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1.
The ground motion hazard for Sumatra and the Malaysian peninsula is calculated in a probabilistic framework, using procedures developed for the US National Seismic Hazard Maps. We constructed regional earthquake source models and used standard published and modified attenuation equations to calculate peak ground acceleration at 2% and 10% probability of exceedance in 50 years for rock site conditions. We developed or modified earthquake catalogs and declustered these catalogs to include only independent earthquakes. The resulting catalogs were used to define four source zones that characterize earthquakes in four tectonic environments: subduction zone interface earthquakes, subduction zone deep intraslab earthquakes, strike-slip transform earthquakes, and intraplate earthquakes. The recurrence rates and sizes of historical earthquakes on known faults and across zones were also determined from this modified catalog. In addition to the source zones, our seismic source model considers two major faults that are known historically to generate large earthquakes: the Sumatran subduction zone and the Sumatran transform fault. Several published studies were used to describe earthquakes along these faults during historical and pre-historical time, as well as to identify segmentation models of faults. Peak horizontal ground accelerations were calculated using ground motion prediction relations that were developed from seismic data obtained from the crustal interplate environment, crustal intraplate environment, along the subduction zone interface, and from deep intraslab earthquakes. Most of these relations, however, have not been developed for large distances that are needed for calculating the hazard across the Malaysian peninsula, and none were developed for earthquake ground motions generated in an interplate tectonic environment that are propagated into an intraplate tectonic environment. For the interplate and intraplate crustal earthquakes, we have applied ground-motion prediction relations that are consistent with California (interplate) and India (intraplate) strong motion data that we collected for distances beyond 200 km. For the subduction zone equations, we recognized that the published relationships at large distances were not consistent with global earthquake data that we collected and modified the relations to be compatible with the global subduction zone ground motions. In this analysis, we have used alternative source and attenuation models and weighted them to account for our uncertainty in which model is most appropriate for Sumatra or for the Malaysian peninsula. The resulting peak horizontal ground accelerations for 2% probability of exceedance in 50 years range from over 100% g to about 10% g across Sumatra and generally less than 20% g across most of the Malaysian peninsula. The ground motions at 10% probability of exceedance in 50 years are typically about 60% of the ground motions derived for a hazard level at 2% probability of exceedance in 50 years. The largest contributors to hazard are from the Sumatran faults.  相似文献   

2.
In this article, we review the significant recent results of geophysical studies and discuss their implications on seismotectonics, magmatism, and mantle dynamics in East Asia. High-resolution geophysical imaging revealed structural heterogeneities in the source areas of large crustal earthquakes, which may reflect magma and fluids that affected the rupture nucleation of large earthquakes. In subduction zone regions, the crustal fluids originate from the dehydration of the subducting slab. Magmatism in arc and back-arc areas is caused by the corner flow in the mantle wedge and dehydration of the subducting slab. The intraplate magmatism has different origins. The continental volcanoes in Northeast Asia (such as Changbai and Wudalianchi) seem to be caused by the corner flow in the big mantle wedge (BMW) above the stagnant slab in the mantle transition zone and the deep dehydration of the stagnant slab as well. The Tengchong volcano in Southwest China is possibly caused by a similar process in BMW above the subducting Burma microplate (or Indian plate). The Hainan volcano in southernmost China seems to be a hotspot fed by a lower-mantle plume associated with the Pacific and Philippine Sea slabs’ deep subduction in the east and the Indian slab’s deep subduction in the west down to the lower mantle. The occurrence of deep earthquakes under the Japan Sea and the East Asia margin may be related to a metastable olivine wedge in the subducting Pacific slab. The stagnant slab finally collapses down to the bottom of the mantle, which may trigger upwelling of hot mantle materials from the lower mantle to the shallow mantle beneath the subducting slabs and cause the slab–plume interactions. Some of these issues, such as the origin of intraplate magmatism, are still controversial, and so further detailed studies are needed from now.  相似文献   

