EARTHQUAKE-CAUSED SEISMIC VOLCANIC ROCKS AND THIXOTROPIC DEFORMATION OF SOFT SEDIMENTS IN THE UPPER CRETACEOUS SHIJIATUN MEMBER,JIAOZHOU CITY

A lot of seismic volcanic rocks and strong earthquake-induced thixotropic deformation structures in soft mud-sandy sediments(seismites)were identified from the Upper Cretaceous Shijiatun Member of the Hongtuya Formation for the first time in Jiaozhou City of the Zhucheng Sag, eastern China. Seismic volcanic rocks are volcanic rocks with co-seismic deformation structures which were produced by major earthquakes destroying volcano ejecta. Seismites are sediment layers with soft-sediment deformation structures formed by strong earthquake triggering saturated or semi-consolidated soft sediments to produce liquefaction, thixotropy, faults, cracks and filling and so forth. The Shijiatun Member of the Hongtuya Formation mainly consists of basaltic volcano rocks interbedded with mud-sandy(muddy sand and sandy mud)deposition layers of the river-lake facies. In the Shijiatun Member, main types of seismic volcanic rocks are shattered basalts with co-seismic fissures and seismic basaltic breccias. The thixotropic deformations of soft mud-sandy sediments mainly include thixotropic mud-sandy veins and thixotropic mud-sandy layers with tortuous boundaries. Under the strong earthquake action, saturated mud-sandy sediments could not be liquefied, instead resulting in thixotropy, i.e. their texture can be damaged and their flow-ability or rheology becomes strong. Because basaltic volcano rocks were damaged(shattered, seismic broken), a major earthquake can lead to thixotropic mud-sandy sediments flowing along seismic fissures in basalts, resulting in the formation of deformation structure of thixotropic veins, and boundaries between volcano rock and mud-sand layer became quite winding. Under the koinonia of gravity and vibration force, seismic breccia blocks sunk into thixotropic mud-sandy layers, resulting in the formation of inclusions of thixotropic mud-sandy sediments. Seismic intensity reflected by these strong earthquake records during the end stage of the Late Cretaceous was about Ⅶ to more than X degrees. The Shijiatun Member is mainly distributed in the south of the Baichihe fault in the northern Zhucheng Sag, and the fault has generated many strong tectonic and earthquake activities at the end of the late Cretaceous, also provided the channel for intrusion and eruption of basaltic magma then. At the end of the late Cretaceous, intermittent intrusion and eruption of basaltic magma took place along the Baichihe fault, meanwhile the volcano earthquakes took place or tectonic earthquakes were generated by the Baichihe fault which caused the deformation of the volcano lava and underlying strata of red saturated muddy-sand, resulting in the formation of various seismo-genesis deformations of volcanic rocks interbedded with mud-sandy sediment layers. Therefore, strong seismic events recorded by them should be responses to strong tectonic taphrogenesis of the Zhucheng Sag and intense activity of the Baichihe fault in the end of Late Cretaceous. In addition, these seismogenic deformation structures of rock-soil layers provide new data for the analysis of the failure effect produced by seismic force in similar rock-soil foundations.     

A multi-scale 3-D crust velocity model in the Hefei-Chao Lake area around the southern segment of Tanlu Fault Zone

