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1.
Upper mantle anisotropy beneath central and southwest Japan: An insight into subduction-induced mantle flow 总被引：1，自引：0，他引：1
Analysis of seismic anisotropy in the crust and mantle wedge above subduction zones gives much information about the dynamic processes inside the Earth. For this reason, we measure shear wave polarization anisotropy in the crust and upper mantle beneath central and southwestern Japan from local shallow, intermediate, and deep earthquakes occurring in the subducting Pacific slab. We analyze S phases from 198 earthquakes recorded at 42 Japanese F-net broadband seismic stations. This data set yields a total of 980 splitting parameter pairs for central and southwestern Japan. Dominant fast polarization directions of shear waves obtained at most stations in the Kanto–Izu–Tokai areas are oriented WNW–ESE, which are sub-parallel to the subduction direction of the Pacific plate. However, minor fast polarization directions are oriented in NNE–SSW directions being parallel to the strike of the Japan Trench, especially in the north of Izu Peninsula and the northern Tokai district. Generally, fast directions obtained at stations located in Kii Peninsula and the Chubu district are oriented ENE–WSW, almost parallel to the Nankai Trough, although some fast directions have NW–SE trends. The fast directions obtained at stations in northern central Honshu are oriented N–S. Delay times vary considerably and range from 0.1 to 1.25 s depending on the source depth and the degree of anisotropy along the ray path. These lateral variations in splitting character suggest that the nature of anisotropy is quite different between the studied areas. Beneath Kanto–Tokai, the observed WNW–ESE fast directions are probably caused by the olivine A-fabric induced by the corner flow. However, the slab morphology in this region is relatively complicated as the Philippine Sea slab is overriding the Pacific slab. This complex tectonic setting may induce lateral heterogeneity in the flow and stress state of the mantle wedge, and may have produced NNE–SSW orientations of fast directions. The ENE–WSW fast directions in Kii Peninsula and the Chubu district are more coherent and may be partly induced by the subduction of the Philippine Sea plate. The N–S fast directions in northern central Honshu might be produced by the trench-parallel stretching of the wedge due to the curved slab at the arc–arc junction. 相似文献
2.
对华北地震科学台阵的200个宽频带和甚宽带地震台站所记录的远震SKS(SKKS)波形资料作偏振分析,采用最小切向能量的网格搜索法和叠加分析方法求得每一个台站的SKS(SKKS)快波偏振方向和快、慢波的时间延迟,并结合已发表的固定台站的结果,获得了华北上地幔各向异性图像.从得到结果看,华北东部各向异性快波方向基本为NWW-SEE方向,而西部的快波方向转到NW-SE或NNW-SSE.快、慢波时间延迟范围是0.50~1.47 s,华北西部的平均快、慢波时间延迟小于华北东部.在华北东部,快波方向与绝对板块运动(APM)方向基本一致,预示了NWW向的软流圈地幔流是引起该区域上地幔各向异性的主要原因,它使得上地幔橄榄岩等晶体的晶格优势取向沿地幔物质流动方向,从而导致了NWW趋向的快波方向.然而,在稳定的西部,快波方向既不与绝对板块运动方向一致,也不与构造走向一致,这种弱各向异性很可能是遗留在古老克拉通的厚的岩石圈内的"化石"各向异性. 相似文献
3.
Vanessa Andrews Joann Stock Carlos Ariel Ram��rez V��zquez Gabriel Reyes-D��vila 《Pure and Applied Geophysics》2011,168(8-9):1331-1338
On 22 January 2003, the M w?=?7.6 Tecomán earthquake struck offshore of the state of Colima, Mexico, near the diffuse triple junction between the Cocos, Rivera, and North American plates. Three-hundred and fifty aftershocks of the Tecomán earthquake with magnitudes between 2.6 and 5.8, each recorded by at least 7 stations, are relocated using the double difference method. Initial locations are determined using P and S readings from the Red Sismológica Telemétrica del Estado de Colima (RESCO) and a 1-D velocity model. Because only eight RESCO stations were operating immediately following the Tecomán earthquake, uncertainties in the initial locations and depths are fairly large, with average uncertainties of 8.0?km in depth and 1.4?km in the north?Csouth and east?Cwest directions. Events occurring between 24 January and 31 January were located using not only RESCO phase readings but also additional P and S readings from 11 temporary stations. Average uncertainties decrease to 0.8?km in depth, 0.3?km in the east?Cwest direction, and 0.7?km in the north?Csouth direction for events occurring while the temporary stations were deployed. While some preliminary studies of the early aftershocks suggested that they were dominated by shallow events above the plate interface, our results place the majority of aftershocks along the plate interface, for a slab dipping between approximately 20° and 30°. This is consistent with the slab positions inferred from geodetic studies. We do see some upper plate aftershocks that may correspond to forearc fault zones, and faults inland in the upper plate, particularly among events occurring more than 3?months after the mainshock. 相似文献
4.