3.
蛇纹石脱水与大洋俯冲带中源地震(70~300km)的关系   总被引：4，自引：2，他引：4  
余日东  金振民 《地学前缘》2006,13(2):191-204
蛇纹石脱水致裂作用是诱发大洋俯冲带中源地震(70~300km)的一种重要成因机制,它与中等深度双地震带的形成有很密切的关系。双地震带在冷俯冲带中是一种常见现象,它由上下相距20~40km的两个平行地震层组成。上地震层位于俯冲洋壳中,可能是洋壳蓝片岩脱水形成榴辉岩的系列脱水反应诱发了地震;下地震层位于大洋俯冲地幔中,可能是部分交代的地幔橄榄岩脱水控制着中源地震的分布。蛇纹岩在高温高压条件下的变形实验证实蛇纹石在脱水过程中引起岩石弱化和脆性破裂,这已经得到了对蛇纹石脱水过程中岩石物理性质和变形后样品的显微构造等理论研究上的支持。在蛇纹石脱水过程中,产生的流体与固体残留物分离,形成了大量的I型(张性)微裂隙,最终导致岩石破裂和形成断层。根据叶蛇纹石脱水反应相图,理论上在大洋俯冲带中蛇纹石脱水位置会出现双层结构,但只有平行于俯冲板块顶层等温线的一支才可能脱水诱发地震,并对应于双地震带的下地震层。下地震层所处的位置具有低的vp/vs值,暗示岩石圈大洋地幔顶层发生了部分交代。但它的交代机制尚不清楚,可能是海水通过洋底转换断层和/或沿着在外海沟隆起中形成的断层渗入大洋地幔顶层,并发生了洋壳和大洋地幔交代。双地震带在120~200km深度合一以后,冷俯冲带中所发生的中源地震可能与蛇纹石脱水有关,在热俯冲带中更可能与“湿”榴辉岩脱水有关。  相似文献   

4.
Akio Yoshida 《Tectonophysics》1987,140(2-4):131-143
Seismic activity in the region surrounding the foci is investigated for three severe earthquakes (two with a magnitude of 6.1 and one with a magnitude of 5.3) which have occurred in Japan in recent years. The most conspicuous feature commonly noticed is precursory activation of seismic belts which include the focal regions of main shocks. The repetition of the same pattern in the space-time distribution of earthquake occurrence along the seismic belt is also observed for each case. The precursory activity of seismic belts terminates in rather a short period and, after that, the area around the focus of the forthcoming large earthquake becomes quiescent, which demonstrates the appearance of the seismic gap of the second kind (Mogi, 1979). The periods of seismic quiescence for the cases investigated in this paper are longer than those which are given by the regression relationship between earthquake magnitude and precursor time proposed for example, by Sekiya (1977). However, our definition of anomalous seismic activity is clear, and it is possible to give a physical meaning to it as an increase in the local stress field in the seismic belt. We propose that a kind of coupling between intraplate tectonic blocks, analogous to interplate coupling in the subduction region, is responsible for the formation of the stress field relevant to these earthquakes. Although this is at present only one of the possible viewpoints on the formation of the focal region of large intraplate earthquakes, it may be worthwhile to study various precursory phenomena in-connection with this hypothesis.  相似文献   

5.
Transition from subduction of normal to thickened oceanic crust occurs in the central portion of the Costa Rican margin, where large interplate earthquakes (M ~ 7) and abundant interseismic seismicity have been associated with subduction of bathymetric highs. We relocated ~1,300 earthquakes recorded for 6 months by a combined on- and offshore seismological network using probabilistic earthquake relocation in a 3D P-wave velocity model. Most of the seismicity originated at the seismogenic zone of the plate boundary, appearing as an 18° dipping, planar cluster from 15 to 25–30 km depth, beneath the continental shelf. Several reverse focal mechanisms were resolved within the cluster. The upper limit of this interseismic interplate seismicity seems to be controlled primarily by the overlying-plate thickness and coherency, which in turn is governed by the erosional processes and fluid release and escape at temperatures lower than ~100 to 120 °C along the plate boundary. The downdip limit of the stick–slip behaviour collocates with relative low temperatures of ~150 to 200 °C, suggesting that it is controlled by serpentinization of the mantle wedge. The distribution of the interseismic interplate seismicity is locally modified by the presence of subducted seamounts at different depths. Unlike in northern Costa Rica, rupture of large earthquakes in the last two decades seems to coincide with the area defined by the interseismic interplate seismicity.  相似文献   

