汤加-克马德克俯冲带的地质构造与地震火山特征

2022年1月15日西南太平洋的洪阿哈阿帕伊岛海底火山发生了爆炸式的剧烈喷发，吸引了全球的关注。洪阿哈阿帕伊岛海底火山位于汤加-克马德克俯冲带，综合前期研究结果，对汤加-克马德克俯冲带的地质构造特征、地震和火山分布进行初步分析，发现：（1）从汤加-克马德克俯冲带弧前向海方向直到俯冲的太平洋板块，构造上主要表现为大规模正断层。（2）路易斯维尔海山链的俯冲将汤加-克马德克俯冲带分为北部的汤加俯冲带和南部的克马德克俯冲带，沿汤加俯冲带板块汇聚率为67～84 mm/a，沿克马德克俯冲带板块汇聚率为41～58 mm/a，板块俯冲速度的差异造成汤加俯冲带和克马德克俯冲带目前俯冲深度的不同。（3）在路易斯维尔海山链以北，太平洋板块上覆沉积物厚度不足0.4 km，而在南侧达到1 km左右，由于俯冲板块上覆沉积物厚度的差异而造成北部的汤加俯冲带和南部的克马德克俯冲带孕育地震能力的差异。这些认识对研究该俯冲带的火山喷发机制、大地震成因机理及其灾害风险具有重要意义。     

西北太平洋地震层析剖面及地球动力学启示

具有较高分辨率的日本海沟-长白山-东乌珠穆沁旗的地震层析成像剖面,显示了太平洋板块俯冲板片的形貌特征和长白山、大兴安岭下方的岩石圈结构特征,表明太平洋板块以26°倾角俯冲到660 km之后明显下插,在其西侧伴有一显著低速的热上涌体,不存在俯冲板块继续向西深入的迹象.根据太平洋板块俯冲时间及俯冲作用引起岩浆活动的时间差,可认为中国东部中生代岩浆活动与太平洋板块俯冲无直接的成因联系.同时还强调地震层析剖面反映的是现存的深部结构,它可以保留某些中生代的岩石圈结构特征,据此可推测华北中生代岩浆的演化与大陆板块内软流圈上涌体的活动有关.     

四川汶川大地震与C型俯冲的关系和防震减灾的建议

四川汶川大地震发生在龙门山冲断带,属构造地震.龙门山冲断带与川西前陆盆地是中国西部典型大陆构造,属C型俯冲(陆内俯冲)模式.C型俯冲不仅控制油气资源分布,还孕育着发震机制.作者从龙门山冲断带地史演化、变形特征、深部地球物理信忠,建立起龙门山C型俯冲构造运动模式.汶川大地震发震与此模式的地质构造背景关系密切,是现今发生的陆内俯冲引起的地震.当时可能发生了两次强烈地震,这才可能是北川县城遭到毁灭性破坏的原因.汶川大地震可能发生在上地壳底至中地壳深12～24 km的高导层上,属中国陆内俯冲型地震,很可能是太平洋板块推挤中国大陆的远端效应触发作用所引起,与印度板块推挤作用关系不大.     

孟加拉湾若开盆地构造特征及演化

根据地震、钻井等基础资料建立主干剖面,结合平面构造展布及剖面构造特征研究孟加拉湾若开盆地的构造特征和演化。若开盆地是一个主动大陆边缘海沟背景下的增生楔盆地,经历了晚白垩世-古新世早期发育期、始新世-渐新世缓慢碰撞期和中新世-上新世盆地定型期3个区域演化阶段。平面上划分为东部的若开褶皱带和西部的海底平原带。若开褶皱带具有南北分段特征,根据构造特征差异分为2段:(1)北段挤压褶皱带,主要发育NNW走向、带状分布褶皱。北段挤压褶皱带可分为高陡褶皱带和低缓褶皱带2个次一级构造单元,变形强度从东往西逐渐减弱。(2)南段走滑褶皱带,褶皱表现为宽度较窄,并发育明显走滑构造。主干剖面构造演化史分析认为:研究区构造变形开始于晚中新世末,上新世末-第四纪变形强烈;构造变形从东向西传播,东部变形时间早,变形强烈,而西部变形时间晚,变形较弱;在中新统沉积后,区域构造发生翘板式逆时针旋转,沉积中心从东部迁移到西部。若开盆地的构造形成与演化主要受印度板块向欧亚板块斜向B型俯冲碰撞控制,由于俯冲碰撞接触带在若开盆地呈弧状,南北两段应力场不同,导致南北两段构造变形存在明显差异。     

闽台区域地球动力学及地震活动效应研究

该文主要依据近30多年以来对福建地块的地震地质背景、地壳变形观测、构造应力场、地震活动性以及震源动力学参数等资料,结合台湾学者对台湾岛弧地球动力学、强震构造等的研究成果,探索海陆板块俯冲一碰撞地球动力学特征及其对板内活动地块影响,进而揭示板间-板缘-板内强震活动关联性和动力学性状的异同,以期为闽台区域的强震预测和防震减灾对策提供新思路.     

