以精定位背景地震活动与震源机制解研究郯庐断裂带中南段现今活动习性

郯庐断裂带是中国东部板内一条规模最大的强构造变形带与地震活动断裂带,其断裂结构与历史地震活动性具明显的分段活动性。文中通过沿郯庐断裂带中南段的历史地震活动性、精定位背景地震活动性与震源机制解分析,讨论了断裂带的深部几何结构与现今活动习性。现今地震活动在中段主要沿1668年郯城MS 8地震破裂带线性分布,线性条带在泗洪-诸城间延伸约340 km长,为1668年地震长期缓慢衰减的余震序列活动。大震地表破裂遗迹与精定位地震分布都揭示出郯庐断裂带中段的两条全新世活动断裂昌邑-大店断裂与安丘-莒县断裂以右阶斜列的形式共同参与了1668年郯城MS 8地震破裂。精定位震源剖面刻画出两条断裂结构面呈高角度相背而倾,其中昌邑-大店断裂倾向SE,安丘-莒县断裂倾向NW,两条断裂在深部没有合并汇聚。余震活动所代表的1668年地震震源破裂带是郯庐断裂带中现今尚未闭锁的安全段落,对应于高b值段。而未发生破裂的安丘以北段,小震活动不活跃,b值低,现今可能已成为应力积累的闭锁段。震源机制解揭示的断裂应力状态在中段以NE向主压应力为主,表现为右旋走滑活动性质,且存在少量正断分量,南段转为以NEE至近EW向为主,存在少量的逆冲分量。在中段与南段的转折处宿迁-嘉山段,主压应力方向垂直断裂带走向呈NWW向,反映出局部以挤压为主的应力特征,其中泗洪-嘉山段也是历史地震未破裂段,现今小震活动不活跃,因此该段可能更易于应力积累。精定位小震活动在郯庐断裂与北西向断裂相交汇处聚集,反映出北西向断裂的新活动性,以及郯庐断裂带现今的逆冲作用。在断裂带南端,精定位背景地震活动沿与其相交汇的襄樊-广济断裂带东段呈北西向线性分布,表明了该段的现今活动性。     

从地震信息看新生代郯庐断裂带的展布与活动

郯庐前新近纪断裂带为中国东部滨太平洋地区一条巨型张扭走滑构造带。利用地震信息分析地质结构,可将其分为南、中、北三大段和七亚段。按其影响范围的规模可划分为广义和狭义两类:广义郯庐前新近纪断裂带地质结构是“两凹夹一隆”,狭义郯庐前新近纪断裂带则指高垒带与两侧(邻)的断层组合。断裂带内外都发育有反转、花状构造。狭义郯庐前新近纪断裂带走向为NNE,仅在嘉山以南为NE向,其两侧的前新近纪断层大体上都为EW、NE和NEE走向,与狭义郯庐断裂呈锐角相切。古近纪构造层中,由南到北郯庐断层两侧箕状断陷边界都由生长断层所组成,说明郯庐断裂两侧的古近纪断陷应属拉张盆地,并非拉分盆地,郯庐古近纪断裂动力学是拉张而非拉分构造应力场。古近纪构造层在拉张翘倾运动中叠加了右旋走滑地质应力,这种右旋走滑应力是太平洋板块向西俯冲至欧亚板块之下的结果。中生代郯庐断裂的动力学表现为库拉板块向北俯冲至欧亚板块之下形成的左旋平移现象。郯庐断裂NE向平移走滑与北西向隐蔽断裂的相交处是发生破坏性地震的构造部位,因此,应特别重视对高丽营北东向生长断层与渤中-孙河-南口北西向隐蔽断层相交处地震动向的监控。     