Regional high-precision velocity models of the crust are an important foundation for examining seismic activity, seismogenic environments, and disaster distribution characteristics. The Hefei-Chao Lake area contains the main geological units of Hefei Basin, with thick sediments and the Chao Lake depression. Several major concealed faults of the southern NNE-trending Tanlu Fault Zone cross this area. To further explore the underground distribution characteristics of the faults and their tectonic evolutionary relationship with adjacent tectonic units, this study used ambient noise data recorded by a seismic array deployed in Hefei City and Chao Lake, constructing a 3-D velocity model at the depth of 1–8 km. Then a multi-scale high-resolution 3-D velocity model of this area was constructed by this new upper crustal velocity model with the previous middle and lower crustal model. The new model reveals that a high-velocity belt is highly consistent with the strike of the Tanlu Fault Zone, and a low-velocity sedimentary characteristic is consistent with the Hefei Basin and Chao Lake depression. The distribution morphology of high and low velocity bodies shows that the sedimentary pattern of Hefei-Chao Lake area is closely related to the tectonic evolution of the Tanlu Fault Zone since the Mesozoic. This study also identifies multiple low-velocity anomalies in the southeastern Hefei City. We speculate that strong ground motion during the 2009 Feidong earthquake (magnitude of 3.5) was related to amplification by the thick sediments in the Hefei Basin. We also discuss further applications of multi-scale high-resolution models of the shallow layer to strong ground motion simulations in cities and for earthquake disaster assessments.     

Features and origin time of Mesozoic strike-slip structures in the Yilan-Yitong Fault Zone

The NE-striking Yilan-Yitong Fault Zone(YYFZ) with a length of ca. 900 km is an important major fault zone in northeastern China. Its origin has been a controversial issue for a long time. Detailed field investigation and comprehensive analyses show that strike-slip faults or ductile shear belts exist as the origination structures on the both shoulders of the Cretaceous-Paleogene grabens. These strike-slip structures are dominated by brittle transcurrent faults, and appear as ductile shear belts only in the Weiyuanpu-Yehe and Shulan parts in the south and middle of the fault zone, respectively. The shear belts strike NE-SW and show steep mylonitic foliation and gentle mineral elongation lineation. Outcrop structures, microstructures and quartz c-axis fabrics demonstrate a sinistral shear sense with minor reverse component for the ductile shear belts. The microstructures suggest deformation temperatures of 400–450°C for the Weiyuanpu-Yehe shear belts and 350–400°C for the Shulan shear belt. A series of zircon U-Pb dating results for deformed and undeformed plutons or dikes in the shear belts constrain the strike-slip motion to the time between 160 and 126 Ma. It is further inferred from ages of main geological events in this region that the fault zone originated in the earliest Early Cretaceous. It is suggested therefore that the southern and middle parts of the Tan-Lu Fault Zone, which originated in Middle Triassic, propagated into northeastern China along the sinistral YYFZ under the earliest Early Cretaceous regional compression that is referred to as the Yanshan B event. The earliest Early Cretaceous initiation of the YYFZ results from both the high-speed oblique subduction of the Izanagi Plate and the final closure of the Mongol-Okhotsk Ocean, but the Izanagi Plate subduction played a major dynamic role in the fault zone origin.     

Seismic event, sequence and tectonic significance in Canglangpu Stage in Paleo-Tanlu Fault Zone

The Canglangpu Stage of Lower Cambrian Series is widely distributed along both sides of the Tanlu (Tancheng-Lujiang) Fault Zone in the Jiao-Liao-Xu-Huai regions. In the Liaodong Peninsula, the Canglangpu Stage consists of three formations, i.e. Gejiatun, Dalinzi and Jianchang formations in ascending order (lying on the eastern side of the Tanlu Fault Zone). The Dalinzi Formation, developing in a littoral Sabkha environment, is full of catastrophic event records of violent seism, such as liquefied muddy-sandy veins, hydroplastic folds, hydroplastic micro-faults (three forming an organic whole), liquefied crinkled deformations, liquefied breccia and sandy dikes. Based on such records, the seismic liquified sequence of argillaceous rocks in Sabkha is built up. In northern Jiangsu and Anhui provinces, however, there hardly observe seismic records in the Canglangpu Stage, which consists of Jinshanzhai and lower Gouhou and upper Gouhou formations (lying on the western side of the Tanlu Fault Zone). Even if the Gouhou Formation, developing in a lagoon-dry environment, is in the same climate zone as the Dalinzi Formation, and 4 depositional sequences have been identified in the Canglangpu Stage in Northern Jiangsu and Anhui provinces, however, in the same stage in the Liaodong Peninsula, there exist only 3 ones. Therefore, it is not supported by the above mentioned evidence (such as catastrophic events, sequences stratigraphy and lithologic correlation of formations) that the Canglangpu Stage in the Liaodong Peninsula came from northern Jiangsu and Anhui provinces through a long-distance, about hundreds kilometers, left-hand displacement of the Tanlu Fault in the Mesozoic era.     

