A new tectonic model for Abu-Dabbab seismogenic zone (Eastern Desert,Egypt): evidence from field-structural,EMR and seismic data

Abu-Dabbab area is the most active seismic zone in the central Eastern Desert of Egypt, where seismic activities are daily recorded. The reported earthquakes are microearthquakes of local magnitudes (ML < 2.0). A spatial distribution of these microearthquakes shows that the earthquakes of the area follow an ENE–WSW trending pattern, which is nearly perpendicular to the Red Sea Rift. Focal mechanisms of different fault styles were recognized with dominant normal faulting (with a strike-slip component) events characterized by focal depths greater than 7 km and reverse ones of shallower focal depths. Several lines of evidence indicating that the brittle-ductile transition zone underlies the Abu-Dabbab area occurs at a relatively shallow depth (10–12 km) and it is acting as a low-angle normal shear zone (LANF). Field-structural, EMR and seismic data (this study) reveal that the maximum compressive stress (σ1) in the area is perturbed from the regional NW–SE direction to ENE–WSW orientation. This stress rotation is evidently akin to the reactivation of the crustal scale Najd Fault System (NFS), where such reactivation is attributed to the ongoing activity/opening of the Red Sea. Our tectonic model proposes that the continuous activity on the brittle-ductile transition zone including the LANF led to stress localization, which triggering a brittle deformation in the upper crustal-levels and associated shallow dipping thrusts. Such bimodal tectonic model suggests that the deep earthquakes are owing to the tectonic movement on the LANF (transtension), whereas the shallow earthquakes are related to a brittle deformation inside the fault blocks of the upper crust (transpression). Deformation creep along this zone didn’t permit continuous accumulation of strain and hence reduce the possible occurrence of large earthquakes.     

Tectonic significance of an earthquake sequence in the Zacoalco half-graben, Jalisco, Mexico

We studied a sequence of small earthquakes that occurred during the months of April and May of 1997, in Jalisco, southwestern Mexico. The earthquakes were located along a set of active faults that form the Zacoalco half-graben (La Lima fault system), west of Lake Chapala, within the rift–rift–rift triple junction. A total of 33 events were located, with magnitudes ranging from 1.5 to 3.5, recorded by a portable array of broadband seismographs. We identified two groups of events: one corresponding to a shallow normal fault, synthetic to La Lima fault system, and another group associated with a deeper fault. The events that occurred on the synthetic fault show normal faulting oriented on a NW–SE plane, dipping shallowly towards the SW. The other group of mechanisms showed either a normal fault oriented NW–SE and dipping steeply to the NE, or a very shallow-dipping normal fault, dipping to the SW. Earthquake distribution and fault plane solutions suggest that the Zacoalco half-graben developed from blocks that rotate as slip occurs on listric faults. These mechanisms could represent the type of motion expected for larger earthquakes in the area, like the one that occurred in 1568.     

Modeling Cenozoic crustal deformation in Gorny Altai

The Altai lithospheric structure has been generally understood due to available high-resolution digital models. As a further step in modeling, we have simulated the structure of southeastern Altai as interaction of eight blocks which comprise or surround the Chuya and Kurai basins, proceeding from the basic configuration of blocks and earthquake mechanisms. Should the stresses in the system remain invariable, the western periphery of the Kurai basin will deform to let the Uimen-Sumulta fault join the Chuya (western end of Tolbonur) fault and evolve further as a single shear zone. The best fit model was one with slip along a single border fault in the middle of the area between two rheologically different terranes. This setting corresponds to a fault boundary between the more plastic Gorny Altai and more rigid Teletskoe-Chulyshman domains, which is consistent with current crustal movements from GPS data. In addition to scientific significance, models of this kind have practical applications as they highlight areas of stress buildup prone to release in large earthquakes. The new approach was applied to simulate the stress and strain patterns of central and southeastern Gorny Altai, and the models were tested against available geomorphological and seismotectonic data.     