郯庐断裂带中南段闭锁特征与地震危险性分析

利用华北地区1999～2007、2013～2017两期GPS水平运动速度场数据,采用块体负位错模型,分别反演了郯庐断裂带中南段不同段的断层闭锁程度和滑动亏损速率分布;结合地表应变结果,综合分析了郯庐断裂带前后两期的变形差异特征,并探讨了其与日本3·11地震间的可能关系。研究结果表明:日本地震后,郯庐断裂带中南段郯城以北的段落闭锁程度有所减弱,中南段东部地区主张应变率增强,处于拉张状态;日本大地震的发生对郯庐断裂带中南段的应变积累起到一定的缓解作用。2013～2017最新一期反演结果显示莒县以北断层闭锁程度仍较高,闭锁深度较深,为右旋挤压亏损,是1668年郯城地震的未破裂段;莒县以南到泗洪附近断层闭锁程度较低,无滑动亏损积累;泗洪以南到嘉山段断层闭锁程度较高,是历史地震的未破裂段,同时该地区小震不活跃,易于应力积累,地震危险性值得关注。     

郯庐断裂带附近地壳浅层现今构造应力场

郯庐断裂带是中国东部大陆规模最大的第四纪活动构造带和地震活动带,断裂带附近现代构造应力场明显控制了其运动方式、活动强度和地震活动,深入研究郯庐断裂带附近地壳浅层现今构造应力场,对于探讨断裂带当前活动性具有重要的科学意义。在参考前人研究成果的基础上,依据郯庐断裂带附近(山东—环渤海—辽宁段)6个600~1 000 m深孔地应力实测数据,初步揭示了断裂带附近地壳浅层应力分布规律,并分析其在不同活动段之间的差异。结合研究区已有其他应力数据(震源机制解、钻孔崩落、应力解除及断层滑动矢量反演数据等),重绘断裂带及邻区地壳现代构造应力场图,基于此详细分析了现今构造应力场主压应力方位特征及其对断裂活动方式的影响。初步结果表明:(1)在构造应力积累水平上,郯庐断裂带山东段西南端应力积累最高,渤海段东、西两侧之辽东半岛应力积累强度次之,辽宁段东北端本溪地区略低于渤海段,山东段北端和河北昌黎两地应力积累强度最低。(2)断裂带山东段南端地应力环境为逆冲型,而北端以正断型为主;渤海段之辽东半岛和河北昌黎及邻区主要为正断型;辽宁段东北端以正断型为主,兼具走滑型应力状态,地壳浅部地应力状态与断裂各段第四纪以来的运动学特征具有较好的一致性。(3)郯庐断裂带山东段及邻区现今构造应力场主压应力优势方位为N70°E,渤海段及邻区为N68°E,辽宁段及邻区为N72°E,各段之间差异不明显,该应力作用方式有利于断裂带产生右旋走滑活动,同时也表明郯庐断裂带所处的华北及辽宁大部地区具有较统一构造应力环境。     

郯庐断裂带中段第四纪活动及其分段特征

郯庐断裂带是中国东部一条岩石圈尺度的构造不连续带。位于江苏和山东培内的郯庐断裂带中段，在新构造运动时期强烈右旋走滑复活，形成地貌形迹显著的走滑活动断裂带。笔者在断层活动形迹的野外调查和观测的基础上，结合TM遥感影像特征解译和地震震源机制解资料，分析了郯庐断裂中段第四纪活动的分段行为特征。位于嘉山－潍坊之间的郯庐断裂带中段可以进一步划分为3段，北段安丘－茅埠亚段，中段汪湖－宿迁亚段，南段宿迁－嘉山亚段，这三段可能分别是独立的地震破裂段。观测表明，新构造变形主要集中在宿迁以北的中、北段，是历史强震的发生段，而南段变形相对较弱，嘉山以南安徽境内郯庐断裂新构造变形更弱。郯庐断裂带新构造走滑变形的走向分段行为是华北地区不同块体新构造运动位移调节的结果。     