利用收集到的各种来源共计7 959组的地震各向异性观测数据和21 750组应力场数据,结合板块绝对运动模型计算给出的各板块的运动规律,分别统计分析了板块运动与地震各向异性及应力场的相关性,并对板块运动对地震各向异性及应力场特征产生的影响进行了分析． 统计结果表明,阿拉伯、 加勒比、 胡安德富卡、 北美、 纳兹卡、 太平洋和南美板块上地震各向异性与板块运动均具有较好的相关性,而非洲、 南极洲、 澳大利亚、 欧亚、 印度和菲律宾板块上二者的相关性则相对较差. 讨论分析发现,板块运动拖动软流圈流动、 橄榄岩晶格优选方位、 化石各向异性和地幔流动或岩石圈流动等因素均在一定程度上控制并影响着地震各向异性与板块运动的一致性． 而板块基底拖曳力、 洋脊推力、 浮力作用和碰撞及俯冲作用等多种因素共同制约了板块运动与应力场的相关性,使得非洲、 可可斯、 欧亚、 胡安德富卡、 北美、 纳兹卡、 菲律宾和南美板块上二者的相关性较好,其它板块上其相关性则较差． 对于俯冲带地区,由于俯冲机制的复杂性和软流圈、 岩石圈地幔流动方向的不确定性,其板块运动与地震各向异性及应力场的相关性图像表现复杂,需要结合具体的俯冲带构造进行近一步研究． 相似文献
5.
6.
We have investigated the application of the F-ratio test, a standard statistical technique, to the results of relative plate motion inversions. The method tests whether the improvement in fit of the model to the data resulting from the addition of another plate to the model is greater than that expected purely by chance. This approach appears to be useful in determining whether additional plate boundaries are justified. We confirm previous results favoring separate North American and South American plates with a boundary located between 30°N and the equator. Using Chase's global relative motion data, we show that in addition to separate West African and Somalian plates, separate West Indian and Australian plates, with a best-fitting boundary between 70°E and 90°E, can be resolved. These results are generally consistent with the observation that the Indian plate's internal deformation extends somewhat westward of the Ninetyeast Ridge. The relative motion pole is similar to Minster and Jordan's and predicts the NW-SE compression observed in earthquake mechanisms near the Ninetyeast Ridge. 相似文献
7.
中国东北地区广泛发育新生代板内火山,晚中生代以来岩石圈遭受过多期拉张作用.作为中国唯一的深震孕育区,中国东北地区受到太平洋板块的西向俯冲,使得其成为研究岩石圈变形、板块俯冲和板内火山成因及其相互作用关系的天然实验室.通过分析架设在中国东北地区北部的147个流动和固定台站的SKS波形数据,共计得到了377对各向异性参数和251个无效分裂结果.结果表明,中国东北地区东西两侧具有不同的各向异性分布:西部地区各向异性方向变化范围为N143-199°E,平均N169°E,与晚中生代岩石圈伸展方向一致;其各向异性延迟时间平均值约为0.8s,说明来自地幔的各向异性比较微弱,主要由残留在岩石圈中的古老变形所引起.同时,在松辽盆地和佳木斯地块部分区域,观测到延迟时间较小的各向异性(~0.4s),可能是由于岩石圈的拆沉和热地幔物质的上涌侵蚀了保留在岩石圈的古老形变所致.在研究区东部,NNW-SSE朝向的各向异性被观测到,并伴随较大的延迟时间(大于1.0s),可能与太平洋板块撕裂回撤而产生的地幔流动有关.此外,近W-E方向的各向异性只在佳木斯地块被观测到,而太平洋板块在地幔过渡带中的俯冲可能是其产生的主要成因. 相似文献
8.