断块大地构造与地震活动的构造物理研究

断块大地构造理论几乎涉及地震活动的各个方面： 1）地震记录表明不但是强震,大多数6级以上地震也分布在构造块体边界上,构造块体控制了地震分布; 2）地震活动规律体现在块体整体活动中。例如,鄂尔多斯地块周边单个断陷带的地震活跃期与平静期长短不一,无明显规律。但当把鄂尔多斯地块周边作为一个整体,其地震活动在时间上显示了准周期性; 3）地块运动通过周边断层交替活动实现。从断层活动相互作用的时间间隔和错动形式出发可把它分为强震交替活动型（又可分长时间间隔和短时间间隔两类）和强震与弱震或断层蠕动交替活动型。强震交替活动型中时间间隔很短的双震活动较早被发现。强震交替活动型中时间间隔很长的类型虽然不易识别,但是依赖于中国历史地震目录,还是发现鄂尔多斯地块周边山西断陷带与渭河断陷带在历史上的3次交替活动等; 强震与弱震或断层蠕动型的交替活动型很不容易被发现,仅在台网较密,观测条件较好的北京地区观测到。4）利用一些实验结果讨论了交替活动的规律。此外,结合断块大地构造理论对一些地震现象进行了讨论。     

河套地震带的震源机制类型时空分布特征

基于地质构造背景分析,收集2000年以来发生在河套地震带的M_L≥2.8级地震作为研究对象,综合运用基于P波初动的振幅比方法(APAS)和基于波形拟合的CAP反演方法求出256次地震事件的震源机制解。以断层节面滑动角作为判定指标,分区域给出了河套地震带的震源机制类型空间和时间分布图像,从断层滑动角度呈现河套地震带应力场时空变化过程。结果显示:临河盆地断层节面滑动角主要在水平±20°方向存在优势分布,走滑型特征显著;具体来讲,狼山-色尔腾山山前断裂带、临河断裂、乌拉山山前断裂等主要以纯走滑型地震为主,巴彦乌拉山断裂与磴口-本井断裂之间的区域多分布正走滑型地震。呼包盆地断层节面解虽然也呈现出走滑型为主的特征,但滑动角分布较为离散,优势分布方向不明显;呼包盆地西侧的包头至西山咀凸起一带表现出以走滑为主的小范围震源应力场特征,呼包盆地内部及东侧由于显著的区域垂直差异运动,正断层和逆冲型地震所占比例较大,震源机制类型整体呈现出与构造相依的分布特点。分析认为,2000年以来,河套地震带的应力场存在一定的时空非均匀性变化,研究结果更多表现了河套地震带的震源应力场变化过程,而研究资料时间不够长和震级不够大是引起这种应力场非均匀性暂态特征的主要因素。     

基于形变观测分析中国台湾东部花东纵谷断层运动特征

花东纵谷断层是中国台湾动力作用和地壳运动变形最强烈的断层之一,其断层运动特征和强震危险程度一直备受学者的关注。文中分别以同震地表位移、1992-1999年震间形变数据为约束,反演2003年成功MW 6.8地震同震位错分布和花东纵谷断层震间运动特征。结果表明：花东纵谷断层北段处于强闭锁状态（闭锁率高达0.9）,闭锁深度深（约27 km）;南段闭锁程度较弱（闭锁率约0.5）,闭锁深度较浅（约12 km）;中段闭锁程度与闭锁深度介于南北段之间。另一方面,2003年成功MW 6.8地震微观震中位于震间无震滑移区与闭锁区的过渡带附近。依据同震位错、震间断层运动反演结果,以及历史强震破裂分布特征,分析认为,花东纵谷断层南北段运动方式存在差异性,北段主要以强震形式运动,南段以蠕滑和地震两种形式运动。自1951年花莲-台东ML 7.3地震序列后,花东纵谷断层南段、中段和北段至2016年所累积的矩能量分别等价MW 6.4、MW 7.0、MW 7.4地震;若发生级联破裂,整个断层至2016年所累积的矩能量等价MW 7.5地震。     

