鲜水河断裂带的断错地貌及其地震学意义

断层的近代位错必将在地貌形态上留下深刻的印记,因而凭借断错地貌现象有可能得到关于活断层上地震活动性的重要信息。鲜水河断裂带上的断错地貌现象表明,断层以间歇性的地震滑动为特征,而且服从于大小相当的地震在原地重复发生的特征地震模式。断错水系的演化图象则进一步提供了在一定时段内,同一地点滑动速率基本稳定的线索。于是,可以把断错地貌的研究作为地震活动性研究的重要手段     

天山全新世活动断裂及古地震研究

横亘亚洲腹地的天山山脉近代构造活动十分强烈。规模较大的全新世活动断裂有２０多条，多为近东西走向的倾滑型逆断裂，常与活动褶皱相伴生。活动褶皱为无根的断裂扩展褶皱和滑脱褶皱，它的生长是受地下活动的盲断裂所控制，往往是褶皱地震潜在的地区。天山古地震活动遗迹很多，归纳其标志有：多重断层陡坎、古断塞塘、崩积楔、填充楔、推覆楔、地震断错台地和断裂扩展褶皱等。近几年对１０条全新世活动断裂进行开挖研究，已取得大地     

中国大陆古地震研究的关键技术与案例解析(2)——汶川地震地表变形特征与褶皱逆断层古地震识别

汶川MS8.0地震是近代少有的大陆褶皱逆断层型巨大地震,其地表破裂带是研究和解剖褶皱逆断层地表同震变形样式,并以此探讨古地震遗迹的不可多得的现实案例。在整理和分析汶川地震地表破裂带地质地貌调查资料的基础上,选择可能仅记录1次事件的平通、邓家(北川-映秀断裂)和九龙(江油-灌县断裂)等地为例,分析同震变形的特点和类型,并结合映秀、桂溪等地的古地震研究成果,讨论褶皱逆断层型古地震识别的技术要点。结果显示:地表变形主要包括逆断层直接位错、折曲位错变形和弯曲褶皱变形等类型;崩积楔、断层与地层切盖关系是分析断错地表型古地震事件的可行依据,而折曲位错变形型和弯曲褶皱变形型古地震识别则强调在上盘是否存在侵蚀不整合面,下盘是否存在生长地层,以及标志地层在断层两盘位差的突然增减;断层陡坎高度的倍数关系在一定程度上与古地震次数相关,但不能简单地用同震位移量除以陡坎高度的方法确定古地震期次;对于低角度逆断层的古地震识别,薄长状崩积楔、断层与堆积地层的切错关系和不同标志地层在断层两侧的累积位差的突变是重要的标志。识别古地震应因地制宜、思考多种因素的影响、用多种证据相互印证。     

郯庐断裂带赤山段中晚更新世之交的史前地震遗迹

赤山地处安徽省泗县东北部,为郯庐断裂带西支断裂通过处。以往研究认为,郯庐断裂带赤山段第四纪以来有较为明显的活动。此次在垂直于赤山段开挖的多个探槽中均揭露出史前地震遗迹(多期逆断型崩积楔),具体表现为断层上盘砖红色上白垩统砂岩或黄白色中更新统砾石层向东叠置于褐黄色中、上更新统黏土之上。在楔状体根部均发现向NW陡倾的断裂,陡立的断面向上转变为缓倾的上翻垮落面。多条探槽揭示的断裂点的连线总体呈NNE走向,断裂通过处的上盘后缘大部分残留了凸起地貌,部分EW向冲沟被断裂右旋扭错。多条探槽两壁都一致显示出从西向东的推挤垮塌痕迹。其中Tc1～Tc4表现为砖红色砂岩(K₂)逆冲垮落于黄褐色含铁锰结核黏土(Q_p2-3)之上,Tc5则表现为红砂岩(K₂)和黄白色砾石层(Q_p2)逆冲垮塌于上述的黏土之上。在几个探槽的逆断型崩积楔上均揭示出此段曾有多次逆断型崩积事件发生,个别探槽揭露楔体较少,可能与其所处的位置、揭露深度和剥蚀残存的高度有关。对开挖探槽及光释光测年结果的详细分析初步得出,郯庐断裂带赤山段在中、晚更新世之交曾多次发生大规模逆断型黏滑事件,即曾多次发生史前大地震。     