Yu. V. Gabsatarov G. M. Steblov D. I. Frolov 《Izvestiya Physics of the Solid Earth》2013,49(3):411-415
The question concerning the integrity of major tectonic plates is still unclear for several regions covering the plate junction zones. The Northeast Asia is one such region, where there is no common concept of the configuration of plate boundaries. From the classical viewpoint, the dynamics of Northeast Asia is determined by the superposition of the relative rotations of the three major plates (Eurasian, North American and Pacific). According to the alternative viewpoint, the fragments that were split from these plates rotate independently in the form of microplates (Bering, Okhotsk, and Amur). The analysis of kinematics for the GPS stations located in eastern Chukotka, western Alaska, and on the Bering Sea islands suggests the existence of the Bering microplate rotating clockwise relative to the North American plate. 相似文献
9.
Jan Vondrák 《Studia Geophysica et Geodaetica》1994,38(3):256-265
Summary The latitude variations observed by optical astrometry at five International Latitude Service (ILS) stations since the beginning of the century are analyzed to estimate the secular motion of the pole and the correction of the proper motions of the observed stars. The geophysical model of tectonic plate motions NUVEL-1 is used to correct the observed latitude drifts, thus referring the derived secular polar motion to the "no net rotation" terrestrial reference system. It is shown that one of the ILS stations, Ukiah (near the U.S. west coast), very probably exhibits an anomalous drift which differs significantly from the motion of both the Pacific and North American plates. 相似文献
10.
对布设在南北构造带南段的中国地震科学探测台阵项目一期350个宽频带流动台站和中国地震台网90个宽频带固定台站记录的远震XKS(SKS、SKKS和PKS)波形资料作偏振分析,采用最小切向能量的网格搜索法和"叠加"分析方法求得每一个台站的XKS波的快波偏振方向和快、慢波的时间延迟,获得了南北构造带南段上地幔各向异性图像.结果显示研究区的各向异性具有明显的南北分区特征,北部的快波方向为近N-S方向,而南部主要表现为近E-W方向,且北部的平均时间延迟小于南部.分析表明,具有厚岩石圈的北部的各向异性主要由岩石圈变形引起,是一种垂直连贯变形模式;具有薄岩石圈的南部的各向异性主要由软流圈地幔流引起,缅甸和巽达板片的后撤/回转作用产生了指向西南的软流圈地幔流,在岩石圈底部和软流圈之间产生了一个水平差异运动,产生了一个与简单剪切一致的软流圈变形结构,从而产生了南部观测的各向异性. 相似文献
11.
中国大陆及邻区上地幔P波各向异性结构 总被引：1，自引：1，他引：0
利用分布在中国大陆及邻近地区的213个地震台站记录到的远震P波走时数据和弱各向异性条件下P波速度扰动调和分析方法,研究了中国大陆上地幔P波各向异性结构.研究结果表明中国大陆西部上地幔变形主要受印度大陆俯冲的影响.印度大陆的P波快波方向总体为NNE方向,与绝对板块运动方向一致,这表明印度大陆上地幔流动方向与板块运动方向一致.青藏高原内部、东天山的P波快波方向与主压应力方向接近,而在青藏高原南缘、北缘及东北缘等块体边界地区P波快波方向与主压应力方向垂直.中国大陆东部上地幔变形主要受菲律宾板块和太平洋板块俯冲的影响.在扬子板块内部P波快波方向为SE方向,这与绝对板块的运动方向一致.华北地区的各向异性结构较为复杂,可能与华北克拉通裂解有关.中国大陆东北的东部平均方向为SE,而在兴安岭一侧为SSW方向,即平行于构造线方向.根据各向异性的倾角,中国大陆及邻区上地幔各向异性结构大体可分为三块:1)青藏新疆地区的各向异性倾角接近水平,推测该区形变力源主要为上地幔物质水平流动.2)南北带地区的各向异性倾角较大,特别是在青藏东缘地区的倾角约为40°,这可能是由于青藏向东挤出过程中受华南地块和鄂尔多斯地块的阻挡,在板块边界地区产生了垂直变形.3)中国东部地区各向异性结构较为复杂,在中国大陆东北部各向异性倾角接近水平,这可能是该区上地幔变形主要受太平洋板块俯冲的影响,而在太行山、大别-苏鲁地区各向异性倾角较大,这表明该区上地幔以垂直变形为主. 相似文献
12.