锦州及葫芦岛地区的地震灾害及未来地震趋势分析

锦州和葫芦岛地区位于东北地震区,毗邻华北地震区的郯庐地震带和华北平原北部地震带。上述地区的地震活动频度和强度并不高,陆地部分的最大地震只有5级,但是附近强震带的地震活动对该区多次造成破坏,成为强震受害区。本文在总结地震活动特点和地震构造条件的基础上对该区未来的地震趋势作了分析。     

由地壳速度结构判断郯庐断裂带江苏段未来大震位置

用多震相地震走时成像法反演郯庐断裂带鲁苏皖段及邻区三维地壳速度结构。一些地区如郯庐断裂带临沭到定远及以东地区在中地壳的20~25km出现低速层,一些地区莫霍面埋深有变化。浅层速度结构的分段与断裂活动的分段相一致,表明新沂到泗洪是活动断裂的闭锁段。对比1668年山东郯城8级地震区和研究区的深部速度结构,结合与郯庐带相交的断裂、地震活动、活动断裂的闭锁段、中地壳低速层及莫霍面深度变化,综合判断郯庐断裂带江苏段未来可能发生大震的地区为33.4°~34.1°N,118.2°~118.8°E,重点是宿迁、沭阳、泗阳和泗洪。震级估计可达8级。     

NEW EVIDENCES FOR LATE QUATERNARY ACTIVITY IN THE SOUTHERN SEGMENT OF THE YISHU-TANGTOU FAULT,THE TAN-LU FAULT ZONE,AND ITS TECTONIC IMPLICATION

The Tan-Lu Fault Zone(TLFZ), a well-known lithosphere fault zone in eastern China, is a boundary tectonic belt of the secondary block within the North China plate, and its seismic risk has always been a focus problem. Previous studies were primarily conducted on the eastern graben faults of the Yishu segment where there are historical earthquake records, but the faults in western graben have seldom been involved. So, there has been no agreement about the activity of the western graben fault from the previous studies. This paper focuses on the activity of the two buried faults in the western graben along the southern segment of Yishu through combination of shallow seismic reflection profile and composite drilling section exploration. Shallow seismic reflection profile reveals that the Tangwu-Gegou Fault(F₄)only affects the top surface of Suqian Formation, therefore, the fault may be an early Quaternary fault. The Yishui-Tangtou Fault(F₃)has displaced the upper Pleistocene series in the shallow seismic reflection profile, suggesting that the fault may be a late Pleistocene active fault. Drilling was implemented in Caiji Town and Lingcheng Town along the Yishui-Tangtou Fault(F₃)respectively, and the result shows that the latest activity time of Yishui-Tangtou Fault(F₃)is between(91.2±4.4)ka and(97.0±4.8)ka, therefore, the fault belongs to late Pleistocene active fault. Combined with the latest research on the activity of other faults along TLFZ, both faults in eastern and western graben were active during the late Pleistocene in the southern segment of the Yishu fault zone, however, only the fault in eastern graben was active in the Holocene. This phenomenon is the tectonic response to the subduction of the Pacific and Philippine Sea Plate and collision between India and Asian Plate. The two late Quaternary active faults in the Yishu segment of TLFZ are deep faults and present different forms on the surface and in near surface according to studies of deep seismic reflection profile, seismic wave function and seismic relocation. Considering the tectonic structure of the southern segment of Yishu fault zone, the relationship between deep and shallow structures, and the impact of 1668 Tancheng earthquake(M=8(1/2)), the seismogenic ability of moderate-strong earthquake along the Yishui-Tangtou Fault(F₃)can't be ignored.     