云南大理苍山东麓晚第四纪地震断层及古地震年代

大理盆地是滇西北主要的强震区之一。笔者1986—1987年对大理盆地进行了详细的1/5万活断层地质填图,在苍山东麓断裂带上发现了多处地貌陡坎,通过槽探,揭露出较为典型的古地震断层。1.上关龙首墙西地震断层(照1) 该断层保存有一条长100余米,高7—14米,近南北向延伸的陡坎(照2)。已开挖出两个断层剖面。其中一个剖面上见有两个“崩积楔”和切穿两个崩积楔的裂缝充填物,标志着这     

二台活断层的多次粘滑破裂和古地震复现期

研究二台活断层的水平断距与断尾沟长度的关系发现,该断层自全新世以来至少破裂过5次。每次粘滑破裂一般都发生8级地震,其古地震复现期平均为1800年左右。预计公元3700年二台断层将再次破裂发震,地点在活断层的南端。这种估计已为多级地震断层、多级断崖、多个地震崩积楔及开挖工作证实。     

新疆古地震研究

新疆古地震研究是地震科技联合基金会资助课题,从1987年6月至1989年3月近两年的工作,对新疆境内的昭管站、博罗霍洛、卡兹克阿尔特、秋立塔格和博格达—哈尔里克五条断裂重点地段进行了古地震普查与开挖工作,至此,已完成该课题研究任务,取得如下成果: 1.对地震断错地貌作了较系统的研究,把地震断错地貌分为地震断层崖、地震断错—挠     

正断层崩积楔粒度特征及沉积环境

通过对崩积楔沉积物进行粒度特征分析,研究崩积楔形成时的沉积条件和环境特征。沿系舟山北麓断裂,具体分析了水峪剖面、湖村剖面上各期崩积楔共21个样品的粒度及粒度参数。在崩积楔体中,砾、砂、黏土等各个粒级均有较大范围的分布,其中砾级总体概率含量达50%～70%。粒度分布为多峰态,分选差,偏度为极正偏,尖峰度>1,说明崩积楔是以砾级为主体的非均一性混杂堆积。崩积楔样品与作为物源的断层下盘样品的粒度分析对比表明,作为一种近源堆积,崩积楔的粒度特征与其下盘沉积物在粒度分布上有一定的继承性     

天全-荥经断裂晚第四纪活动的地貌证据

天全-荥经断裂是青藏高原东南缘的1条晚第四纪且活动资料较少的断裂，在2008年汶川M 8.0地震之后，龙门山断裂带西南端未来的地震危险性受到关注。对天全-荥经断裂晚第四纪活动特征的获取有助于理解该区地震危险性的评价。通过遥感影像解译，结合野外调查和断错地貌测量，分析了天全-荥经断裂在荥经下坝村至桂花村切过荥经河河谷晚第四纪地貌区的活动证据。断裂沿线形成冲积扇断错、阶地坎断错和断坎等地貌，并沿断裂发育滑坡。晚第四纪以来断裂以左旋走滑活动为主，其中T2/T2'阶地坎被左旋断错22—24m。利用荥经河阶地与青衣江河流阶地对比，认为该断裂20—40ka以来左旋走滑速率为0.6—1.1mm/a。仍需要从古地震等方面开展工作，来进一步确定天全-荥经断裂的地震危险性。     

论地震地貌

本文论述了地震断层崖、地震断错地貌、地震脊形地貌、地震洼地、地震堆积地貌、地震滑坡及其成因机制。这些由地震内营力造成的地震地貌,是活动构造地貌的一个重要的组成部分。最后讨论了这类地貌在外营力的削蚀作用下的演变过程,识别这种演变地形,对古地震的存在提供判别标志。     