K. M. Fischer M. J. Fouch D. A. Wiens M. S. Boettcher 《Pure and Applied Geophysics》1998,151(2-4):463-475
—We have obtained constraints on the strength and orientation of anisotropy in the mantle beneath the Tonga, southern Kuril, Japan, and Izu-Bonin subduction zones using shear-wave splitting in S phases from local earthquakes and in teleseismic core phases such as SKS. The observed splitting in all four subduction zones is consistent with a model in which the lower transition zone (520–660 km) and lower mantle are isotropic, and in which significant anisotropy occurs in the back-arc upper mantle. The upper transition zone (410–520 km) beneath the southern Kurils appears to contain weak anisotropy. The observed fast directions indicate that the geometry of back-arc strain in the upper mantle varies systematically across the western Pacific rim. Beneath Izu-Bonin and Tonga, fast directions are aligned with the azimuth of subducting Pacific plate motion and are parallel or sub-parallel to overriding plate extension. However, fast directions beneath the Japan Sea, western Honshu, and Sakhalin Island are highly oblique to subducting plate motion and parallel to present or past overriding plate shearing. Models of back-arc mantle flow that are driven by viscous coupling to local plate motions can reproduce the splitting observed in Tonga and Izu-Bonin, but further three-dimensional flow modeling is required to ascertain whether viscous plate coupling can explain the splitting observed in the southern Kurils and Japan. The fast directions in the southern Kurils and Japan may require strain in the back-arc mantle that is driven by regional or global patterns of mantle flow. 相似文献
13.
本文对布设在华北克拉通东西两块体交界区域的宽频带流动地震观测台阵和部分固定台站的远震波形记录开展了SKS波分裂研究.结果显示,鄂尔多斯块体内部的各向异性比较弱,剪切波分裂导致的时间延迟一般小于0.7s.鄂尔多斯块体东缘的山西断陷带和太行山以及华北平原西部均表现出了比较强的各向异性,时间延迟大于1.0s.特别是在太行山地区观测到的ENE趋向的快波偏振方向明显不同于鄂尔多斯块体和华北平原地区的近E-W和ESE方向的快波偏振方向.在华北克拉通东西两块体交界过渡带的太行山地区观测到的显著上地幔各向异性及变化可能对应于围绕鄂尔多斯块体东南角的局部软流圈绕流,而后者可能起因于鄂尔多斯块体的逆时针旋转以及青藏高原软流圈沿秦岭大别造山带向东的流动. 相似文献
14.
We investigate the regional thermal structure of the crust in Mexico using Curie Point Depth (CPD) estimates. The top and bottom of the magnetized crust were calculated using the power-density spectra of the total magnetic field from the freely available ??Magnetic Anomaly Map of North America??. We applied this method to estimate the regional crustal thermal structure in overlapping square windows of 2°?×?2°. The CPD estimates range between 10 and 40?km and show several regions of relatively shallow and deep magnetic sources, with a general inverse correlation with measured heat flow. A deep CPD region (20?C30?km) is located in the fore-arc area where the subducting Cocos plate has a flat-slab geometry. This deep region is bound to the NW and SE by shallow CPD areas beneath the states of Michoacan (CPD?=?12?C16?km) and Oaxaca (CPD?=?~16?km), respectively. There is a good spatial correlation between this deep CPD area and two main fracture zones located on the incoming Cocos plate (Orozco and O??Gorman fracture zones), suggesting that subduction plays an important role in setting apart different CPD provinces along the Mexican coast. Another deep CPD (16?C32?km) area corresponds to the region where the Rivera plate subducts beneath Jalisco block. The Trans-Mexican Volcanic Belt is characterized by a decrease in Curie depths from west (16?C20?km) to east (10?C12?km). Finally, several deep CPD areas are situated in the back-arc region where old Mesozoic terrains are present. Our results suggest that the main control on the crust??s regional thermal structure in the fore-arc and volcanic arc regions is due to the subduction of the Cocos and Rivera plates beneath Mexico. 相似文献
15.
