龙门山断裂带南段阶地变形定量研究

发布时间：2019-06-10 10:54

【摘要】：2008年龙门山断裂带中段突发汶川Ms8.0级大地震,时隔5年后的2013年4月20日在同一条断裂带的南段再次发生芦山Ms7.0级强烈地震,震源相距85km。本次地震没有产生连续的地表破裂带,其发震构造不明确。龙门山断裂带南段第四系沉积差,晚第四纪构造活动性资料零星,结论也存在差异。本文通过遥感解译、野外地质调查、对河流阶地进行差分GPS精确测量及详细的研究,建立了龙门山南段陇东镇至芦山盆地凤禾乡的河流阶地位相图,通过构造-气候旋回估计了河流阶地的废弃年龄,并结合石油物探资料和流动地震台站CCP成像结果、芦山地震余震精定位结果以及1:20万区域地质调查报告等多种资料,研究了龙门山断裂带南段的构造活动性,取得如下认识:1)青衣江宝兴县陇东镇至芦山县凤禾乡段主要发育6级河流阶地,T1阶地主要为堆积阶地,阶地结构完整;T2-T6级阶地为残留的基座阶地,河流相堆积层缺失或残存,但基座保留较完整,上覆第四系沉积物,前缘往往基座出露。依据构造-气候旋回方法,推测T1至T6阶地的废弃年龄分别约8-15ka、20-27ka、50-57ka、95-105ka、130-140ka、750-770ka,建立了晚第四纪以来的地貌时间标尺。2)晚更新世以来盐井-五龙断裂的逆冲活动速率约0.7mm/a。构造类比可知,龙门山断裂带南段的盐井-五龙断裂与龙门山断裂带中北段的中央断裂结构类似,虽然活动性弱,但具有发生特大地震的潜能。3)大川-双石断裂的活动未造成T2阶地的基座发生垂直断错,其在T6阶地形成以来也无明显的垂向活动,未造成其上盘的中央块体掀斜变形。4)龙门山断裂带南段山前背形山晚更新世以来平均缩短速率约0.015mm/a。5)芦山地震区域构造环境是包括大川-双石断裂、底板逆冲断层、芦山主发震断层和芦山反冲断层之间的冲起构造组成的构造体系。主发震断层为大川-双石断裂前缘的芦山主发震断层和芦山反冲断层。用GPS观测到的本次芦山地震的垂直断裂的最大水平位移代表每次地震龙门山山前背形的水平缩短量,则芦山地震的复发间隔约2000-2600a。芦山地震的发震特点是震级小,频率高。6)尽管芦山地震和汶川地震的震源相距仅85km,时隔仅5年,汶川地震的发震断层为中央断裂和前山断裂,而芦山地震的发震断层与龙门山南段前缘褶皱相关,两次地震为相互独立的地震事件,因为不在相同的断裂上,所以不具有相互填补的关系。因此,龙门山断裂带南段未来仍有大震危险性,该区的大震发震构造可能是以逆冲性质为主的盐井-五龙断裂。南段的应力释放方式,也可能以龙门山山前一些规模相对较小的次级断裂释放,如本次芦山地震的发震断裂。
[Abstract]:In 2008, a Wenchuan Ms8.0 earthquake broke out in the middle part of the Longmenshan fault zone, and a strong earthquake of magnitude Ms7.0 occurred again in the southern section of the same fault zone on April 20, 2013, five years later, with a focal distance of 85 km. There is no continuous surface rupture zone in this earthquake, and its seismogenic structure is not clear. The Quaternary sedimentary data in the southern segment of Longmenshan fault zone are poor, and the late Quaternary tectonic activity data are sporadic, and the conclusions are also different. In this paper, by means of remote sensing interpretation and field geological survey, the differential GPS accurate measurement and detailed study of river terraces are carried out, and the phase diagram of river rank from Longdong Town in the southern section of Longmen Mountain to Fenghe Township in Lushan Basin is established. The abandoned age of river terraces is estimated by tectonic-climatic cycles, combined with petroleum geophysical data and CCP imaging results of mobile seismic stations, fine location results of aftershocks of Lushan earthquake and 1: 200000 regional geological survey report. The tectonic activity of the southern section of Longmenshan fault zone is studied, and the following understandings are obtained: 1) from Longdong Town to Fenghe Township Section of Baoxing County, Qingyi River, there are mainly 6 grade river terraces, T1 terraces are mainly accumulation terraces, and the terraces structure is complete; The T2-T6 terrace is a residual pedestal terraces, and the fluvial accumulation layer is missing or residual, but the pedestal is preserved completely, the Quaternary sediments are overlying, and the front edge is often exposed. According to the tectonic-climatic cycle method, it is inferred that the abandoned ages of terraces T1 to T6 are about 8: 15ka, 20 / 27ka, 50 / 57ka, 95 / 105 ka, 130 / 140ka and 750 / 770ka, respectively. The geomorphological time scale since the late Quaternary has been established. 2) the thrust activity rate of the Yanchi-Wulong fault since the late Pleistocene is about 0.7 mm / a. The structural analogy shows that the Yanchi-Wulong fault in the southern section of the Longmenshan fault zone is similar to the central fault structure in the central and northern segment of the Longmenshan fault zone, although the activity is weak. However, it has the potential to occur a large earthquake. 3) the activity of the Dachuan-Shuangshi fault does not cause the vertical fault of the base of the T2 terraces, and there is no obvious vertical activity since the formation of the T6 terraces. There is no oblique deformation of the central block in the upper plate. 4) the average shortening rate of the Piedmont back mountain in the southern segment of the Longmenshan fault zone since the late Pleistocene is about 0.015mm/a.5) the regional tectonic environment of the Lushan earthquake includes the Dachuan-bimolith fault. The structural system composed of bottom plate thrust fault, Lushan main seismic fault and Lushan recoil fault. The main seismic faults are Lushan main seismic fault and Lushan recoil fault at the leading edge of Dachuan-Shuangshi fault. The maximum horizontal displacement of the vertical fault of the Lushan earthquake observed by GPS represents the horizontal shortening of the front shape of Longmen Mountain in each earthquake, then the recurrence interval of the Lushan earthquake is about 2000 脳 2600a. The earthquake of Lushan earthquake is characterized by small magnitude and high frequency. 6) although the source distance between the Lushan earthquake and the Wenchuan earthquake is only 85km and the interval is only five years, the seismic fault of the Wenchuan earthquake is the central fault and the Qianshan fault. The seismic fault of Lushan earthquake is related to the front fold of the southern segment of Longmenshan section. The two earthquakes are independent seismic events, because they are not on the same fault, so they do not have the relationship of filling each other. Therefore, there is still the risk of large earthquakes in the southern section of Longmenshan fault zone in the future, and the seismic structure in this area may be the Yanchi-Wulong fault, which is dominated by thrust. The mode of stress release in the southern section may also be the release of some relatively small secondary faults in front of Longmen Mountain, such as the seismic fault of the Lushan earthquake.
【学位授予单位】：中国地震局地震预测研究所
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：P315.2

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