6.
青藏高原板内地震震源深度分布规律及其成因   总被引：6，自引：0，他引：6  
青藏高原板内地震以浅源地震为主, 下地壳基本上没有地震, 地震震源多集中在15~40 km的深度范围, 主要在中地壳内, 呈似层状弥散分布.其中30~33 km深度是一个优势层, 与壳内分层有关.总体上青藏高原南、北部的震源面略呈相向倾斜特征.70~100 km深度区间出现了比较集中的震级较小的地震, 可能与壳幔过渡带的拆离作用有关.高原内部的正断层系与板内地震密切相关, 是板内浅源地震的主控构造.总之, 青藏高原地震震源沿着活动的上地壳脆性层与软弱层之间的脆-韧性过渡带分布.这些板内地震活动属于大陆动力学过程, 与板块碰撞和板块俯冲无关.初步认为青藏高原浅层到深层多震层的成因分别是韧性基底与脆性盖层、韧性下地壳与脆性上地壳、韧性下地壳与脆性上地幔的韧-脆性转换、拆离和解耦的产物.   相似文献   

7.
长白山火山的起源和太平洋俯冲板块之间的关系   总被引：6，自引：0，他引：6  
近年来，尽管不同学科通过不同手段对长白山火山进行过广泛研究，然而，目前人们对它的起源仍不清楚。利用全球地震层析成像和区域层析成像结果，综合分析了长白山火山的起源。结果表明，它的起源既不同于夏威夷等板内热点火山，也不同于日本等岛弧火山，而是一种与太平洋俯冲板块在地幔转换带内的滞留和深部脱水等过程密切相 关的弧后板内火山。  相似文献   

8.
Jianshe Lei  Dapeng Zhao 《Tectonophysics》2005,397(3-4):281-295
We present the first seismic image of the upper mantle beneath the active intraplate Changbai volcano in Northeast Asia determined by teleseismic travel time tomography. The data are measured at a new seismic network consisting of 19 portable stations and 3 permanent stations. Our results show a columnar low-velocity anomaly extending to 400-km depth with a P-wave velocity reduction of up to 3%. High velocity anomalies are visible in the mantle transition zone, and deep-focus earthquakes occur at depths of 500–600 km under the region, suggesting that the subducting Pacific slab is stagnant in the transition zone, as imaged clearly by global tomography. These results suggest that the intraplate Changbai volcano is not a hotspot like Hawaii but a kind of back-arc volcano related to the deep subduction and stagnancy of the Pacific slab under Northeast Asia.  相似文献   

9.
Three-dimensional P-wave velocity structure beneath the Changbai and other intraplate volcanic areas in Northeast Asia is determined by inverting 1378 high-quality P-wave arrival times from 186 teleseismic events recorded by 61 broadband seismic stations. Low-velocity (low-V) anomalies are revealed beneath the Changbai, Longgan, Xianjindao volcanoes. High-velocity (high-V) anomalies are found in the mantle transition zone, where deep-focus earthquakes under Hunchun occur at depths of 500–600 km. The high-V anomaly reflects the deep subduction of the Pacific slab under NE Asia which may have contributed to the formation of the Changbai, Longgang, Xianjindao and Jingpohu intraplate volcanoes. A low-V anomaly is also revealed in the mantle transition zone, which may have a close relationship with the occurrence of deep earthquakes under the Hunchun area. Our results support the Big Mantle Wedge (BMW) model by Zhao et al. [Zhao, D., Lei, J., Tang, Y., 2004. Origin of the Changbai volcano in northeast China: evidence from seismic tomography, Chin. Sci. Bull. 49, 1401–1408; Zhao, D., Maruyama, S., Omori, S., 2007. Mantle dynamics of western Pacific and East Asia: insight from seismic tomography and mineral physics. Gondwana Res. 11, 120–131.] who proposed that the intraplate volcanoes in NE Asia are caused by the back-arc magmatism associated with the deep dehydration process of the subducting slab and convective circulation process in the BMW above the stagnant Pacific slab.  相似文献   