大陆动力学若干重要问题的探讨

本文通过对大陆构造变形的基础与过程、板块运动与大陆碰撞造山带的关系以及大陆地震特征的分析,探讨了大陆构造的特点与大陆变形的一般规律.     

南海和东海板块构造演化的几个问题

本文就南海、东海及台湾板块构造演化的若干问题展开讨论。我们认为南海海盆是在被动大陆边缘的基础上张开的，而不是弧后扩张的产物；台湾不是活动岛弧．而是典型的岛弧‘大陆碰撞带，并将台湾与鲁马拉雅地区的构造作了对比；吕宋至台湾的板块边缘显示出板块俯冲至碰撞的转化．南海东缘板块边界演化的基本趋势是．从俯冲到碰撞．进而形成极性相反的新俯冲带．南海海盆终将成为被地块全面包围的残留盆地；南海的开合表现为威尔逊旋回的复杂形式。认为冲绳海槽尚未出现伴有新洋壳形成的海底扩张作用．冲绳海槽是弧后裂陷．而非弧后扩张的产物。     

吉林汪清7.2级深源地震前后延边地震台前兆异常的探讨

本文介绍了吉林省延边地震台所处的地质构造位置以及2002年6月29日吉林汪清发生7.2级深震前后,延边地震台出现的显著的地形变和地下流体前兆异常.针对深源地震前地形变和地下流体前兆异常的基本特征.研究了西太平洋板块俯冲应力应变的传递与演化动力学过程.结果证明前兆异常可能与西太平洋板块俯冲有关.     

华北板块南部高变质煤条带成因机制

 为确定华北板块南缘高变质煤条带形成机制与形成时代,通过研究华北板块南缘中二叠统山西组二1煤层煤级分布和岩浆岩空间展布特征,结合华北板块与扬子板块碰撞历史,利用地质学"重力均衡"原理和日本岩石学家都城秋穗提出的"双变质带"理论,建立碰撞热流模型并合理解释导致高变质煤条带形成的热流来源,得出:1)扬子板块向华北板块的俯冲作用,在华北板块南缘前陆盆地形成高温低压带,促进了煤化作用,形成高变质煤条带;2)结合地质资料和区域构造特征分析,发现高变质煤带走向与印支期构造走向(东-西)一致,该高变质煤条带形成于印支期。燕山期岩浆控制了华北板块南缘大部分煤变质作用,但是在晋城-济源一带其煤变质是由于印支期板块碰撞形成的异常热作用的结果。     

全球地震及火山的分布特征及其变化关系

根据全球地震和火山的分布特征,发现全球的地震和火山具有相似的活动区,其活动区与板块运动密切相关.全球的地震和火山活动区大致可以分为大陆活动区、大洋活动区和俯冲-碰撞带.根据近一个世纪内地震和火山发生强度的大小,将全球的地震和火山活动划分为四个时期,通过分析全球地震和火山分布的平均维度曲线,发现地震和火山在时空上具有同步变化的特征,这很可能与板块的运动密切相关,但局部却显示出不一致性.     

Faulting induced by precipitation of water at grain boundaries in hot subducting oceanic crust

Dehydration embrittlement has been proposed to explain both intermediate- and deep-focus earthquakes in subduction zones. Because such earthquakes primarily occur at shallow depths or within the core of the subducting plate, dehydration at relatively low temperatures has been emphasized. However, recent careful relocation of subduction-zone earthquakes shows that at depths of 100-250 km, earthquakes continue in the uppermost part of the slab (probably the former oceanic crust that has been converted to eclogite) where temperatures are higher. Here we show that at such pressures and temperatures, eclogite lacking hydrous phases but with significant hydroxyl incorporated as defects in pyroxene and garnet develops a faulting instability associated with precipitation of water at grain boundaries and the production of very small amounts of melt. This new faulting mechanism satisfactorily explains high-temperature earthquakes in subducting oceanic crust and could potentially be involved in much deeper earthquakes in connection with similar precipitation of water in the mantle transition zone (400-700 km depth). Of potential importance for all proposed high-pressure earthquake mechanisms is the very small amount of fluid required to trigger this instability.     

