthquake. The 4 trenches reveal that the dislocation of the marker strata of the second terrace on both sides of fault is twice of that of the Wenchuan earthquake, which shows that there was an earthquake event with the same scale of surface deformation of this Ms8.0 Wenchuan earthquake between the formation time of the second terrace in Lomhmenshan area and the May 12 Wenhucan earthquake.