10.
李涛  王宗秀 《地学前缘》2005,12(3):125-136
与洋陆俯冲关系不同,在板内汇聚过程中,大陆岩石圈固有的多圈层、多界面结构的特点,使得地块的俯冲变形伴有多圈层顺层拆离解耦的行为,使变形结构复杂化。虽然多圈层界面拆离解耦所引发的地震点群空间分布不像洋陆俯冲关系那么规则完美,但是依据地震群与破裂位置、破裂与岩石圈分层力学特性的依次控制关系,运用深度/频次、平面密度等统计方法,再以各种地球物理实测手段得到的岩石圈结构构造数据作为界面标定依据,还是能够得出诸如拆离解耦的界面深度、界面归属和区域层间变形范围等重要的几何学信息,这些变形几何学、运动学数据是构建大陆岩石圈板内汇聚造山特别是盆山耦合模式时的关键性的依据。文中通过对塔里木盆地及周缘造山带的相关研究,在岩石圈层拆离解耦状态及其与盆山构造格局之间的关系方面得出以下几点认识:(1)塔里木盆地及周缘造山带岩石圈的主拆离解耦层均发育于中地壳,但随各区中地壳的具体深度位置不同而有所差别;(2)塔西南/西昆仑盆山构造耦合关系是构建于岩石圈尺度上的,塔北/南天山盆山耦合关系是构建于地壳尺度上的;(3)地震活动的密集程度及密集带的展布与天山的变形强度、隆升状态和地貌阶段类型的变化规律有着近乎完美的精确匹配关系;(4)塔北/南天山和塔西南/西昆仑对应于岩石圈的强拆离解耦区,塔东北/东天山和塔东南/阿尔金山之间无耦合关系,其边缘带对应于岩石圈弱拆离解耦和无拆离解耦区;(5)塔里木盆地总体上的弱变形状态与其岩石圈弱或未拆离解耦类型占据总面积90%的情形相适应;(6)塔里木地块以驱动、阻挡约束、平移滚筒约束和克拉通过渡等多重“身份”存在于相邻单元“包围”的力学环境中。  相似文献   

11.
Although subduction zones around the world are known to be the source of earthquakes and/or tsunamis, not all segments of these plate boundaries generate destructive earthquakes and catastrophic tsunamis. Costa Rica, in Central America, has subduction zones on both the Pacific and the Caribbean coasts and, even though large earthquakes (Mw = 7.4–7.8) occur in these convergent margins, they do not produce destructive tsunamis. The reason for this is that the seismogenic zones of the segments of the subduction zones that produce large earthquakes in Costa Rica are located beneath land (Nicoya peninsula, Osa peninsula and south of Limón) and not off shore as in most subduction zones around the world. To illustrate this particularity of Costa Rican subduction zones, we show in this work the case for the largest rupture area in Costa Rica (under the Nicoya peninsula), capable of producing Mw ～ 7.8 earthquakes, but the tsunamis it triggers are small and present little potential for damage even to the largest port city in Costa Rica.The Nicoya seismic gap, in NW Costa Rica, has passed its ～50-year interseismic period and therefore a large earthquake will have to occur there in the near future. The last large earthquake, in 1950 generated a tsunami which slightly affected the southwest coast of the Nicoya Peninsula. We present here a simulation to study the possible consequences that a tsunami generated by the next Nicoya earthquake could have for the city of Puntarenas. Puntarenas has a population of approximately eleven thousand people and is located on a 7.5 km long sand bar with a maximum height of 2 m above the mean sea level. This condition makes Puntarenas vulnerable to tsunamis.  相似文献   

12.
西藏多龙矿集区是班公湖-怒江缝合带北缘在增生楔基础上发育的超大型岛弧斑岩铜金矿床。对多龙增生杂岩中的辉长岩进行了LA-ICP-MS锆石U-Pb定年及主量、微量元素分析和Sr-Nd同位素组成研究,获得了多龙增生杂岩中辉长岩的成岩时代。由LA-ICP-MS测得的辉长岩锆石~(206)Pb/~(238)U年龄为246.0±1.4Ma,代表了岩浆的结晶年龄。辉长岩以低钛、富钠、富镁、贫钾、富集轻稀土元素和大离子亲石元素(Ba、U、La、Sr)、亏损高场强元素Nb和Ta为特征,具有与岛弧玄武岩相似的主量和微量元素组成,相对于原始地幔具高Sr、低Nd的同位素组成和古老的Nd同位素二阶段模式年龄(T_(DM2)=0.54~0.99Ga),属于增生楔基础上发育的板内岛弧辉长岩。由此认为,多龙矿集区早三叠世末岩浆活动很可能是班公湖-怒江洋向北俯冲诱发洋壳物质与岩石圈地幔物质相互作用的结果。该发现和认识为班公湖-怒江洋早三叠世向北俯冲,提供了重要的岩浆作用证据。  相似文献   