TYPE AND DISPLACEMENT CHARACTERISTICS OF LINGSHAN M6¾ EARTHQUAKE SURFACE RUPTURE ZONE IN 1936, GUANGXI

On April 1, 1936, an M6¾ earthquake occurred on the Fangcheng-lingshan Fault. So far, the Lingshan M6¾ earthquake is the biggest one in South China. There are some reports about the Lingshan earthquake fissures, but its surface rupture hasn't been systemically studied. Based on the geological mapping and measurement of the right-lateral displacement and vertical offset, the surface rupture zone caused by the Lingshan M6¾ earthquake was found, which contains two secondary surface rupture zones in the east and west respectively, its strike varies from N55°E to N60°E with en echelon-like distribution along the north section of Lingshan Fault, and its total length is about 12.5km. The western surface rupture zone locates intermittently along Gaotang-Xiatang-Liumeng, about 9.4km in length, with a right-lateral displacement of 0.54~2.9m and a vertical offset of 0.23~1.02m; the other one appears between Jiaogenping and Hekou, about 3.1km in length, with a right-lateral displacement of 0.36~1.3m and a vertical offset of 0.15~0.57m. The maximum right-lateral displacement and vertical offset are 2.9m and 1.02m, appearing at the east of Xiatang reservoir. The types of surface rupture mainly contain earthquake fault, earthquake scarp, earthquake fissure, earthquake colluvial wedge, earthquake caused landslide and liquefaction of sand and so on. The earthquake fault develops at the east of Xiatang and Jiaogenping, earthquake scarp appears at Xiaoyilu and Xiatang, earthquake fissure locates at Xiatang, there are multiple earthquake landslides along the surface rupture zone, and the trench LSTC03 exposes the earthquake colluvial wedge. In order to further investigate the Lingshan earthquake surface rupture zones, the author compares the parameters of Lingshan M6¾ earthquake with the similar typical earthquakes in western China, the results show that the parameters of Lingshan earthquake are similar to the typical earthquakes in western China. The length of Lingshan earthquake surface rupture is shorter, but the dislocation is bigger. The author considers that this is mainly related with the parameters of Lingshan earthquake, site condition and structural environment of surface rupture zone, the symbols of dislocation measuring, human activity and weather condition and so on. The research of surface rupture zone features and analysis of Lingshan M6¾ earthquake provides important and basic data for exploring the seismogenic structure of Lingshan M6¾ earthquake, and it has important scientific significance.     

断层崖研究与地震危险性估计——以贺兰山东麓断层崖为例

本文研究了断层崖的形成条件及演化过程,提出了必须对断层崖同时进行几何学和沉积学的研究,并根据我们对贺兰山山前断裂带的断层崖的研究结果,初步总结了断层崖崩积楔的若干特征及其在大地震重复性研究中的意义。根据对宁夏红果子沟和苏峪口断层崖几何学和沉积学的研究结果,确定了贺兰山山前断裂自全新世以来曾发生过4次快速错动事件.从老至新分别命名为 A、B、C、D 事件。由于第二期崩积楔下部物质的 C~(14)年龄为距今5745±90年,所以,B 事件大约发生在距今约6000年左右。第四次事件(D)发生在距今400年以内,以致使明代长城发生错动,西错断点的垂直断距为0.35米,东错断点为0.95米。由此估计4次断层错动事件的重复间隔约为2000—2500年。根据三期古崩积楔高度估计三次古错动的垂直位移幅度分别为:红果子沟西部断层崖为0.25—0.5米,东部断层崖为0.9—1.2米,苏峪口为0.8—1.6米,它们均与串件 D 相当。若假定事件 D 与1739年平罗地震相关,则上述4次断层错动的重复间隔即为7—7~(1/2)级以上地震的重复间隔。此数据与根据相同地段断层滑动速率计算的大地震平均重复间隔相当。宁夏北部贺兰山东麓断层为右旋走滑正断层,全新世以来垂直滑动速率北段为0.2—0.25毫米/年,中段为0.5—0.63毫米/年。而宁夏南部南、西华山断裂第四纪以来为左旋走滑断裂,20000年以来的走滑速率最大可达28.65毫米/年。这是因为二者分属于华北和青藏两个不同的构造区,因而具有不同的构造活动性及地震重复率。     