KINEMATICS OF QUATERNARY EXTENSION IN THE PAMIR

KINEMATICS OF QUATERNARY EXTENSION IN THE PAMIR1　BurtmanVS ,MolnarP .GeologicalandgeophysicalevidencefordeepsubductionofcontinentalcrustbeneaththePamir[J].SpecPapGeolSocAm ,1993,2 81:76 . 2　StreckerMR ,FrischW ,HamburgerMW ,etal.QuaternarydeformationintheEasternPamirs ,TadzhikistanandKyrgyzs tan[J].Tectonics,1995 ,14(5 ) :10 6 1～ 10 79.…     

云南川西地区地震地质基本特征的探讨

云南川西地区是多震的地方(图1),从公元前116年到1974年6月,据记载共发生M≥4.75级的地震481次,其中6≤M<7的99次,7≤M<8的14次,M≥8的3次。震源深度一般小于30公里,均为浅源地震。本区强震多发生在深大断裂带上,而且都有一定的地质标志可循,与近代地壳运动、板块构造在成因上有着密切关系。     

内蒙古狼山地区新生代断层活动特征：对正断层生长的限定

阿拉善地块东北缘的狼山地区新生代发育有3期构造,分别为中新世NW-SE向挤压形成的逆断层,NNE向挤压形成的左行走滑断层以及晚新生代NW-SE向伸展形成的高角度正断层。结合阿拉善地块东缘的新生代构造,认为狼山地区新生代断层的活动与青藏高原东北缘的逐步扩展、应力场逐渐调整有关。狼山山前正断层目前是一条贯通的断层,其演化基本符合恒定长度断层生长模型,断层中间部位滑动速率最大,向断层两侧逐渐递减。从不同方法得出的滑动速率来看,进入全新世以来,断层滑动速率有逐渐变小的趋势。结合阿拉善地块内部及东缘断层震源机制解以及断层的几何学、运动学特征,认为河套—吉兰泰盆地和银川盆地属于两个性质不同的伸展盆地,两者通过构造转换带相连,转换区内断层表现为右行走滑。转换区5级以上地震可能是受区域性NE-SW向挤压,近南北向右行断层活动的表现。     

柴北缘大柴旦地区山前带构造建模及演化研究

针对柴北缘大柴旦地区北东、北西向两组逆冲推覆断裂交汇,构造变形极其复杂,构造解析困难的问题,充分利用野外地质调查、地震、重磁电、钻井(孔)等资料,理清了研究区主要断裂体系及其组合特征与展布规律。采用地表和地下构造、浅部和深部构造、地震和非震资料相结合的方法,开展了山前带构造建模研究与构造解析。通过研究,确定了研究区发育NW和近WE向两组断层和盆缘逆冲、盆内逆冲、盆内挤压走滑等3类断裂体系,平面上具有分区、分带性;建立了盆缘、盆内不同构造变形机制的构造解释模型;共识别出了挤压、伸展和走滑等3大类8种构造样式,明确了构造样式组合模式及其分布规律,理清了研究区“南北分带、东西分区”的构造变形特征;南西北东向构造演化剖面分析明确了盆缘、盆内推覆构造与盆内反冲构造后展式演化时序及其对中生界残留分布的控制作用。     

龙门山前山断裂大地震原地重复发生的地质地貌证据

龙门山前山断裂大地震是原地重复发生的吗？针对这一问题,对前山断裂的同震地表形变带进行了追踪,并选择5.12汶川地震区九龙、白鹿以及汉旺等3个地点开展了古地震探槽研究工作。调查结果是： 前山断裂5.12地震地表形变带基本沿1 ∶200000地质图上先存基岩断裂发育,但仅限于通济－安昌以南段; 白鹿－八角一带同震地表形变带继承先存断层陡坎或断层沟谷发育; 偏离先存断裂而被一些学者视为断裂全新世活动重要证据的永安附近T1阶地上线性陡坎实为废弃河岸; 白鹿、汉旺探槽剖面揭示这两个地点5.12汶川地震之前至少有过一次古地震事件。这些研究结果表明前山断裂大地震是沿晚第四纪活动断裂原地重复发生的,此次地震是沿先存活动断裂又一次发生的大地震事件,为大地震原地复发模型提供了一个新的震例支持。在沿该断裂周边进行规划建设时应当合理避让。     