The origin of El Chichón volcano is poorly understood, and we attempt in this study to demonstrate that the Tehuantepec Ridge (TR), a major tectonic discontinuity on the Cocos plate, plays a key role in determining the location of the volcano by enhancing the slab dehydration budget beneath it. Using marine magnetic anomalies we show that the upper mantle beneath TR undergoes strong serpentinization, carrying significant amounts of water into subduction. Another key aspect of the magnetic anomaly over southern Mexico is a long-wavelength (∼ 150 km) high amplitude (∼ 500 nT) magnetic anomaly located between the trench and the coast. Using a 2D joint magnetic-gravity forward model, constrained by the subduction P–T structure, slab geometry and seismicity, we find a highly magnetic and low-density source located at 40–80 km depth that we interpret as a partially serpentinized mantle wedge formed by fluids expelled from the subducting Cocos plate. Using phase diagrams for sediments, basalt and peridotite, and the thermal structure of the subduction zone beneath El Chichón we find that ∼ 40% of sediments and basalt dehydrate at depths corresponding with the location of the serpentinized mantle wedge, whereas the serpentinized root beneath TR strongly dehydrates (∼90%) at depths of 180-200 km comparable with the slab depths beneath El Chichón (200-220 km). We conclude that this strong deserpentinization pulse of mantle lithosphere beneath TR at great depths is responsible for the unusual location, singularity and, probably, the geochemically distinct signature (adakitic-like) of El Chichón volcano. 相似文献
16.
The 10 January 2018 MW7.5 Swan island, Honduras earthquake occurred on the Swan island fault, which is a transform plate boundary between the North American and Caribbean plates. Here we back-project the rupture process of the earthquake using dense seismic stations in Alaska, and find that the earthquake ruptured at least three faults (three stages) for a duration of ~40 s. The rupture speed for the longest fault (stage 3) is as fast as 5 km/s, which is much faster than the local shear wave velocity of ~4 km/s. Supershear rupture was incidentally observed on long and straight strike-slip faults. This study shows a supershear rupture that occured on a strike-slip fault with moderate length, implying that supershear rupture might commonly occur on large strike-slip earthquakes. The common occurrence of supershear rupture on strike-slip earthquakes will challenge present understanding of crack physics, as well as strong ground motion evaluation in earthquake engineering. 相似文献
17.
Anisotropy beneath the Iberian Peninsula: The Contribution of the ILIHA-NARS Broad-band Experiment 总被引：1，自引：0，他引：1
—The presence of anisotropy beneath the Iberian Peninsula and its main distinctive features can be established through the analysis of teleseismic shear-wave splitting observed in the ILIHA-NARS experiment. In this experiment, an homogeneous data set is provided by a network of 14 broad-band stations deployed over the entire peninsula for about one year. Even if technical problems led to an amount of data smaller than expected, significant variations in the inferred fast velocity direction are observed for stations located in different Iberian domains. The stations in Central and East Iberia show a fast velocity direction oriented roughly E–W, coincident with previous results in Toledo. A clearly different NE–SW direction is observed in the Ossa-Morena zone, supporting the image from a previous regional experiment. The observed delay times lie between 0.5 and 1 s. Although large-scale mechanisms, such as the absolute plate motion of Eurasia, can be invoked to explain the origin of anisotropic features in many sites, the regional variations observed in some domains imply that differentiated origins of the anisotropy have to be considered, probably related to the particular tectonics in the area. An interesting example of this fact is provided by the stations in the Betic chain; the fast velocity direction inferred for a station located in the limit of the External Betics (South Iberian domain), oriented N80°E, is clearly different from the N15–35°E direction observed in the Internal Betics (Alboran crustal domain), the origin of which has to be related to the Alpine building of the chain. 相似文献
18.