THE RESPONSE OF SAG POND SEDIMENT TO THE PALEOEARTHQUAKE EVENT ON THE XIADIAN FAULT ZONE

The wedge-shaped deposit formed in front of fault scarp is called colluvial wedge. Repeated faulting by faults may produce multiple colluvial wedges, each of which represents a paleoseismic event. When there are two or more colluvial wedges, the new colluvial wedge is in sedimentary contact with the fault, while the old ones are in fault contact with the fault. The shape of colluvial wedge is usually in the form of horizontal triangle, and the sedimentary facies is usually of binary structure. The overall grain size decreases gradually from bottom to top. Soil layer generally develops on the top, and different types of soil are developed under different climate or soil environments. Another deposit in front of fault scarp is the sag pond graben. The graben in front of sag pond is generally a set of sedimentary assemblages of colluvial facies, alluvial diluvial facies and swamp facies. The area close to the fault, especially the main fault, is of colluvial facies, while the area away from the fault is of alluvial and pluvial facies and marshy facies. In an accumulative cycle, the size of the deposit decreases from bottom to top, and soil layers develop on the top or surface. Multiple pile-ups may be a marker for identifying multiple faulting events. The pile-up strata such as colluvial wedge and fault sag pond can be used as identification markers for paleoseismic events. Colluvial wedge and sag pond, as the identification markers for paleoearthquake, have been well applied to practical research. However, there is still lack of detailed research on the lithological structure and genetic evolution in the interior of colluvial wedge and sag pond sediment, meanwhile, there is still a deficiency in the analysis of the completeness and the regional characteristics of paleoearthquake by using colluvial wedge and sag pond sediment. This paper discusses the method of identifying paleoearthquake by using sag pond sediments and colluvial wedge. We discuss the lithologic combination and sedimentary evolution of sag pond and choose the surface rupture zone of the 1679 M8.0 earthquake on the Xiadian Fault as the research area. In this paper, the distribution range and filling sequence of sag pond are analyzed, using borehole exploration. Four paleoearthquake events are identified since 25ka to 12ka, based on the sag pond sediments and colluvial wedge. The in situ recurrence interval of these seismic events is 480a, 510a, 7 630a and 2 830a, respectively. The lithologic combination and sedimentary evolution law of the sag pond sediments caused by an ancient earthquake are discussed. The sag pond distribution range and filling sequence are determined by the surface elevation survey and drilling exploration. The exploratory trench exposes the sag pond filling strata sequence and lithologic combination. Based on this, we analyze the three sedimentation stages of sag pond sediments formed by a paleoearthquake event near the earthquake fault. It is believed that the filling sequence is composed from bottom to top of the colluvial wedge, the erosion surface or unconformity surface, the fine detrital sediments(containing biological debris)and paleosols. For the fault-sag ponds formed by active faults, the paleoearthquakes occurred near the unconformity or erosion surface of the sediments of the fault-plug ponds. An ancient earthquake event includes the combination of organic deposits such as sediments, clastic deposits, bioclasts, burrow, plant roots and other organic deposits on the vertical scour surface or unconformity. The time interval between two paleoseismic events is defined by two adjacent unconformities(or scour surfaces). According to the vertical facies association and chronological test results of the sediments in the Pangezhuang trough of the Xiatan Fault, four paleo-seismic events are identified since the late Pleistocene period of 25~12ka BP, with recurrence intervals of 480a, 510a, 7 630a and 2 830a, respectively.     

Calculation of shielding factors for production of cosmogenic nuclides in fault scarps

The distribution of concentration of cosmogenic nuclides in fault scarps is used to determine slip histories. The complicated part is the calculation of cosmic radiation shielding by the escarpment body and the overlying wedge of the colluvial sediment. To improve reconstruction of earthquake ages and slip histories, we developed a mathematical model and corresponding MATLAB^® code for computation of shielding factor profiles in fault scarp geometry. In the model, cosmic radiation received by a point of footwall is represented as unit rays attenuated exponentially in scarp geometry. This approach allows producing very precise results both for the fault scarp and the sloped surface. The code is presented as a m-function and as a stand-alone program with a user-friendly interface. Shielding factors are calculated by the code for fast neutrons or for muons and include all general shieldings: topographical, sloped surface, fault scarp surface, colluvium cover, snow cover and self-shielding. A variety of input parameters enables one to adjust the model and the code to almost all possible shielding cases. The code and stand-alone version are provided as supplementary materials and equipped with help and explanatory notes.     