http://www.sciencedirect.com/science/article/pii/S1674987114000024

We propose that the brittle-ductile transition(BDT) controls the seismic cycle.In particular,the movements detected by space geodesy record the steady state deformation in the ductile lower crust,whereas the stick-slip behavior of the brittle upper crust is constrained by its larger friction.GPS data allow analyzing the strain rate along active plate boundaries.In all tectonic settings,we propose that earthquakes primarily occur along active fault segments characterized by relative minima of strain rate,segments which are locked or slowly creeping.We discuss regional examples where large earthquakes happened in areas of relative low strain rate.Regardless the tectonic style,the interseismic stress and strain pattern inverts during the coseismic stage.Where a dilated band formed during the interseismic stage,this will be shortened at the coseismic stage,and vice-versa what was previously shortened,it will be dilated.The interseismic energy accumulation and the coseismic expenditure rather depend on the tectonic setting(extensional,contractional,or strike-slip).The gravitational potential energy dominates along normal faults,whereas the elastic energy prevails for thrust earthquakes and performs work against the gravity force.The energy budget in strike-slip tectonic setting is also primarily due elastic energy.Therefore,precursors may be different as a function of the tectonic setting.In this model,with a given displacement,the magnitude of an earthquake results from the coseismic slip of the deformed volume above the BDT rather than only on the fault length,and it also depends on the fault kinematics.     

丹江断裂东段第四纪活动性及地震地质涵义

丹江口地区地震地质因修建丹江水库而得到深入研究,但针对断裂进行的系统性研究较少。通过对丹江断裂东段进行1：1万大比例条带状活动断裂填图,综合采用地质地貌调查、岩样年龄测试、地质钻探、浅层地震勘探等多种手段进行研究,认为：1）丹江断裂东段第四纪以来累积左旋走滑量在250~500 m之间,上更新统地层垂直断距为10~20 m;2）断裂最新活动时代为晚更新世,活动性质以由NE向SW的逆冲为主,兼具有左旋走滑分量;3）断裂活动性由山地向盆地迁移。丹江断裂晚更新世以来活动表明,南襄盆地西缘活动性较强。该认识可为进一步评估丹江口地区乃至南襄盆地西缘的地震构造环境提供依据。     

Acceleration response spectra for the earthquakes of the southwestern flank of the Baikal Rift Zone

The acceleration response spectra of earthquakes with M = 4–6.5 in the southwestern part of the Baikal Rift Zone have been studied. The absorption properties of the medium and the attenuation of seismic signals in the study area were determined. Average acceleration response spectra were obtained for regional earthquakes. A comparative analysis of the acceleration response spectra was made for earthquake focal mechanisms with different senses of motion: reverse fault, reverse slip, strike slip, and oblique slip. The effect of the sense of fault motion in the seismic source on acceleration response spectra was determined.     

Kinematics of the eastern Caucasus near Baku, Azerbaijan

The potential for large, shallow earthquakes and their associated seismic hazard in the eastern Caucasus, an area of dense population and sensitive industrial infrastructure, remains speculative based on historical precedent and current geologic and seismologic observations. Here we present updated and expanded results from a GPS network between the northern edge of the Lesser Caucasus and Greater Caucasus, providing geodetic constraints to the problem. A significant strain rate is observed in a profile over a distance of about 150?km across the Kura Basin. We attribute this to inter-seismic strain accumulation on buried fault structures and present simple elastic dislocation models for their plausible geometry and slip rate based on the known geology, seismicity and the GPS velocities. Due to the close proximity of the strain anomaly to Baku, further observations are needed to determine whether observed contraction is due to inter-seismically locked faults and, if so, implications for the seismic hazard in the region.     