K–Ar ages have been determined for 14 late Miocene to Pliocene volcanic rocks in the north of the Kanto Mountains, Japan, for tracking the location of the volcanic front through the time. These samples were collected from volcanoes located behind the trench–trench–trench (TTT) triple junction of the Pacific, Philippine Sea, and North American plates. This junction is the site of subduction of slabs of the Pacific and the Philippine Sea plates, both of which are thought to have influenced magmatism in this region. The stratigraphy and K–Ar ages of volcanic rocks in the study area indicate that volcanism occurred between the late Miocene and the Pliocene, and ceased before the Pleistocene. Volcanism in adjacent areas of the southern NE Japan and northern Izu–Bonin arcs also occurred during the Pliocene and ceased at around 3 Ma with the westward migration of the volcanic front, as reported previously. Combining our new age data with the existing data shows that before 3 Ma the volcanic front around the TTT junction was located about 50 km east of the preset‐day volcanic front. We suggest that northward subduction of the Philippine Sea Plate slab ended at ～3 Ma as a result of collision between the northern margin of the plate with the surface of the Pacific Plate slab. This collision may have caused a change in the subduction vector of the Philippine Sea Plate from the original north‐directed subduction to the present‐day northwest‐directed subduction. This indicates that the post ～3 Ma westward migration of the volcanic front was a result of this change in plate motion. 相似文献
19.
S. Goes F.A. Capitanio G. Morra M. Seton D. Giardini 《Physics of the Earth and Planetary Interiors》2011,184(1-2):1-13
The dynamics of plate tectonics are strongly related to those of subduction. To obtain a better understanding of the driving forces of subduction, we compare relations between Cenozoic subduction motions at major trenches with the trends expected for the simplest form of subduction. i.e., free subduction, driven solely by the buoyancy of the downgoing plate. In models with an Earth-like plate stiffness (corresponding to a plate–mantle viscosity contrast of 2–3 orders of magnitude), free plates subduct by a combination of downgoing plate motion and trench retreat, while the slab is draped and folded on top of the upper-lower mantle viscosity transition. In these models, the slabs sink according to their Stokes’ velocities. Observed downgoing-plate motion–plate-age trends are compatible with >80% of the Cenozoic slabs sinking according to their upper-mantle Stokes’ velocity, i.e., subducting-plate motion is largely driven by upper-mantle slab pull. Only in a few cases, do young plates move at velocities that require a higher driving force (possibly supplied by lower-mantle–slab induced flow). About 80% of the Cenozoic trenches retreat, with retreat accounting for about 10% of the total convergence. The few advancing trench sections are likely affected by regional factors. The low trench motions are likely encouraged by low asthenospheric drag (equivalent to that for effective asthenospheric viscosity 2–3 orders below the upper-mantle average), and low lithospheric strength (effective bending viscosity ～2 orders of magnitude above the upper-mantle average). Total Cenozoic trench motions are often very oblique to the direction of downgoing-plate motion (mean angle of 73°). This indicates that other forces than slab buoyancy exert the main control on upper-plate/trench motion. However, the component of trench retreat in the direction of downgoing plate motion (≈ slab pull) correlates with downgoing-plate motion, and this component of retreat generally does not exceed the amount expected for free buoyancy-driven subduction. High present-day slab dips (on average about 70°) are compatible with largely upper-mantle slab-pull driven subduction of relatively weak plates, where motion partitioning and slab geometry adjust to external constraints/forces on trench motion. 相似文献
20.
Benjun Wu Clinton P. Conrad Arnauld Heuret Carolina Lithgow-Bertelloni Serge Lallemand 《Earth and Planetary Science Letters》2008,272(1-2):412-421
Although subducting slabs undergo a bending deformation that resists tectonic plate motions, the magnitude of this resistance is not known because of poor constraints on slab strength. However, because slab bending slows the relatively rapid motions of oceanic plates, observed plate motions constrain the importance of bending. We estimated the slab pull force and the bending resistance globally for 207 subduction zone transects using new measurements of the bending curvature determined from slab seismicity. Predicting plate motions using a global mantle flow model, we constrain the viscosity of the bending slab to be at most ~ 300 times more viscous than the upper mantle; stronger slabs are intolerably slowed by the bending deformation. Weaker slabs, however, cannot transmit a pull force sufficient to explain rapid trenchward plate motions unless slabs stretch faster than seismically observed rates of ~ 10− 15 s− 1. The constrained bending viscosity (~ 2 × 1023 Pa s) is larger than previous estimates that yielded similar or larger bending resistance (here ~ 25% of forces). This apparent discrepancy occurs because slabs bend more gently than previously thought, with an average radius of curvature of 390 km that permits subduction of strong slabs. This gentle bending may ultimately permit plate tectonics on Earth. 相似文献