LATE QUATERNARY ACTIVITY OF THE SOUTHEASTERN SECTION OF ZHONGDIAN-DAJU FAULT

The southeast section of Zhongdian-Daju Fault is located in the northern part of Haba and Yulong Snow Mountain, belonging to the southwestern boundary of the secondary block in northwestern Sichuan, an important boundary fault striking 310°~320° on the whole. The nature of the fault, the age of its activity and the slip rate are of great significance for the analysis of the secondary block movement in the northwestern Sichuan and the intersection relationship with the eastern piedmont fault of Yulong Mountains. Based on the 1 ︰ 5 million-scale active fault geological mapping, this paper studies in detail the stratigraphic landform, scarp landform, surface rupture, typical fault profile and river terrace along the fault. Based on the research results, we divide the southeastern section of Zhongdian-Daju Fault into two sub-segments, the Majiacun-Daju sub-segment and the Daju-Dadong sub-segment, according to the geometric structure, fault landforms and fault activity. (1)Fault scarp:In the Majiacun-Daju sub-segment, the fault parallelly controls the two sides of the Haba fault depression. It cuts the late Pleistocene moraine deposits, forming a fault scarp of about 4.5km long and(14±2)m high. The continuity of the scarp is very good, and it is also very obvious in the remote sensing image. In the Daju-Dadong sub-segment, a scarp with a height of about 2m is formed, and an optical luminescence dating sample is collected from the upper part of the gravel layer on the second-order terrace to obtain an age of(22±2.2)ka. (2)Horizontal dislocation:In the Majiacun-Daju sub-segment, through the analysis of the development of outwash fans in the area and the measurement and induction of the gully dislocations, it is considered that there are at least three stages of outwash fans developed in the area and there may be four phases of faulting. That is, the earliest-stage outwash fan and gully are horizontally dislocated about 1km; the second-stage outwash fan and gully are horizontally dislocated about 47m, and the vertical dislocation is about(14±2)m; the gully in the third stage outwash fan is horizontally dislocated twice, the first dislocation formed a beheaded gully with a dislocation of 22m, and the second formed a beheaded gully with a dislocation of 8.5m. It is further proved that the fault has strong activity since the Holocene in the Majiacun to Daju area. In the Daju-Dadong sub-segment, there are no obvious horizontal dislocations in the alluvial deposits since the Holocene. Only 3~4 gullies are found to be offset right-laterally in the ridges east of Wenhe Village, with the maximum dislocation of 210m, which may be the older phase dislocation. (3)Surface rupture:In the northwest direction of Dabazi Village on the T3 terrace in the basin between Majiacun and Daju, an earthquake surface rupture zone is found, extending in the NW direction. The rupture zone left clear traces on the about 1m-thick, hard T₃ terrace surface formed by calcification of sand gravels, and the overburden either upwarps and bulges, or ruptures, generates ground fissures, or forms small pull-apart "depressions" locally. However, the rupture zone is not large in size, about 350m long, 60m wide at the widest point, and 0.3~1.5m high. It is partially en-echelon or obliquely arranged, dominated by compressive ruptures. Through observation, the possibility of artificial transformation is ruled out for these upwarping bulges, ruptures or ground fissures. The fault section is found in the southeast direction of the rupture zone. The slickensides at the section show that the fault is dominated by right-lateral strike-slip with a small amount of thrust. In the eastern sub-segment, only intermittently distributed surface ruptures are found in the northern part of the village, and the scale is small. In summary, through the field geological survey, it is found that the Majiacun-Daju sub-segment is a Holocene active segment. Though the Daju-Dadong sub-segment also offset the late Pleistocene to Holocene strata, it is considered that its Holocene activity is weak in terms of either the dislocation amount or the slip rate of this segment. By analyzing the geological and geomorphological evidences, such as fault scarps, horizontal dislocation and surface ruptures along the fault, it is considered that the Majiacun-Daju sub-segment is a right-lateral strike-slip fault with a normal faulting component, and its vertical slip rate since the late Pleistocene is(0.4~0.8)mm/a, the horizontal slip rate is 1.5~2.4mm/a. The Daju-Dadong sub-segment is dominated by right-lateral strike-slip with a normal faulting component, and its vertical slip rate since the late Late Cenozoic is 0.1mm/a. The formation of the NW-trending surface rupture zone found in the Daju Basin is very young, where there are only two major earthquakes, namely, the M_S6.4 1966 Zhongdian earthquake and the 1996 Lijiang M_S7.0 earthquake, and both earthquakes produced NW-oriented surface rupture zones. Therefore, it cannot be ruled out that the rupture zone is a product of the 1966 Zhongdian M_S6.4 earthquake or the 1996 Lijiang M_S7.0 earthquake.     