泛长三角地区主要活动断裂及地震活动基本特征

在系统收集整理前人成果资料基础上,结合基础地质和遥感解译,对泛长三角地区的主要活动断裂及其活动性进行了全面梳理和分析,并根据区域主要断裂的活动性与历史地震活动特征,进一步分析指出了该区需要特别关注的潜在震源区和地壳稳定性相对较差的区域。研究结果表明,该区主要发育38条活动性相对显著的活动断裂带,并以北东和北西走向两组活动断裂为主,前者以右旋走滑活动为主,后者以左旋走滑活动为主;其次为近东西走向,活动性较弱。进一步结合历史地震活动信息分析发现,其中的北东/北北东向断裂是区域上主要的控震构造和发震断层,其次是北西向断裂,这两组不同走向断裂带的交汇部位常常是地震发生部位,其中最显著的控震断裂是北东向的郯庐断裂与北西向的无锡-宿迁断裂。      

利用高精度航磁资料研究滁州——六合地区断裂构造

利用最新的1∶2.5万高精度航磁资料,辅以1∶20万重力资料,对滁州—六合地区断裂构造进行了重新厘定和推断。结果显示,区内基本构造格架由北北东向、北东向、北西向3组断裂构成,它们是区内主要的控岩、控盆、控矿断裂。中部郯庐断裂带(苏皖段)与黄—破深断裂与两侧的构造形迹具有明显差异,形成了类似中部"树干"、两侧"树枝"的独特结构特征。笔者还简要讨论了近东西向、北北东向次级断裂对蚌埠隆起东段矿产的控制作用,以及郯庐断裂带(苏皖段)与合肥盆地东北部演化的关系等问题。     

海峡西岸长乐—南澳断裂带中段现今原位地应力测量

为查明长乐—南澳断裂带现今地应力状态和活动现况,在该断裂带中段泉州市区北侧开展了600 m深度钻探工程和随深度系统水压致裂现今原位地应力测量工作,在钻探深度内成功完成了21段地应力大小测试和4段最大水平主应力方向测试。随深度系统的地应力测量结果表明:1水平主应力量值处于高值状态且三个主应力大小之间关系为SHShSV,表明区域构造作用强烈且水平应力起主导作用,易于逆断层发生。2现今最大水平主应力方向为北西西向,反映北东向的长乐—南澳断裂带中段现今活动具有右行走滑的特征。3基于实测数据计算得到μm和R,μm=0.44~0.76,反映了该区域相对很高的应力积累水平;R=0.44~0.65,说明此应力状态有利于该区域内断层的活动。依据实测地应力结果表明,长乐—南澳断裂带中段的应力状态已经达到断层活动的临界值,断裂活动进入临界状态,具有逆断层发生滑动的潜在危险性,值得重点关注和深入研究。研究成果为长乐—南澳断裂带中段现今活动性研究补充了新的动力学数据,为地震地质研究和城市安全评价提供了依据。     

渤海海域新近纪断层成因与动力学状态

新近纪是渤海海域新构造运动的重要阶段,具有不同于第四纪以来的构造活动特征和盆地格局。大量的盆地勘探资料显示,海域新近系内发育了大量张性、张扭性断层,主要为近东西向断层,其次为北北东向的郯庐断裂带和局部的大型北西向断层。这一断层格局与下伏古近系相似,但断层活动强度明显较弱,属于坳陷型盆地内的断层活动。断层的平、剖面特征表明,近东西向者为正断层,北北东与北西向断层分别为右行、左行平移正断层。各类断层的运动学特征指示,海域古近纪处于南北向的弱伸展环境。近东西向断层小型者多为新生正断层,大型者常是早期断层的复活,皆为区域拉张作用的直接结果。北北东向的郯庐断裂带与北西向断层皆为继承性活动,其走滑是斜向拉张的结果。郯庐断裂带较强的张扭性活动一方面使旁侧派生了一系列北东-北东东向的小型正断层,另一方面使其旁侧各类断层密度明显增大。海域新近纪的弱伸展环境结束于新近纪末一次区域性挤压事件。     

郯庐断裂带在辽北地区三叠纪左行韧性剪切带的发现及变形特征

郯庐断裂带位于辽北地区的部分为区域上郯庐断裂北延分支依兰-伊通断裂带的一段,在1：5万区调中发现有北东向展布的三叠纪左行韧性剪切带,此韧性剪切带控制一系列三叠纪侵入岩的分布并对古生代地质体的展布进行改造.     