Instability structures, synsedimentary faults and turbidites, witnesses of a Liassic seismotectonic activity in the Dauphiné Zone (French Alps): A case example in the Lower Pliensbachian at Saint-Michel-en-Beaumont

Instability structures, synsedimentary faults and turbidites have been studied in the Lower Pliensbachian succession of Saint-Michel-en-Beaumont, belonging to the Taillefer block, an ancient half-graben emplaced during the Liassic Tethyan rifting. Geometrical and mechanical analyses demonstrate that the instability structures occurred thanks to movements along spineless synsedimentary normal faults, when the turbiditic and limestone layers were already case-hardened and partly fractured by tension gashes even when the mudstones were still unlithified. Both the tension gashes and the synsedimentary faults are homogeneous in strike with the major regional faults and are in good agreement with the regional direction of extension for this period. The characters of the turbiditic beds, with erosive base, graded bedding, and incomplete Bouma sequence, are in favour of a seismic origin. Instability structures, spineless synsedimentary faults and turbiditic inflows are thus considered as seismites and interpreted as the result of high seismicity periods including some events with M > 5 in the general extensive ambiance of the Liassic Tethyan rifting. The analysis of the geometrical relationships between all these sedimentary features allows to distinguish the successive stage of occurrence of an instability structure, from the sedimentation of alternating marls and limestones, and sudden turbiditic inflows, then early case-hardening of the turbidites, until the important seismotectonic event generating the spineless normal faults, themselves triggering the fall of indurated blocks and locally the forming of breccias. The Ornon Fault, which constitutes the border of the Taillefer block, 15 km eastward, played a major role during the Liassic sedimentation and may represent the major seismic fault related to the seismites occurrence in the Beaumont basin.     

华北地区的背景地震活动及区域未来强震危险性

华北地区的历史强震活动非常频繁。 然而, 自1998年张北M_S6.2地震以来, 该地区已经经历了10多年的地震平静期, 中强震平静现象比较突出。 本文利用1970年至2009年的小震资料, 对华北地区的背景地震活动进行分析, 给出了地震活动性参数b值、 最大震级以及强震复发间隔和强震年平均发生概率的空间分布。 b值空间分布表明, 山西构造带的运城地区, 郯庐断裂带的宿迁地区以及太行山块体内部的石家庄地区的b值较低。 地震活动性参数的综合空间图像表明华北地区的地震活动主要受区域深部动力因素所控制。     

粤东滨海断裂带第四纪活动特征研究

我国东南沿海滨海断裂带是1条活动强烈的地震构造带,位于珠江口盆地北缘的粤东滨海断裂带是其重要组成部分,确定该断裂带的几何展布位置与最新活动特征对科学评价华南沿海地区地震危险性、地震构造和地球动力学具有重要科学意义.通过综合分析近年来南海东北部海域地质地貌、地震反射剖面、深部探测、地震活动等方面的研究成果,总结了粤东滨海...     

郯庐断裂中段两侧坳陷的新生代构造-热演化特征

郯庐断裂带两侧分布众多的坳陷,其构造－热演化历史对于这些含油气坳陷的油气成藏起到重要的作用.本文利用镜质组反射率和磷灰石裂变径迹古温标模拟计算了郯庐断裂带中段两侧6个坳陷120口单井的热演化历史,在此基础上得到各坳陷新生代的地温梯度演化特征.总体上,郯庐断裂两侧坳陷的古地温梯度在新生代是逐渐降低的,在古近纪较高而新近纪仅比目前的地温梯度略高;结合构造沉降史的恢复结果,认为其反映了研究区由断陷向坳陷转化的构造演化特征.根据各坳陷古地温梯度的演化模拟结果,距离郯庐断裂带不同位置的坳陷其古热场特征存在差异.位于断裂带内的辽河盆地、渤中和昌潍坳陷古热场较高(第三纪初期的地温梯度达到57～59 ℃/km),且辽河盆地和昌潍坳陷在第三纪地温梯度一直处于较高的状况,而渤中坳陷新近纪时期的较低地温梯度是由于其大的构造沉降所致.远离郯庐断裂带的冀中和临清坳陷的古热场及其演化程度均是较低的,第三纪初期的地温梯度分别为53 ℃/km和50 ℃/km.由此推测郯庐断裂带在第三纪以来的活动性对其两侧地区热场的影响较大.郯庐断裂带两侧坳陷地温场的差异在宏观上受控于郯庐断裂的演化,在局部地区受控坳(盆)断裂等构造的影响.     