13.
The Andaman-Sumatra subduction zone is seismically one of the most active and complex subduction zones that produced the 26 December 2004 mega thrust earthquake (Mw 9.3) and large number of aftershocks. About 8,000 earthquakes, including more than 3,000 aftershocks (M ≥ 4.5) of the 2004 earthquake, recorded during the period 1964–2007, are relocated by the EHB method. We have analysed this large data set to map fractal correlation dimension (Dc) and frequency-magnitude relation (b-value) characteristics of the seismogenic structures of this ~3,000-km-long mega thrust subduction zone in south-east Asia. The maps revealed the seismic characteristics of the Andaman-Sumatra-Java trenches, West Andaman fault (WAF), Andaman Sea Ridge (ASR), Sumatra and Java fault systems. Prominent N–S to NW–SE to E–W trending fractal dimension contours all along the subduction zone with Dc between 0.6 and 1.4 indicate that the epicentres mostly follow linear features of the major seismogenic structures. Within these major contours, several pockets of close contours with Dc ~ 0.2 to 0.6 are identified as zones of epicentre clusters and are inferred to the fault intersections as well as asperity zones along the fault systems in the fore arc. A spatial variation in the b-value (1.2–1.5) is also observed along the subduction zone with several pockets of lower b-values (1.2–1.3). The smaller b-value zones are corroborated with lower Dc (0.5–0.9), implying a positive correlation. These zones are identified to be the zones of more stress or asperity where rupture nucleation of intermediate to strong magnitude earthquakes occurred.  相似文献   

14.
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15.
Seismic coupling and uncoupling at subduction zones   总被引：1，自引：0，他引：1  
Seismic coupling has been used as a qualitative measure of the “interaction” between the two plates at subduction zones. Kanamori (1971) introduced seismic coupling after noting that the characteristic size of earthquakes varies systematically for the northern Pacific subduction zones. A quantitative global comparison of many subduction zones reveals a strong correlation of earthquake size with two other variables: age of the subducting lithosphere and convergence rate. The largest earthquakes occur in zones with young lithosphere and fast convergence rates, while zones with old lithosphere and slow rates are relatively aseismic for large earthquakes. Results from a study of the rupture process of three great earthquakes indicate that maximum earthquake size is directly related to the asperity distribution on the fault plane (asperities are strong regions that resist the motion between the two plates). The zones with the largest earthquakes have very large asperities, while the zones with smaller earthquakes have small scattered asperities. This observation can be translated into a simple model of seismic coupling, where the horizontal compressive stress between the two plates is proportional to the ratio of the summed asperity area to the total area of the contact surface. While the variation in asperity size is used to establish a connection between earthquake size and tectonic stress, it also implies that plate age and rate affect the asperity distribution. Plate age and rate can control asperity distribution directly by use of the horizontal compressive stress associated with the “preferred trajectory” (i.e. the vertical and horizontal velocities of subducting slabs are determined by the plate age and convergence velocity). Indirect influences are many, including oceanic plate topography and the amount of subducted sediments.All subduction zones are apparently uncoupled below a depth of about 40 km, and we propose that the basalt to eclogite phase change in the down-going oceanic crust may be largely responsible. This phase change should start at a depth of 30–35 km, and could at least partially uncouple the plates by superplastic deformation throughout the oceanic crust during the phase change.  相似文献   