Bending-related faulting and mantle serpentinization at the Middle America trench

The dehydration of subducting oceanic crust and upper mantle has been inferred both to promote the partial melting leading to arc magmatism and to induce intraslab intermediate-depth earthquakes, at depths of 50-300 km. Yet there is still no consensus about how slab hydration occurs or where and how much chemically bound water is stored within the crust and mantle of the incoming plate. Here we document that bending-related faulting of the incoming plate at the Middle America trench creates a pervasive tectonic fabric that cuts across the crust, penetrating deep into the mantle. Faulting is active across the entire ocean trench slope, promoting hydration of the cold crust and upper mantle surrounding these deep active faults. The along-strike length and depth of penetration of these faults are also similar to the dimensions of the rupture area of intermediate-depth earthquakes.     

甘孜-理塘断裂带构造演化与金矿的关系

论述了川西甘孜—理塘断裂带构造演化与金矿床成矿作用的关系,总结了区内金矿化的特征和金矿床类型。研究表明,甘孜—理塘断裂带是由韧性剪切带、逆冲断裂带、断陷盆地带、推覆构造带、脆性破碎带和平移走滑带相互叠加、改造而形成的复杂断裂带。其演化历史主要经历了晚三叠世卡尼期－诺利早期的洋壳俯冲、晚三叠世诺利晚期－瑞替期弧－陆碰撞、侏罗－白垩纪陆内会聚和喜马拉雅期断陷、推覆和平移剪切构造发育演化阶段。区内金矿化、金矿床（点）和金矿体的形成和分布,明显受断裂带的控制。金矿的成矿作用与该断裂带构造发展演化密切相关。     

Seismic reflection imaging of two megathrust shear zones in the northern Cascadia subduction zone

At convergent continental margins, the relative motion between the subducting oceanic plate and the overriding continent is usually accommodated by movement along a single, thin interface known as a megathrust. Great thrust earthquakes occur on the shallow part of this interface where the two plates are locked together. Earthquakes of lower magnitude occur within the underlying oceanic plate, and have been linked to geochemical dehydration reactions caused by the plate's descent. Here I present deep seismic reflection data from the northern Cascadia subduction zone that show that the inter-plate boundary is up to 16 km thick and comprises two megathrust shear zones that bound a >5-km-thick, approximately 110-km-wide region of imbricated crustal rocks. Earthquakes within the subducting plate occur predominantly in two geographic bands where the dip of the plate is inferred to increase as it is forced around the edges of the imbricated inter-plate boundary zone. This implies that seismicity in the subducting slab is controlled primarily by deformation in the upper part of the plate. Slip on the shallower megathrust shear zone, which may occur by aseismic slow slip, will transport crustal rocks into the upper mantle above the subducting oceanic plate and may, in part, provide an explanation for the unusually low seismic wave speeds that are observed there.     

A great earthquake doublet and seismic stress transfer cycle in the central Kuril islands

Temporal variations of the frictional resistance on subduction-zone plate boundary faults associated with the stick-slip cycle of large interplate earthquakes are thought to modulate the stress regime and earthquake activity within the subducting oceanic plate. Here we report on two great earthquakes that occurred near the Kuril islands, which shed light on this process and demonstrate the enhanced seismic hazard accompanying triggered faulting. On 15 November 2006, an event of moment magnitude 8.3 ruptured the shallow-dipping plate boundary along which the Pacific plate descends beneath the central Kuril arc. The thrust ruptured a seismic gap that previously had uncertain seismogenic potential, although the earlier occurrence of outer-rise compressional events had suggested the presence of frictional resistance. Within minutes of this large underthrusting event, intraplate extensional earthquakes commenced in the outer rise region seaward of the Kuril trench, and on 13 January 2007, an event of moment magnitude 8.1 ruptured a normal fault extending through the upper portion of the Pacific plate, producing one of the largest recorded shallow extensional earthquakes. This energetic earthquake sequence demonstrates the stress transfer process within the subducting lithosphere, and the distinct rupture characteristics of these great earthquakes illuminate differences in seismogenic properties and seismic hazard of such interplate and intraplate faults.     