Seismic faults in the Aegean area

Reliable fault plane solutions of shallow earthquakes and information on surface fault traces in combination with other seismic, geomorphological and geological information have been used to determine the orientation and other properties of the seismic faults in the Aegean and surrounding area.Thrust faults having an about NW-SE strike occur in the outer seismic zone along western Albania-westernmost part of mainland of Greece-Ionian Sea-south of Crete-south of Rhodes.The inner part of the area is dominated by strike-slip and normal faulting. Strike-slip with an about NE-SW slip direction occurs in the inner part of the Hellenic arc along the line Peloponnesus-Cyclades-Dodecanese-southwest Turkey as well as along a zone which is associated with the northern Aegean trough and the northwesternmost part of Anatolia. All other regions in the inner part of the area are characterized by normal faulting. The slip direction of the normal faults has an about SW-NE direction in Crete (N38°E) and an about E-W direction (N81°E) in a zone which trends N-S in eastern Albania and its extension to western mainland of Greece. In all other regions (central Greece-southern Yugoslavia and Bulgaria, western Turkey) the slip of the normal faults has an about N-S direction.     

Faults in the Baikal region: morphostructural and structure-genetic features (case study of the Buguldeika fault junction)

Lineament analysis is applied to map the pattern of the Obruchev fault system in the Buguldeika Village area, where several fault zones (Olkhon, Primorsky, Prikhrebtovyi, Buguldeika, and Kurtun) make up a junction. As inferred from the predominant directions of genetically related lineaments, the Olkhon, Primorsky, and Prikhrebtovyi faults originated under NW-SE extension and compression. The extension and compression settings within these zones are reconstructed by analysis of lineaments that have prominent and poor geomorphic expression, respectively. However, the pattern of lineaments well expressed in the surface topography within a weakly deformed block corresponds to reverse slip, while that of poorly pronounced lineaments corresponds to left-lateral strike slip. The study confirms the idea that the latest extension (rifting) stage in the Baikal region reactivated fault zones but did not deform blocks. The blocks store record of residual deformation produced by previous settings of regional compression and shear. The obtained results agree with earlier tectonophysical analysis of faults and fractures in the area and prove the applicability of the suggested approach to map the fault patterns and reconstruct their respective stress settings in areas that underwent multiple deformation events of different ages.     

胶莱盆地及其邻区白垩纪－古新世沉积构造演化历史及其区域动力学意义

基于野外和钻孔测井资料分析、火山岩同位素年代学分析 (40Ar-39Ar and SHRIMP U-Pb)、地震剖面的构造解释、断层运动学的野外分析结果,综合研究了胶莱盆地及其邻区白垩纪－古新世沉积构造演化历史。岩性地层分析表明,胶莱断陷盆地由三套地层单元所充填：早白垩世莱阳群和青山群、晚白垩世－古新世王氏群。青山群火山岩的同位素年代学测试结果给出了该火山岩的喷发时代在120～105 Ma。地震剖面的构造解译结果揭示胶莱盆地伸展构造受到深部两个拆离构造系统控制：一个发育于盆地南部地区,拆离断面位于深部8~10 km,向南缓倾于苏鲁造山带之下;另一个拆离系统由一系列北倾的犁式断层组成、分布于宽阔的胶莱盆地北部地区,主拆离面向北倾。这两个拆离系统分别形成于早白垩世莱阳群和晚白垩世-古新世王氏群沉积阶段。通过对不同地层单元断层滑动矢量的野外测量和古构造应力场反演,以及地层时代和同位素年代学测试结果的制约,建立了白垩纪－古新世构造应力场演替的年代序列。结果表明,胶莱盆地在白垩纪-古新世之间经历了伸展－挤压应力体制的交替演化。早白垩世伸展作用经历了两个不同的阶段：早期NW-SE向伸展和晚期近W-E向伸展。在早白垩世末期至晚白垩世初期,盆地遭受NW-SE向挤压,导致了胶莱盆地的缩短变形和郯庐断裂带的左旋走滑活动。晚白垩世－古新世时期,构造应力场转变为N-S向伸展,直到古新世末期,构造应力场转换为NE－SW向挤压。胶莱盆地和沂沭裂谷系白垩纪－古新世沉积构造演化历史对华北地区岩石圈减薄过程的动力学背景提供了重要的构造地质学制约。笔者推断,早白垩世两期引张应力作用是分别对华北地区增厚地壳或岩石圈的重力垮塌和岩石圈拆沉的响应,而早白垩世末期NW-SE向挤压记录了古太平洋板块与亚洲陆缘俯冲碰撞产生的远程效应。晚白垩世-古新世的引张伸展作用完全不同于早白垩世伸展构造,它指示了沿NNE向郯庐断裂带的右旋走滑活动及其拉分作用,在动力学上受到青藏地区块体的陆－陆碰撞产生的远程效应和古太平洋板块向亚洲大陆俯冲作用的联合应力场控制。     