利用远震接收函数研究辽宁地区的地壳厚度及泊松比

文中利用辽宁省数字地震遥测台网15个台站记录的远震P波波形资料,用频率域反褶积方法提取接收函数,由H-Kappa叠加方法得到了各台站下方的地壳厚度和泊松比。研究结果表明,辽宁地区的地壳泊松比在0.25～0.29之间,地壳厚度介于31～36km之间,东部褶隆带的地壳厚度从北向南由31km增至35km。松辽构造盆地地壳厚度变化不大,平均厚度为31km。辽西褶隆带与东部褶隆带的地壳厚度均比哈尔滨构造盆地厚约2～4km     

郯庐断裂带莱州湾段的构造特征

本文利用海上浅层地震勘探剖面分析了郯庐断裂带莱州湾段的上更新统、全新统和活动构造的某些特征。晚更新世末期发生的构造运动使上更新统产生断裂与褶皱,沿郯庐断裂带东主干断裂发育了狭长的背斜构造,在西主干断裂两侧次级横向(东西向)断裂十分发育,这些横向断裂是一些高角度的张性正断层。     

Lithological,structural, and chronological relationships between the Sanbagawa Metamorphic Complex and the Cretaceous Shimanto Accretionary Complex on the central Kii Peninsula,SW Japan

It is essential to clarify the lithological, structural, and chronological relationships between the Sanbagawa Metamorphic Complex (MC) and the Cretaceous Shimanto Accretionary Complex (AC) for understanding the tectonic evolution of SW Japan. To this end, we carried out a detailed field survey of the Sanbagawa MC and the Cretaceous Shimanto AC on the central Kii Peninsula, where they are in direct contact with each other. We also conducted U–Pb dating of detrital zircons from these complexes. The field survey showed that the boundary between the Iro Complex of the Sanbagawa MC and the Mugitani Complex of the Shimanto AC, Narai Fault, shows a sinistral sense of shear with a reverse dip‐slip component, and there are significant differences in the strain intensity and the degree of recrystallization between the two complexes across this fault. Detrital zircon U–Pb dating indicates that the Iro Complex in the hanging wall of the Narai Fault shows a significantly younger maximum depositional age than the Mugitani Complex in the footwall of the fault, and an apparently large gap in the MDA of ca. 35 Myr exists across this fault. This large age gap across the Narai Fault suggests that this fault is an essential tectonic boundary fault within the Cretaceous accretionary–metamorphic complexes on the Kii Peninsula, and is considered to be an out‐of‐sequence thrust. In addition, a similar shear direction and a large age gap have been identified across the Ui Thrust, which marks the boundary between the Kouyasan and Hidakagawa belts of the Cretaceous Shimanto AC. The Cretaceous accretionary–metamorphic complexes record the large‐scale tectonic juxtapositions of complexes, and these juxtaposed structures had been caused by sinistral–reverse movements on the tectonic boundary faults such as the Narai Fault and the Ui Thrust.     