第四纪断层上断裂活动的群集及迁移现象

对许多第四纪断层上形成地表破裂的巨大滑动事件及错动位移状况的具体研究表明,在不同时间段它们的数量和强度是变化的,不均匀的。这种断层活动的不均匀性主要表现在两个方面:一是滑动事件的发生往往集中于一定的地段和一定的时间段,并与相对平静的阶段相交替,即所谓断裂活动的群集性;另一方面的表现是集中发生滑动事件段落(或区域)在空间上的迁移变化。这是中国大陆内部第四纪断层活动所表现出的主要特征之一。这方面的实例可见于许多活断层上。如沿郯庐断层、鲜水河断层以及河西走廊断裂带、阿尔金断裂带等。深入研究第四纪断层活动的这一不均匀特性对正确评估活断层的活动程度和潜在地震危险性具有实际应用意义。     

Active Faulting Pattern,Present‐day Tectonic Stress Field and Block Kinematics in the East Tibetan Plateau

Abstract: This paper examines major active faults and the present-day tectonic stress field in the East Tibetan Plateau by integrating available data from published literature and proposes a block kinematics model of the region. It shows that the East Tibetan Plateau is dominated by strike-slip and reverse faulting stress regimes and that the maximum horizontal stress is roughly consistent with the contemporary velocity field, except for the west Qinling range where it parallels the striking of the major strike-slip faults. Active tectonics in the East Tibetan Plateau is characterized by three faulting systems. The left-slip Kunlun-Qinling faulting system combines the east Kunlun fault zone, sinistral oblique reverse faults along the Minshan range and two major NEE-striking faults cutting the west Qinling range, which accommodates eastward motion, at 10–14 mm/a, of the Chuan-Qing block. The left-slip Xianshuihe faulting system accommodated clockwise rotation of the Chuan-Dian block. The Longmenshan thrust faulting system forms the eastern margin of the East Tibetan Plateau and has been propagated to the SW of the Sichuan basin. Crustal shortening across the Longmenshan range seems low (2–4 mm/a) and absorbed only a small part of the eastward motion of the Chuan-Qing block. Most of this eastward motion has been transmitted to South China, which is moving SEE-ward at 7–9 mm/a. It is suggested from geophysical data interpretation that the crust and lithosphere of the East Tibetan Plateau is considerably thickened and rheologically layered. The upper crust seems to be decoupled from the lower crust through a décollement zone at a depth of 15–20 km, which involved the Longmenshan fault belt and propagated eastward to the SW of the Sichuan basin. The Wenchuan earthquake was just formed at the bifurcated point of this décollement system. A rheological boundary should exist beneath the Longmenshan fault belt where the lower crust of the East Tibetan Plateau and the lithospheric mantle of the Yangze block are juxtaposed.     

Deformation Stages and Ar‐Ar Age Data of the Wan‐Zhe‐Gan Tectonic Zone,Southeast China,and Their Tectonic Significance

The major tectonic zone that passes through the border regions of the Anhui, Zhejiang, and Jiangxi Provinces in southeast China has been commonly referred to as the Wan-Zhe-Gan fault zone. Geologically, this zone consists of several regional fault belts of various ages and orientations. We have categorized the faults into four age groups based on field investigations. The Neoproterozoic faults are northeast striking. They start from the northeast Jiangxi Province and extend northeastward to Fuchuan in Anhui Province, the same location of the northeast Jiangxi-Fuchuan ophiolite belt. The faults probably acted? during the Neoproterozoic as a boundary fault zone of a plate or a block suture with mélange along the faults. The nearly east-west- or east-northeast-striking faults are of Silurian ages (40Ar/39Ar age 429 Ma). This group includes the Qimen-Shexian fault and the Jiangwang-Jiekou ductile shear belt. They represent a major tectonic boundary in the basement because the two sides of the fault have clear dissimilarities. The third group of faults is north-northeast striking, having formed since the early-middle Triassic with 40Ar/39Ar ages of 230–254 Ma. They form a fault belt starting from Yiyang in northern Jiangxi and connect with the Wucheng as well as the Ningguo-Jixi faults. This fault belt is a key fault-magmatic belt controlling the formation of Jurassic-Cretaceous red basins, ore distribution, magmatic activity, and mineralization. When it reactivated during the late Cretaceous, the belt behaved as a series of reverse faults from southeast to northwest and composed the fourth fault group. Therefore, classifying the Wan-Zhe-Gan fault zone into four fault groups will help in the analysis of the tectonic evolution of the eastern segment of the Jiangnan orogen since the Neoproterozoic era.     