Variations in earthquake-size distribution across different stress regimes

The earthquake size distribution follows, in most instances, a power law, with the slope of this power law, the 'b value', commonly used to describe the relative occurrence of large and small events (a high b value indicates a larger proportion of small earthquakes, and vice versa). Statistically significant variations of b values have been measured in laboratory experiments, mines and various tectonic regimes such as subducting slabs, near magma chambers, along fault zones and in aftershock zones. However, it has remained uncertain whether these differences are due to differing stress regimes, as it was questionable that samples in small volumes (such as in laboratory specimens, mines and the shallow Earth's crust) are representative of earthquakes in general. Given the lack of physical understanding of these differences, the observation that b values approach the constant 1 if large volumes are sampled was interpreted to indicate that b = 1 is a universal constant for earthquakes in general. Here we show that the b value varies systematically for different styles of faulting. We find that normal faulting events have the highest b values, thrust events the lowest and strike-slip events intermediate values. Given that thrust faults tend to be under higher stress than normal faults we infer that the b value acts as a stress meter that depends inversely on differential stress.     

Some thoughts on seismotectonics of major earthquake occurrence zones in China

A major earthquake occurrence zone means a place where M≥6 events have occurred since the Holocene and similar shocks may happen again in the future. The dynamic context of the major earthquake occurrence zones in China is primarily associated with the NNE-directed push of the India plate,next with the westward subduction of the Pacific plate. The Chinese mainland is a grand mosaic structure of many crust blocks bounded by faults and sutures. When it is suffered from boundary stresses,deformation takes place along these faults or sutures while the block interiors remain relatively stable or intact. Since the Quaternary,for example,left slip on the Xianshuihe-Xiaojiang fault zone in southwestern China has produced a number of fault-depression basins in extensional areas during periods Q1 and Q2. In the Q3,the change of stress orientation and enhancement of tectonic movement made faults of varied trends link each other,and continued to be active till present day,producing active fault zones in this region. Usually major earthquakes occur at some special locations on these active fault zones. During these events,in the epicenter areas experience intensive deformation characterized by large-amplitude rise and fall of neighboring sections,generation of horst-graben systems and dammed rivers. The studies on palaeoearthquakes suggest that major shocks of close magnitudes often repeated for several times at a same place. By comparison of the Chi-Chi,Taiwan event in 1999 and Yuza,Yunnan event in 1955,including contours of accelerations and intensities,destruction of buildings,and in contrast to the Xigeda formation in southwestern China,a sandwich model is established to account for the mechanism of deformation caused by major earthquakes. This model consists of three layers,i. e. the two walls of a fault and the ruptured zone intercalated between them. This ruptured zone is just the loci where stress is built up and released,and serves as a channel for seismic waves.     

Intermediate-depth earthquake faulting by dehydration embrittlement with negative volume change

Earthquakes are observed to occur in subduction zones to depths of approximately 680 km, even though unassisted brittle failure is inhibited at depths greater than about 50 km, owing to the high pressures and temperatures. It is thought that such earthquakes (particularly those at intermediate depths of 50-300 km) may instead be triggered by embrittlement accompanying dehydration of hydrous minerals, principally serpentine. A problem with failure by serpentine dehydration is that the volume change accompanying dehydration becomes negative at pressures of 2-4 GPa (60-120 km depth), above which brittle fracture mechanics predicts that the instability should be quenched. Here we show that dehydration of antigorite serpentinite under stress results in faults delineated by ultrafine-grained solid reaction products formed during dehydration. This phenomenon was observed under all conditions tested (pressures of 1-6 GPa; temperatures of 650-820 degrees C), independent of the sign of the volume change of reaction. Although this result contradicts expectations from fracture mechanics, it can be explained by separation of fluid from solid residue before and during faulting, a hypothesis supported by our observations. These observations confirm that dehydration embrittlement is a viable mechanism for nucleating earthquakes independent of depth, as long as there are hydrous minerals breaking down under a differential stress.     

Reflection signature of seismic and aseismic slip on the northern Cascadia subduction interface

At the northern Cascadia margin, the Juan de Fuca plate is underthrusting North America at about 45 mm x yr(-1) (ref. 1), resulting in the potential for destructive great earthquakes. The downdip extent of coupling between the two plates is difficult to determine because the most recent such earthquake (thought to have been in 1700) occurred before instrumental recording. Thermal and deformation studies indicate that, off southern Vancouver Island, the interplate interface is presently fully locked for a distance of approximately 60 km downdip from the deformation front. Great thrust earthquakes on this section of the interface (with magnitudes of up to 9) have been estimated to occur at an average interval of about 590 yr (ref. 3). Further downdip there is a transition from fully locked behaviour to aseismic sliding (where high temperatures allow ductile deformation), with the deep aseismic zone exhibiting slow-slip thrust events. Here we show that there is a change in the reflection character on seismic images from a thin sharp reflection where the subduction thrust is inferred to be locked, to a broad reflection band at greater depth where aseismic slip is thought to be occurring. This change in reflection character may provide a new technique to map the landward extent of rupture in great earthquakes and improve the characterization of seismic hazards in subduction zones.