Monitoring degradation of the 1995 Nojima earthquake fault scarps at Awaji Island, southwestern Japan

The Nojima fault on the northwestern coast of Awaji Island, south of Kobe, was reactivated during the January 17, 1995 Hyogoken-nanbu earthquake. This fault rupture was dominated by right-lateral offset (max. 1.7 m) along a high-angle reverse fault which has a maximum vertical displacement of 1.3 m on the southeastern side. We repeatedly measured seven profiles across the fault scarp in two areas (Hirabayashi to the northeast and Ogura to the southwest) for approximately 1 year following the earthquake. The original profile of the fault scarp was an overhanging scarp at Hirabayashi and Ogura, corresponding to the 70–80 ° dip of the fault plane. The fault scarp at Hirabayashi displaces Plio-Pleistocene siltstones of the Osaka Group and is overlain by a thin bed of unconsolidated gravel. The Ogura area is entirely underlain by the Osaka Group. Scarp degradation at Hirabayashi occurred by collapse of the gravel bed and proceeded more quickly than at Ogura, where fault scarp degradation proceeded mainly by exfoliation of the Osaka Group siltstones. The degradation occurred at a very fast rate until March at Hirabayashi, and until June or July at Ogura. Since then, the degradation has been very slow. Our data strongly indicate that the scarp profile was initially controlled mainly by the dip of the fault plane, and scarp degradation has been primarily controlled by lithological factors. The degradation of the Nojima earthquake fault scarp proceeded much more quickly than that of normal fault scarps in the western U.S.A., where many observations of the initial stages of scarp degradation have been carried out. The extremely rapid degradation of the Nojima fault scarp in weak late Neogene siltstones might, in combination with rapid cultural modification of the landscape, explain the paucity of geomorphic scarps along the numerous active faults in Japan. This observation may also have implications for tectonic geomorphology and paleoseismicity studies in other countries characterised by weak bedrock and moderate to high rainfall regimes.     

用地貌学方法研究贺兰山山前断层全新世活动状况

本文根据穿过断层的45条冲沟中的裂点和阶地测量资料,论述贺兰山山前断层全新世以来的活动状况。我们认为全新世以来该断层至少有8次活动。最后一次断层活动错断了距今400年的长城,并在冲沟中形成一个裂点,该裂点现今已距断层陡坎约5米远。假定这次断层活动和1739年平罗大地震有关,求出裂点的溯源侵蚀平均速度为2.0—2.5厘米/年.根据两裂点间的距离和裂点的溯源侵蚀平均速度,并考虑到我国近5000年的气候有逐渐变干的趋势,求出断层活动的周期(表3)。从断层陡坎附近的各级阶地面之间的高差,求出断层各次活动的幅度(表4)。     

山东汤头断层全新世右旋走向滑动的新证据

汤头断层的断层陡坎长36公里,坎高1至3米,东升西降。断层陡坎与白垩系砂岩和太古界片麻岩之间的断层位置大致吻合。汤头断层居泉庄处,长1.1公里范围内,有12条近东西向冲沟被三条左阶羽列断层右旋扭错,出现冲沟直角扭曲及断头断尾现象,扭错距离3至25米。断层的扭错量中间大,两端小。位于一冲沟水平扭错处距现今沟底2米处淤泥层的发现,表明该断层的最新走滑活动发生在距今3681年以后