New data on the evolution of the Tan–Lu fault belt: constraints from geological–geophysical surveys in the southern segment

The Tan–Lu fault is a well-known active fault belt in eastern China that has been the focus of geologic studies over the past 40 years. Since the late 1990s, numerous geophysical and geological investigations of this dislocation zone have been carried out by Chinese oil companies, as well as by universities. However, its deep structure, active periods of slip, and fault mechanism remain obscure. This study focuses on the deep structures within the Jiashan–Lujiang segment of the Tan–Lu fault belt, using high-precision geophysical tools, including magnetotelluric and magnetic sounding, and artificial seismic exploration using active source methods. Our results suggest that this segment is composed of several sub-faults. The southern part of the Tan–Lu fault belt, along the Jiashan–Lujiang sub-fault, can be divided into two parts on the basis of contrasting geological features. The Chihe–Taihu sub-fault is taken as the boundary between the two. The region east of the Chihe–Taihu sub-fault is dominated by strike–slip activity along several sub-faults. Only the Jiashan–Lujiang sub-fault is exposed at the surface, forming a large, positive flower structure, the result of late Middle Jurassic to early Late Jurassic strike–slip movement along the dislocation zone. Three sub-faults are present in Dingyuan County, two of which disappear in the southern Hefei Basin. Only the Chihe–Taihu sub-fault extends to the eastern edge of this basin, creating a half-graben depression that formed during the Early Cretaceous. Our results indicate that the present-day deep structure of the southern portion of the Tan–Lu fault zone is the result of a combination of strike–slip and extensional tectonics.     

青藏高原东北缘大陆岩石圈现今变形和位移

川青地块在地貌上为川西高原，亦是青藏高原东北边缘最重要的构造单元。新的GPS监测资料表明，在欧亚框架内，川青地块及其邻近的龙门山带和华南地块西缘的地壳运动水平速度，具有自西向东由25．66mm／a递变下降到6．99mm／a的总趋势。速度矢量表现出顺时针涡旋转动。川青地块内具有局部应变积累的非均一的区域剪切。横切鲜水河断裂带中段新的GPS量结果揭示，两侧地块间的平均左旋滑动速率约8mm／a；由于局部应变积累，断裂系南西侧的主断裂的移动速率为9．3mm／a，其间为拉分盆地和小的横向伸展断裂。鲜水河断裂系的左旋断裂滑动作用，调节了川青地块与／11滇地块之间的相对运动。     

Stress triggering of normal aftershocks due to strike slip earthquakes in compressive regime

A few cases of occurrence of normal aftershocks after strike slip earthquakes in compressive regime have been reported in the literature. Occurrence of such aftershocks is intriguing as they occurred despite the apparent stabilizing influence of compressive plate tectonic stresses on the normal faults. To investigate the occurrence processes of such earthquakes, we calculate change in static stress on optimally oriented normal and reverse faults in the dilational and compressional step over zones, respectively, due to slip on a vertical strike slip fault under compressive regime. We find that change in static stress is much more pronounced on normal faults as compared to that on reverse faults, for all values of fault friction. Change in static stress on reverse fault is marginally positive only when the fault friction is low, whereas for normal faults it is positive for all values of fault friction, and is maximum for high fault friction. We suggest that strike slip faulting in compressive regime creates a localized tensile environment in the dilational step over zone, which causes normal faulting in that region. The aftershocks on such normal faults are considered to have occurred as an almost instantaneous response of stress transfer due to strike slip motion.     

