汶川地震区断层围陷波探测

对汶川地震区开展震后科考和断层围陷波探测,本文主要介绍平通镇断层围陷波探测及初步结果。平通镇处在汶川8．0级地震断层带正上方,地表破坏十分严重。该断层围陷波测线横跨断层,以地震探槽为大致中点,沿NW—SE方向两端延伸,测线长约400m。这次观测结果表明,在汶川8．0级地震的新破裂带中可以观测到断层围陷波,反映了断层带内外的介质在物理性质上有较大的差异。该测线记录的断层围陷波优势频率大约为3—4Hz。探槽附近的台站断层围陷波较强,初步推测,该地段地壳内断层的宽度大约有200m。     

开发地震构造建模技术的研究方法

某油田已注水开发40年,高井网密度条件下仍对某些微构造、断层认识不足。运用三维地震解释资料与开发井资料解释两项技术结合,优势互补,相互验证,形成了一套开发地震可视化构造建模技术,应用该项技术显著提高了断层的首尾部延伸、复杂断裂带断层的组合关系、无井断点控制的小断层识别精度,对油田特高含水期提高钻井成功率、油田精细调整挖潜具有重要意义。     

基于加权一致性的蚁群算法在断层检测中的应用

突出地震数据振幅空间不连续性的断层增强属性体是断层自动解释的基础,而如何压制噪声、地层残余响应、角度不整合接触等伪断层的影响是断层自动检测的关键问题之一.在指纹图像边缘检测处理中,一致性是对指示局部方向场信息各向异性强度的一个度量,本文将其作为一种新的断层增强属性引入到地震数据处理中.为了更有效地压制噪声,本文通过对一致性进行加权处理,再结合蚁群算法,提出了新的断层自动检测方法——基于加权一致性的蚁群算法.理论地震模型和实际地震数据的处理结果表明,对于相同的断层检测过程,对比常规的基于方差属性的蚁群算法和基于C3相干属性的蚁群算法,本文所提方法不仅更有效地压制了噪声和地层残余响应产生的伪断层信息,而且更完整地展现了断层的延伸长度,同时具有更高的稳定性和计算效率.     

GPS揭示的郯庐断裂带中南段闭锁及滑动亏损

利用华北地区2009—2014年GPS水平运动速度场数据,采用块体负位错模型反演了郯庐断裂带中南段断层深部滑动速率、断层闭锁程度分布、断层滑动亏损速率分布及地震矩积累率,结合地表应变率分布,对郯庐断裂带中南段深、浅部形变、应变特征以及华北地区的地壳形变模式进行了分析.结果表明:郯庐断裂中南段的北端主要为右旋走滑特性,南端则表现为右旋走滑兼拉张性运动,断层滑动速率在0.9mm·a~(-1)至1.2mm·a~(-1),且沿断层走向由北至南逐次增大.断层闭锁程度分布沿走向分布不均一,断层闭锁深度由最北端的27km增加到中段的32km,至最南端变为5km,断层闭锁最深处与1668年郯城MS8.5震中位置相对应.断层滑动亏损速率沿走向由0.9mm·a~(-1)增加到1.2mm·a~(-1),沿倾向由地表至深部逐渐减小为0mm·a~(-1).地震矩积累率在郯庐断裂带中南段郯城附近较大,而地表对应区域为第二应不变分量的低值区.华北地区地壳变形以块体运动为主,块体内部应变及断层闭锁产生的负位错效应次之;郯庐断裂带中南段断层形变沿走向呈条带状分布,形变宽度单侧小于50km,形变量不超过1mm·a~(-1),且上盘形变略大于下盘.     

太行山东缘石家庄南部地壳结构及断裂活动性探测

采用深、浅地震反射和钻孔地质剖面相结合的方法,对太行山东缘石家庄南部的地壳结构和隐伏断裂的活动性进行了研究.深地震反射探测结果表明,该区地壳厚度33~38km,莫霍面从华北平原向太行山下倾伏.石家庄—晋县凹陷是受拆离断层控制的盆岭构造,太行山山前断裂为凹陷的西边界断裂,表现为上陡下缓的铲形断裂.石家庄—晋县凹陷中还发育北席断裂和栾城断裂,它们与太行山山前断裂一样受拆离断层的控制,未错断早更新世晚期以来沉积的地层不属于活动断裂.深地震反射剖面的中部还揭示了一个近垂直的穹窿状反射异常体,它可能起源于莫霍面,向上,穿过上、下地壳分界面,并延伸至上地壳.穹窿状反射异常体内部反射波视频率随深度增加而降低,在莫霍面附近的壳幔过渡带也出现明显的频率降低、界面扭曲和变形现象,推断它可能是上地幔岩浆上涌到地壳内部的侵入体.结合电磁测深结果可以发现,上地幔热物质的上涌和东、西向拉张可能是形成石家庄—晋县凹陷的动力学机制.探测结果为深入理解石家庄地区的深部地球动力学过程、华北克拉通破坏机制、深浅构造关系和地震构造提供了依据.     

