活动弯矩断层陡坎的识别、时空演化特征与强震

The catastrophic earthquakes such as 2015 Nepal Mw7.9 and the July 3, 2015 Pishan Mw = 6.4 have focused attention on the hazards posed by blind thrust faults and highlight the importance of understanding the complex behavior of blind faults and their associated folds for seismic hazard assessment. Blind thrusts do not reach the Earth’s surface, complicating assessment of their activity and slip rate. .Bending-moment normal fault (BMNF) scarps results from coseismic folding, which itself is caused by coseismic fault displacement on a reverse fault that lies beneath the fold. Such well-formed normal fault scarps can be trenched and the resulting paleoseismic history could then be used as a surrogate (albeit possibly incomplete) history for the underlying, more poorly expressed blind thrust. Where and how BMNF scarps are present and what factors control their occurrence, however, are typically poorly known..The 2015 Pishan (Mw = 6.4) earthquake provided one of the best examples of coseismic rupture of a segmented Guman blind-thrust system that is manifest at the surface as a series of anticlines with clearly BMNF scarps. This earthquake lacked any surface rupture, but had an uplift signature associated with the anticline, arguing for a direct relationship between blind-thrust faulting, folding, BMNF and surface deformation. Proper definition of the geometry and kinematics of this fault and associated folds is critical to our understanding of blind-thrust earthquakes processes..We plan to investigate the spatial and temporal distribution of the BMNF scarps; try to define the kinematic evolution, folding mechanisms, subsurface geometry, fault slip rates, coseismic behavior, and paleoearthquake histories for the active, Guman fold and blind thrust system in the west Kunlun Shan by observations of entire thrust fault fold system provided by petroleum industry seismic reflection and well data, growth strata and pre-growth strata, the 2015 Pishan seismicity and focal mechanism solutions, and surface geology; through fold growth within the upper few hundred meters observed on flights of deformed fluvial or alluvial terraces recording incremental folding; and to reconstructing the Holocene paleoearthquake chronology of the blind thrust, by trenching the breakaway BMNF scarps. We will present models of the 2015 Pishan earthquake and related BMNF scarps, and assess the future regional seismic hazard.

2015年尼泊尔Mw7.9与新疆皮山Mw6.4等破坏性地震的发生凸显了隐伏逆断型强震的危害性和对其开展研究的迫切性。褶皱作用派生的地表弯矩断层陡坎可能是7级以上强震的指示器，为研究这类强震提供了一窗口和新途径。本申请拟以“活动弯矩断层与断层相关褶皱作用”为主线，选择皮山震中区的西昆仑山前陆盆地固满背斜带、横穿背斜的众多连续分布河流阶地及背斜顶部不同河流阶地面上宽数公里、长逾数十公里的活动弯矩正断层陡坎带，通过地质地貌填图、高分辨率无人机航拍影像与DEM数据分析、古地震槽探与测年、构造平衡剖面分析，研究晚第四纪固满背斜与下伏发震逆断裂的变形特征、褶皱隆升的增量递进变形与速率，查明弯矩正断层陡坎的时空展布与演化规律及位移分布与古地震时序，探讨皮山地震发震机理与弯矩正断层陡坎形成新模式，评价该构造带未来强震危险性，提出定量研究活动隐伏逆断层的新方法，为抗震设防中避让带的确定提供科学依据。

如何研究活动褶皱下伏低角度逆断层的地震危险性和危害性是一挑战。活动褶皱作用派生的地表弯矩正断层陡坎可能是7级以上强震的产物，为研究这类强震提供了窗口。本项目选择曾发生2015 年皮山Mw6.4级地震的西昆山前固满背斜带、众多变形河流阶地及弯矩正断层陡坎带，通过大比例尺填图与差分GPS 测量、高分辨率(0.2m)无人机摄影测量、断错微地貌分析、地震反射剖面解译、年代样品测试，取得了以下主要进展与成果：1)划分、对比了研究区阶地期次并确定了其形成年代，T3a阶地面21Ne深度剖面暴露年龄为3.7-5.2Ma，T1b阶地面10Be深度剖面暴露年龄为413-673ka。2)获取了固满背斜带弯矩正断层陡坎的空间展布及变形特征，发现弯矩正断层陡坎具有多次重复活动的特征，提出了背斜北翼活动轴面随时间的迁移控制了弯矩正断层陡坎的形成与演化这一新认识。3)改进了陡坎非线性模型的扩散系数，据此建立了非线性侵蚀退化陡坎形态学测年方法。4)确认了固满背斜是一膝折带迁移生长机制下的断弯褶皱，而非构造楔褶皱，尚处于背形断弯褶皱变形的初期即背斜隆升期。自250ka以来沿深部断坡的滑动速率约为2.6mm/a，其中约13%的滑动量被固满背斜的缩短吸收，剩余的87%滑动量则继续沿上断坪向北传递，可能被以北约160km处的玛扎塔格褶皱冲断带吸收，据此提出了西昆仑前陆冲断带晚第四纪变形的前锋是玛扎塔格褶皱冲断带这一新观点。5)2015年皮山地震沿深部盲断坡发生了部分破裂并未产生地表破裂。固满背斜地表发育的多条弯矩正断层陡坎可能是7级以上强震产物，下伏盲断坡具有发生7级以上强震的构造条件。6)分析和总结了塔里木周缘已发现的弯矩正断层和弯滑断层陡坎的地貌表现、形成条件(岩性、褶皱几何形态和褶皱机制)及其在褶皱变形中的作用，提出了活动褶皱持续生长过程中弯矩断层和弯滑断层陡坎形成演化的概念模型。