Active faulting offshore SE Spain (Alboran Sea): Implications for earthquake hazard assessment in the Southern Iberian Margin

The southern margin of the Iberian Peninsula hosts the convergent boundary between the European and African Plates. The area is characterised by low to moderate magnitude shallow earthquakes, although large historical events have also occurred. In order to determine the possible sources of these events, we recently acquired swath-bathymetry, TOBI sidescan sonar and high-resolution seismic data on the Almería Margin (Eastern Alboran Sea). The new dataset reveals the offshore continuation of the NE–SW trending Carboneras Fault, a master fault in the Eastern Betic Shear Zone, and its associated structures (N150 and NS faults). These structures are active since they cut the Late Quaternary sedimentary units. The submarine Carboneras Fault zone is 100 km long, 5–10 km wide, and is divided into two N045 and N060 segments separated by an underlapping restraining stepover. Geomorphic features typically found in subaerial strike-slip faults, such as deflected drainage, water gaps, shutter ridges, pressure ridges and “en echelon” folds suggest a strike-slip motion combined with a vertical component along the submarine Carboneras Fault. Considering the NNW–SSE regional shortening axis, a left-lateral movement is deduced for the Carboneras Fault, whereas right-lateral and normal components are suggested for the associated N150 and NS faults, respectively. The offshore portion of this fault is at least twice as long as its onshore portion and together they constitute one of the longest structures in the southeastern Iberian Margin. Despite the fact that present day seismicity in the Almería margin seems to be associated with the N150 to NS faults, the Carboneras Fault is a potential source of large magnitude (M_w ∼7.2) events. Hence, the Carboneras Fault zone could pose a significant earthquake and tsunami hazard to the coasts of Spain and North Africa, and should therefore be considered in any hazard re-evaluation.     

沂沭断裂带内部的差异活动及其成因分析

郯庐断裂带在山东境内被称为沂沭断裂带的部分是其新构造活动最为强烈的一段.许多学者对沂沭断裂带做过大量研究工作,指出沂沭带活动存在差异性,但较少关于差异活动的原因分析.本文在前人研究工作基础上,较为深入地分析了沂沭带5条断裂的主要活动规律及其特点,对沂沭带进行了新的分段,并归纳了沂沭带不同断裂之间、同一断裂不同段落之间的活动差异及其差异的基本特征.在此基础上,从地貌、断裂结构、断裂空间组合、现代运动、深部构造环境等几个方面,分析阐述了造成沂沭带差异活动的可能原因,为更深入地研究沂沭带提供了新鲜的启示和思路.     

Evidence of seismites in coastal Quaternary deposits of western Oranie (northwestern Algeria)

Coastal Quaternary deposits of western Oranie show typical soft-sediment deformations including sedimentary dykes, sand volcanoes, sismoslumps, thixotropic bowls, thixotropic wedges, diapir-like structures, and faults grading. Field observations indicate that these deformations exist at several levels of the studied deposits along the west Oranian coast. This study demonstrated that these structures are earthquakes-related, by analysis of potential trigger sources. Several arguments demonstrate the seismic origin: the depositional environment rich in water that located in an active tectonic region, the thixotropic nature of deformations and their large vertical and horizontal diffusion in the Quaternary series consistent with a seismic recurrence. This allows characterizing these Quaternary soft-sediment deformations as seismites that were triggered by earthquakes.     

郯庐断裂带莒县胡家孟晏地震破裂带的发现

郯庐断裂带是中国东部最主要的一条活动断裂带。在该断裂带中部,沂沭断裂东地堑的潍坊—嘉山段中发育了1条长360km的全新世活动断裂带(F5),在该全新世断裂带的北段和中段分别发生了公元70年的安丘地震和公元1668年的郯城地震。2003年底我们考察沭河断裂带时,在莒县境内发现了1条长约7km的地震破裂带,作为活动断层应该归属于F5断裂带,但其是一条独立的地震破裂段还是归属于1668年郯城8.5级地震破裂带有待于进一步的研究。尽管如此,探槽揭示出的上覆未经破坏的地层的14C年代表明,该破裂带在(2140±190)aBP以来没有过活动,因此我们认为其作为1条独立破裂段的可能性较大     