GEOLOGICAL AND GEOMORPHIC EXPRESSIONS OF LATE QUATERNARY STRIKE-SLIP ACTIVITY ON JINTA NANSHAN FAULT IN NORTHERN EDGE OF QING-ZANG BLOCK

Jinta Nanshan Fault is an important fault in northeast front of Qing-Zang Plateau, and it is crucial for determining the eastern end of Altyn Tagh Fault. However, there is still debate on its significant strike-slip movement. In this paper, we study the Late Quaternary activity of Jinta Nanshan Fault and its geological and geomorphic expressions by interpreting aerial photographs and high-resolution remote sensing images, surveying and mapping of geological and geomorphic appearances, digging and clarifying fault profiles and mapping deformation characteristics of micro-topographies, then we analyze whether strike-slip activity exists on Jinta Nanshan Fault. We get a more complete fault geometry than previous studies from most recent remote sensing images. Active fault traces of Jinta Nanshan mainly include 2 nearly parallel, striking 100°~90° fault scarps, and can be divided into 3 segments. West segment and middle segment form a left stepover with 2~2.5km width, and another stepover with 1.2km width separates the middle and east segment. We summarize geomorphic and geologic evidence relating to strike slip activity of Jinta Nanshan Fault. Geomorphic expressions are as follows:First, fault scarps with alternating facing directions; second, sinistral offset of stream channels and micro-topographies; third, pull-apart basins and compressive-ridges at discontinuous part of Jinta Nanshan Fault. Geologic expressions are as follows:First, fault plane characteristics, including extremely high fault plane angle, unstable dip directions and coexistence of normal fault and reverse fault; second, flower structures. Strike-slip rate was estimated by using geomorphic surface age of Zheng et al.(2013)and left-lateral offset with differential GPS measurements of the same geomorphic surface at field site in Fig. 4e. We calculated a strike-slip rate of (0.19±0.05)mm/a, which is slightly larger than or almost the same with vertical slip rate of (0.11±0.03)mm/a from Zheng et al.(2013). When we confirm the strike-slip activity of Jinta Nanshan, we discuss its potential dynamic sources:First, eastern extension of Altyn Tagh Fault and second, strain partitioning of northeastward extension of Qilian Shan thrust belt. The first one is explainable when it came to geometric pattern of several E-W striking fault and eastward decreasing strike slip rate, but the former cannot explain why the Heishan Fault, which locates between the the Altyn Tagh Fault and Jinta Nanshan Fault, is a pure high angle reverse fault. The latter seems more explainable, because oblique vectors may indeed partition onto a fault and manifest strike-slip activity.     

DISTRIBUTION OF YANGJIA VILLAGE-YAODIAN SECTION OF WEIHE FAULT AND THE CHARACTERISTICS OF ITS LATE QUATERNARY ACTIVITY

The Yangjia Village-Yaodian segment of Weihe Fault, starting from Yangjia Village in the west, passing through Weijiaquan, Jinjiazhuang, Donger Village, Chenjiatai to Yaodian, occurs as a NE-striking fault dipping south with a total length of 33 kilometers. As a syn-depositional normal fault, it extends along the leading and trail edge of T₁, T₂ and T₃ terrace at the northern bank of Weihe River. Results of remote sensing interpretation, shallow seismic exploration, exploratory trench, and drilling show that the Yangjia Village-Yaodian section of Weihe Fault manifests as fault scarps, overlapping with the NE-extending terrace scarp at the northern bank of Weihe River. Weihe Fault broke the T₁ that can be distinguished on the shallow seismic profile and multiple profiles with broken signs from T₁ to the ground, which is the same with the cracks through the Han Tomb at the top of the exploratory trench in Yangjia Village. It shows that the fault may still be active from the late Pleistocene to Holocene. Through composite drilling section and the analysis of exploratory trench, there is no significant difference in activity between the Yangjia Village-Jinjiazhuang and Donger Village-Yaodian section. This segment has experienced a large displacement event since (46.0±3.3)ka BP, approximately 11.0~16.5m, with a vertical slip rate of 0.34~0.45mm/a. The most recent activity occurred approximately around 2.0ka BP. The left-step en echelon fracture zone at Jingjiazhuang separates this section into two minor ones, Yangjia Village-Jinjiazhuang section and Donger Villag-Yaodian section. Yangjia Village-Yaodian section in Weihe Fault and Yaodian-Zhangjiawan section which was found out in the Xi'an active fault detection and seismic risk assessment project can be combined into the Yangjia Village-Zhangjiawan section.     