16.
Intraplate compressional features, such as inverted extensional basins, upthrust basement blocks and whole lithospheric folds, play an important role in the structural framework of many cratons. Although compressional intraplate deformation can occur in a number of dynamic settings, stresses related to collisional plate coupling appear to be responsible for the development of the most important compressional intraplate structures. These can occur at distances of up to ±1600 km from a collision front, both in the fore-arc (foreland) and back-arc (hinterland) positions with respect to the subduction system controlling the evolution of the corresponding orogen. Back-arc compression associated with island arcs and Andean-type orogens occurs during periods of increased convergence rates between the subducting and overriding plates. For the build-up of intraplate compressional stresses in fore-arc and foreland domains, four collision-related scenarios are envisaged: (1) during the initiation of a subduction zone along a passive margin or within an oceanic basin; (2) during subduction impediment caused by the arrival of more buoyant crust, such as an oceanic plateau or a microcontinent at a subduction zone; (3) during the initial collision of an orogenic wedge with a passive margin, depending on the lithospheric and crustal configuration of the latter, the presence or absence of a thick passive margin sedimentary prism, and convergence rates and directions; (4) during post-collisional over-thickening and uplift of an orogenic wedge. The build-up of collision-related compressional intraplate stresses is indicative for mechanical coupling between an orogenic wedge and its fore- and/or hinterland. Crustal-scale intraplate deformation reflects mechanical coupling at crustal levels whereas lithosphere-scale deformation indicates mechanical coupling at the level of the mantle-lithosphere, probably in response to collisional lithospheric over-thickening of the orogen, slab detachment and the development of a mantle back-stop. The intensity of collisional coupling between an orogen and its fore- and hinterland is temporally and spatially variable. This can be a function of oblique collision. However, the build-up of high pore fluid pressures in subducted sediments may also account for mechanical decoupling of an orogen and its fore- and/or hinterland. Processes governing mechanical coupling/decoupling of orogens and fore- and hinterlands are still poorly understood and require further research. Localization of collision-related compressional intraplate deformations is controlled by spatial and temporal strength variations of the lithosphere in which the thermal regime, the crustal thickness, the pattern of pre-existing crustal and mantle discontinuities, as well as sedimentary loads and their thermal blanketing effect play an important role. The stratigraphic record of collision-related intraplate compressional deformation can contribute to dating of orogenic activity affecting the respective plate margin.  相似文献   

17.
We test the sensitivity of seismic hazard to three fault source models for the northwestern portion of Gujarat, India. The models incorporate different characteristic earthquake magnitudes on three faults with individual recurrence intervals of either 800 or 1600 years. These recurrence intervals imply that large earthquakes occur on one of these faults every 266–533 years, similar to the rate of historic large earthquakes in this region during the past two centuries and for earthquakes in intraplate environments like the New Madrid region in the central United States. If one assumes a recurrence interval of 800 years for large earthquakes on each of three local faults, the peak ground accelerations (PGA; horizontal) and 1-Hz spectral acceleration ground motions (5% damping) are greater than 1 g over a broad region for a 2% probability of exceedance in 50 years' hazard level. These probabilistic PGAs at this hazard level are similar to median deterministic ground motions. The PGAs for 10% in 50 years' hazard level are considerably lower, generally ranging between 0.2 g and 0.7 g across northwestern Gujarat. Ground motions calculated from our models that consider fault interevent times of 800 years are considerably higher than other published models even though they imply similar recurrence intervals. These higher ground motions are mainly caused by the application of intraplate attenuation relations, which account for less severe attenuation of seismic waves when compared to the crustal interplate relations used in these previous studies. For sites in Bhuj and Ahmedabad, magnitude (M) 7 3/4 earthquakes contribute most to the PGA and the 0.2- and 1-s spectral acceleration ground motion maps at the two considered hazard levels.  相似文献   