Stress triggering of normal aftershocks due to strike slip earthquakes in compressive regime

A few cases of occurrence of normal aftershocks after strike slip earthquakes in compressive regime have been reported in the literature. Occurrence of such aftershocks is intriguing as they occurred despite the apparent stabilizing influence of compressive plate tectonic stresses on the normal faults. To investigate the occurrence processes of such earthquakes, we calculate change in static stress on optimally oriented normal and reverse faults in the dilational and compressional step over zones, respectively, due to slip on a vertical strike slip fault under compressive regime. We find that change in static stress is much more pronounced on normal faults as compared to that on reverse faults, for all values of fault friction. Change in static stress on reverse fault is marginally positive only when the fault friction is low, whereas for normal faults it is positive for all values of fault friction, and is maximum for high fault friction. We suggest that strike slip faulting in compressive regime creates a localized tensile environment in the dilational step over zone, which causes normal faulting in that region. The aftershocks on such normal faults are considered to have occurred as an almost instantaneous response of stress transfer due to strike slip motion.     

湘东─赣西NNE向走滑构造

湘东─赣西构造岩浆带位于郯庐断裂南延的关键性部位。中生代以来，该区 NNE向左旋走滑构造主要由会聚走滑和 K─ E离散走滑构造叠加而成。会聚走滑作用造就了 NNE向雁列式剪切断裂系、剪切弯曲和旋转构造、压剪性煤盆地以及断层动热变质─剪切重熔型花岗岩；而离散走滑作用则控制了该区广泛发育的张剪性红盆地、盆岭式构造地貌、以及大规模中低温热液矿床的形成。湘东─赣西复杂的平移构造型式很可能与该区地壳结构分层特征、前期断裂构造格局、平移幅度和多期走滑构造作用四个因素有关。     

郯庐断裂走滑活动与辽河盆地构造古地理格局

渐新世晚期 ,郯庐断裂的右行走滑活动控制了辽河盆地的构造古地理格局。研究认为 ,岩石圈断块沿郯庐断裂的走滑活动与大型板块构造的活动方式有一定的相似之处 ,即伴随着郯庐断裂的右行走滑 ,从断裂的增压弯曲部位到断裂的释压拉张部位将发生岩石圈断块的汇聚与离散现象 ,由此造成了走滑断裂带上增压弯曲部位与释压拉张部位局部应力场性质的不同 :增压弯曲部位应力相对集中 ,岩石圈断块发生汇聚、挤压、隆升 ;而释压拉张部位由于应力释放 ,岩石圈断块发生离散、伸展、沉降。岩石圈断块的隆升与沉降造成了渐新世晚期辽河盆地构造古地理格局的巨大差异。     

Relationship between the TanLu Fault and Dabie Orogenic Belt in Eastern China

１．ＩｎｔｒｏｄｕｃｔｉｏｎＦｉｇ．１．　ＴｅｃｔｏｎｉｃｐｏｓｉｔｉｏｎｏｆｔｈｅＤａｂｉｅｏｒｏｇｅｎｉｃｂｅｌｔａｎｄＴａｎＬｕｆａｕｌｔｉｎｅａｓｔｅｒｎＣｈｉｎａ　　ＴｈｅＤａｂｉｅｏｒｏｇｅｎｉｃｂｅｌｔａｎｄＴａｎＬｕｓｔｒｉｋｅｓｌｉｐｆａｕｌｔａｒｅｓｉｔｕａｔｅｄｉｎｃｅｎｔｒａｌａｎｄｅａｓｔｅｒｎＣｈｉｎａ,ｒｅｓｐｅｃｔｉｖｅｌｙ（Ｆｉｇ．１）．Ｔｈｅｉｒｅｖｏｌｕｔｉｏｎａｒｙｐｒｏｃｅｓｓｅｓ,ｒｅｌａｔｅｄｔｏｔｈｅａｄｊａｃｅｎｔｇｅｏｌｏｇｉｃｂｌｏｃ…