THE DISCUSSION FOR THE NEW ACTIVITY OF THE TIANQUAN SEGMENT OF LONGMENSHAN FAULT ZONE AND ITS RELATIONSHIP TO THE 1327 TIANQUAN EARTHQUAKE,SICHUAN

The 2008 Wenchuan earthquake occurred along the Longmen Shan fault zone, only five years later, another M7 Lushan earthquake struck the southern segment where its seismic risk has been highly focused by multiple geoscientists since this event. Through geological investigations and paleoseismic trenching, we suggest that the segment along the Shuangshi-Dachuan Fault at south of the seismogenic structure of the Lushan earthquake is active during Holocene. Along the fault, some discontinuous fault trough valleys developed and the fault dislocated the late Quaternary strata as the trench exposed. Based on analysis of historical records of earthquakes, we suggest that the epicenter of the 1327 Tianquan earthquake should be located near Tianquan and associated with the Shuangshi-Dachuan Fault. Furthermore, we compared the ranges of felt earthquakes(the 2013 M7 Lushan earthquake and the 1970 M_S6.2 Dayi earthquake)and suggest that the magnitude of the 1327 Tianquan earthquake is more possible between 6½ and 7. The southern segment of the Longmen Shan fault zone behaves as a thrust fault system consisting of several sub-paralleled faults and its deep structure shows multiple layers of decollement, which might disperse strain accumulation effectively and make the thrust system propagate forward into the foreland basin, creating a new decollement on a gypsum-salt bed. The soft bed is thick and does not facilitate to constrain fault deformation and accumulate strain, which produces a weak surface tectonic expression and seismic activity along the southern segment, this is quite different from that of the middle and northern segments of the Longmen Shan fault zone.     

ACTIVITY FEATURES OF FAULTS AT THE EAST OF ZHANGSHAN ON THE MIDDLE SEGMENT OF TANLU FAULT ZONE

Given the scarcity of research on the activity of Xinyi-Sihong segment of the Tanlu Fault zone, this paper focuses on the Zhangshan segment where there are quite evident geomorphic features to complement the shortage of the research on the northern part of Xinyi-Sihong segment. This study enriches evidences for the late Quaternary activity and paleoseismological events on the Xinyi-Sihong segment. The Zhangshan segment is located at Xiaodian Town to Jintou Village of Suqian City, stretching towards NE for 7 kilometers with a dip angle of 60~80. Research of tectonic geomorphology shows that gullies in northern part of Zhangshan segment were evidently displaced, while in the southern, two NE-trending right-stepped fault scarps are developed, with an average height of 3 meters, which generally suggests that the fault was dominated by thrust and dextral motion. Two trenches were excavated in the southern part of Zhangshan segment, numbered Mayao trench 1 and Mayao trench 2. Both trenches reveal that:(1)within this segment, Tanlu Fault shows periodic fault activity, that is, normal faulting during Pliocene epoch while thrust faulting in Quaternary period; (2)an event occurred between 15.12ka BP to 11.82 BP; (3)the latest event possibly took place around 3 500 a BP. Based on integrated results of previous studies, we identify the dates of paleoseismic events on the Xinyi-Sihong segment as follows:more than 960 thousands years ago, early to middle period of late Pleistocene, (15.12~11.82)ka BP, (11.76±0.05)ka~(10.53±0.05)ka BP, (10.15±0.05)ka~(8.16±0.05)ka BP and 4 960~3 510a BP.     

PALEOSEISMIC RECORDS OF LARGE EARTHQUAKES ON THE CROSS-BASIN FAULT IN THE SALT LAKE PULL-APART BASIN AND CASCADE RUPTURE EVENTS ON THE HAIYUAN FAULT

Cascade rupture events often occur along large strike-slip fault zone.The 1920 AD M 8¹/₂ earthquake ruptured all 3 segments of the Haiyuan Fault,and the Salt Lake pull-apart basin is the boundary between the west and middle segment of the fault.The data of trenching and drilling reveal 7 events occurring since last stage of late Pleistocene,and the two youngest events are associated with the historical records of 1092 AD (possibly) and 1920 AD respectively.These events are all large earthquakes with magnitude M>8,and the recurrence of them is characterized by earthquake clusters alternating with a single event.Now it is in the latest cluster which may last about 1000 years.Comparison of the paleoseismic sequence of this study and previous results reveals that the cross-basin fault in the Salt Lake pull-apart basin does not always rupture when cascade rupture events occur along the Haiyuan Fault,and likely ruptures only when the magnitude of the events is large (maybe M>8).Though there are many advantages in paleoseismic study in pull-apart basin,we should avoid getting the paleoseismic history of major strike-slip fault zones only depending on the rupture records of inner faults in pull-apart basins with large scale (maybe a width more than 3km).