18.
The Japan Trench subduction zone, located east of NE Japan, has regional variation in seismicity. Many large earthquakes occurred in the northern part of Japan Trench, but few in the southern part. Off Miyagi region is in the middle of the Japan Trench, where the large earthquakes (M > 7) with thrust mechanisms have occurred at an interval of about 40 years in two parts: inner trench slope and near land. A seismic experiment using 36 ocean bottom seismographs (OBS) and a 12,000 cu. in. airgun array was conducted to determine a detailed, 2D velocity structure in the forearc region off Miyagi. The depth to the Moho is 21 km, at 115 km from the trench axis, and becomes progressively deeper landward. The P-wave velocity of the mantle wedge is 7.9–8.1 km/s, which is typical velocity for uppermost mantle without large serpentinization. The dip angle of oceanic crust is increased from 5–6° near the trench axis to 23° 150 km landward from the trench axis. The P-wave velocity of the oceanic uppermost mantle is as small as 7.7 km/s. This low-velocity oceanic mantle seems to be caused by not a lateral anisotropy but some subduction process. By comparison with the seismicity off Miyagi, the subduction zone can be divided into four parts: 1) Seaward of the trench axis, the seismicity is low and normal fault-type earthquakes occur associated with the destruction of oceanic lithosphere. 2) Beneath the deformed zone landward of the trench axis, the plate boundary is characterized as a stable sliding fault plain. In case of earthquakes, this zone may be tsunamigenic. 3) Below forearc crust where P-wave velocity is almost 6 km/s and larger: this zone is the seismogenic zone below inner trench slope, which is a plate boundary between the forearc and oceanic crusts. 4) Below mantle wedge: the rupture zones of thrust large earthquakes near land (e.g. 1978 off Miyagi earthquake) are located beneath the mantle wedge. The depth of the rupture zones is 30–50 km below sea level. From the comparison, the rupture zones of large earthquakes off Miyagi are limited in two parts: plate boundary between the forearc and oceanic crusts and below mantle wedge. This limitation is a rare case for subduction zone. Although the seismogenic process beneath the mantle wedge is not fully clarified, our observation suggests the two possibilities: earthquake generation at the plate boundary overridden by the mantle wedge without serpentinization or that in the subducting slab.  相似文献   

19.
板块俯冲起始与大陆地壳演化   总被引：1，自引：0，他引：1  
组成大陆地壳的物质主要来自两个地质过程:地幔柱活动和板块俯冲。目前大多数研究认为板块俯冲起始于30多亿年前。在板块俯冲起始之前,基性的初始地壳物质受热重熔是大陆地壳生长的主要方式,其中,地幔柱活动是关键。地幔柱不仅向地壳输送玄武质岩浆,同时导致已有玄武质岩石和沉积岩通过部分熔融向中酸性岩石转化。当原始岩石圈强度足够大时,地幔柱会导致岩石圈倾斜、破裂,产生下滑力,诱发板块俯冲。板块俯冲引发岩浆活动,产生大量的岩浆岩,如岛弧安山岩、弧后盆玄武岩等。这些岩浆岩通过喷发、侵位,再经由块体拼贴、增生等过程加入到大陆地壳,是大陆地壳生长的主要途径。同时,板内岩浆活动乃至地幔柱活动等也与板块俯冲有直接或者间接的联系。俯冲再循环物质促进地幔柱发育,也为大陆地壳的生长提供物源和热能。与此同时,大陆地壳不断风化剥蚀,其中一部分沉积物随俯冲板块再循环到地幔,而板块俯冲过程也通过俯冲剥蚀等过程,将仰冲盘岩石圈物质刮削带入地幔。这些是大陆地壳消减的主要途径。目前大陆地壳增生和消减基本处于动态平衡。  相似文献   

20.
北祁连山和柴北缘是典型的早古生代大陆造山带,分别发育有北祁连山大洋型俯冲缝合带和柴北缘大陆型俯冲碰撞带.作为早古生代大洋冷俯冲的典型代表,北祁连山经历了从新元古代-寒武纪大洋扩张、奥陶纪俯冲和闭合及早泥盆世隆升造山的过程.高压变质岩变质年龄为490～440Ma,证明古祁连洋经历了至少50m．y．的俯冲过程.柴北缘超高压变质带是大陆深俯冲的结果,岩石学、地球化学和同位素年代学表明,柴北缘超高压变质带中榴辉岩的原岩分别来自洋壳和陆壳两种环境.高压/超高压变质的蛇绿岩原岩的年龄为517±11Ma,与祁连山蛇绿岩年龄一致.榴辉岩早期的变质年龄为443～473Ma,与祁连山高压变质年龄一致,代表大洋地壳俯冲的时代,而柯石英片麻岩和石榴橄榄岩所限定的超高压变质时代为420～426Ma,代表大陆俯冲的年龄.从大洋俯冲结束到大陆俯冲最大深度的转换时间最少需要20m．y．.自420Ma起,俯冲的大洋岩石圈与跟随俯冲的大陆岩石圈断离,大陆地壳开始折返,发生隆升和造山.北祁连山和柴北缘两个不同类型的高压-超高压变质带反映了早古生代从大洋俯冲到大陆俯冲、隆升折返的造山过程.  相似文献   

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