Large earthquakes in the New Guinea-Solomon Islands area, 1873–1972

A catalogue of 1873–1972 earthquakes with M > 6.9 for the New Guinea—Solomon Islands region (130–165° E) is compiled. There are 152 events listed. Duda's (1965) results for 1900–1968 are improved for the Papua New Guinea area (141–156° E) because of the availability of historical data for that area.Although there is evidence of rapid Holocene uplift in the main seismic zones, there is little historical evidence for visible uplift or subsidence resulting directly from modern major earthquakes. Coastal subsidences commonly reported as a result of earthquakes are of smaller extent and appear to be due to settlement. However, the occurrence of tsunamigenic earthquakes does suggest that surface deformations do take place off-shore.Using Davies and Brune's (1971) method, regional fault slip rates over 5° -segments of the shallow seismic zone are determined from the seismicity catalogue. The slip rate for the island of New Guinea (Gutenberg and Richter's Region 16) is found to be at least 4.4 cm/y which is almost double the very anomalously low rate of 2.3 cm/y found by Davies and Brune (1971). If allowance is made for shear movement without seismicity and for the approximately ratio of dip-slip versus strike-slip faulting indicated by fault plane solutions, the agreement with Le Pichon's (1970) approach value of 10.7 cm/y for the Pacific—India (Australia) plates is reasonable. The fault slip rate in the area between east New Britain and Bougainville at the Pacific—Bismarck—Solomon triple junction is extremely high (20.6 cm/y at least). The smallest slip rate (1.5 cm/y) is found for westernmost New Guinea (130–135° E).Temporal cumulative summation of moments curves show a periodicity of approximately 25 years in the seismic activity at the triple junction (150–155° E). Elsewhere the rate of seismic activity is aperiodic.     

Fine structure of seismotectonics in western Shillong massif,north east India

The epicentral tract of the great Assam earthquake of 1897 of magnitude 8·7 was monitored for about 6 months using an array of portable seismographs. The observed seismicity pattern shows several diversely-oriented linear trends, some of which either encompass or parallel known geological faults. A vast majority of the recorded micro-earthquakes had estimated focal depths between 8–14 km. The maximum estimated depth was 45 km. On the basis of a seismic velocity model for the region reported recently and these depth estimates we suggest that the rupture zone of the great 1897 earthquake had a depth of 11–12 km under the western half of the Shillong massif. Four composite fault plane solutions define the nature of dislocation in three of the seismic zones. Three of them show oblique thrusting while one shows pure dip slip reverse faulting. The fault plane solutions fit into a regional pattern of a belt of earthquakes extending in NW-SE direction across the north eastern corner of the Bengal basin. The maximum principle stress axis is approximately NS for all the solutions in conformity with the inferred direction of the Indian-EuroAsian plate convergence in the eastern Himalaya.