THE COULOMB STRESS CHANGES AND SEISMICITY ON SOME MAJOR FAULTS IN NORTH CHINA

In recent years, the Coulomb stress change induced by large earthquakes has attracted extensive attention in seismology. Many scientists at home and abroad have made remarkable achievements in the research on it. It is well known that North China is densely populated and industrially developed. More importantly, the Chinese capital city, Beijing, lies in the hinterland of North China. At the same time, there are abundant active faults and earthquakes in North China. The capital Beijing is China's political, economic, cultural, and transportation center. It is the center of all social activities and economic activities in the country, and is also a region where population, wealth, and information are highly concentrated. With the integration of Beijing-Tianjin-Hebei and the construction of Xiong'an New District, the consequences of big earthquake in Beijing and surrounding areas are unimaginable. Due to its special geographical location, frequent seismic activities in North China capture much attention. From the physical principle, the occurrence of earthquakes releases the accumulated stress, but the stress does not completely disappear. Some of the stresses are transmitted and transferred to other areas, resulting in stress concentration in some areas, which in turn affects the occurrence of earthquakes in the area. This is the idea of stress triggering of earthquakes. According to this hypothesis, the enhancement of Coulomb stress corresponds to the additional loading of the fault and promotes the occurrence of earthquakes; conversely, the weakening of the Coulomb stress in the stress shadow zone corresponds to partial unloading of the fault, which will delay the occurrence of the earthquake. In order to study the future seismic activity of North China, this paper estimates risks of future strong earthquakes in the region. To this end, we calculate the coseismic Coulomb stress changes and postseismic viscoelastic relaxation stresses of the events with M_S ≥ 6.0 that occurred in the North China region since 1820, using elastic dislocation theory and hierarchical lithosphere model, respectively, in order to examine whether the cumulative Coulomb stress change can explain the spatiotemporal pattern of large earthquakes. Also we project the Coulomb stress change onto the specific active faults in North China and assign the present and future Coulomb stress change state to the faults to provide a dynamics reference for analyzing whether the areas will be hit by strong earthquakes in the future. The simulated results show that the effect caused by the effective friction coefficient changes is not significant on the spatial distribution of Coulomb stress changes induced by coseismic and postseismic viscoelastic relaxation effect of the medium of earthquakes in the North China region. Although the variation of the effective friction coefficient has an impact on the Coulomb stresses for some sections of faults, the general pattern of the spatial distribution of the Coulomb stress changes keeps unchanged. Consequently, 19 of the 24 earthquakes since the 1888 Bohai Bay earthquake have fallen in the positive region of Coulomb stress changes, with a triggering rate of 79%. In particular, considering the seismogeological data and the Coulomb stress calculation results, we assume that Luanxian-Yueting Fault, Panzhuangxi Fault, Dongming-Chengwu Fault, Yuncheng Fault, Longyao Fault of Ninghe-Xinxiang seismic belt, the Yingkou-Weifang Fault of Tanlu seismic belt, the Xiadian Fault, and the Huangzhuang-Gaoliying Fault in the Capital area have higher seismic risk and deserve in-depth study.     

南海北部潮汕坳陷上白垩统盆地原型及其大地构造背景分析

潮汕坳陷MZ-1井揭示的中生界为深入分析南海北部晚中生代的构造演化提供了关键性的资料.基于MZ-1井的标定,开展了系统的地震剖面构造-地层解释,在中生代地层内识别出Tm30区域性不整合面,同位素定年确定该界面发育于早白垩世末至晚白垩世初,落实了潮汕坳陷上白垩统的分布.此外,在研究区西南部识别出大型的兴宁—东沙逆冲推覆带,主要由多条NW—SE向延伸、西倾的叠瓦状逆冲断层及其伴生的不对称褶皱组成,其明显控制了上白垩统厚度分布.由此可见,上白垩统构造层不具有张裂盆地的典型特征,因此南海北部主动陆缘向被动陆缘的转换不会早于晚白垩世末.研究认为,在南海地区特提斯残留洋盆关闭的总背景下,在约80 Ma时期,南海地块与华南陆块强烈碰撞挤压,在靠近碰撞带处的礼乐滩、潮汕坳陷西南部形成褶皱冲断构造体系,进而控制了潮汕坳陷晚白垩世周缘前陆盆地的发育.