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1.
马尼拉俯冲带热结构数值模拟与地震意义 总被引：1，自引：0，他引：1
研究马尼拉俯冲带地震分布的成因机制,根据马尼拉俯冲带最新的莫霍面深度和地壳厚度等地质与地球物理资料,选取3条典型剖面,模拟俯冲带热结构。结果表明:1俯冲带热结构主要受俯冲角度、俯冲速度和俯冲板块本身地质条件等因素影响;2 BB′剖面和CC′剖面属于热俯冲;3当洋壳俯冲至软流圈边界时,俯冲板块温度迅速升高,容易形成地震活动。BB′剖面的俯冲角度和俯冲速度比CC′剖面小,使得BB′剖面发生地震的深度更浅。俯冲洋壳底部温度比顶部低,地震活动也持续到更大的深度。 相似文献
2.
The first P-arrival-time data from 513 local earthquakes were analyzed to study lateral variation of the depth to the Conrad
and Moho discontinuities beneath the Chugoku and Shikoku districts, southwest Japan, as well as to determine earthquake hypocenters
and P-wave station corrections. The depth to the discontinuity was estimated by minimizing the travel-time residuals of more
than 8700 first P arrivals observed at 55 seismic stations. The Conrad and Moho discontinuities are located within depth ranges
of 15–25 km and 30–40 km, respectively. The Moho is deeper under the mountain area than under the Seto Inland Sea area, and
especially deep under the Pacific Coast of the Shikoku district and the mountain area in the Chugoku district. The depth variation
of the Moho is quite similar to the Bouguer gravity anomaly distribution and the lateral variations of the P-wave velocity.
The deep Moho under the southern Shikoku is located at the portion in which the continental Moho under the island arc meets
the oceanic Moho that is the boundary interface between the oceanic crust and the Philippine Sea (PHS) plate dipping toward
the back arc. Although there are high mountains in the northern and middle Shikoku, the Moho is not so deep because subduction
of the PHS plate prevents the Moho from getting deep, while the Moho is deep due to isostatic balance under the mountain area
in the Chugoku district. In addition, we indicated the possibility that the upper boundary of the oceanic crust just above
the high-velocity PHS plate is in contact with the deep Moho under the western Chugoku. The contact of the Moho with the oceanic
crust can explain the markedly negative gravity anomaly observed in the western Chugoku and the later phase that appears just
after the first P arrival from local earthquakes. 相似文献
3.
Tomotaka Iwata Takao Kagawa Anatoly Petukhin Yoshihiro Ohnishi 《Journal of Seismology》2008,12(2):223-234
We constructed a prototype of the basin and crustal structure model for the Kinki area, southwest of Japan, for the simulation
of strong ground motions of hypothetical crustal and subduction earthquakes. We collected results of the deep seismic velocity
profiles obtained by the reflection experiments and seismic imaging results, which were conducted in the Kinki area. The obtained
profiles give underground velocity structures of the crust, from the surface to the subducting slab. We also gather the basin
velocity structure information of the Osaka, Kyoto, Nara, and Ohmi basins. To examine the applicability of the constructed
velocity structure model to the ground motion simulation, we simulated waveforms of an intermediate size event occurred near
the source area of the hypothetical subduction earthquakes. Simulated ground motions using the basin and crustal velocity
structure model are fairly well reproducing the observations at most of stations, and the constructed basin and crustal velocity
structure model is applicable for the long-period ground motion simulations. 相似文献
4.
日本俯冲带与IBM俯冲带位于太平洋板块、菲律宾海板块和欧亚板块三者的交汇地带,是典型的"俯冲工厂"地区,具有重要的研究意义.本文利用震源分布资料与卫星重力数据对日本俯冲带与IBM俯冲带进行了研究.通过空间重力异常反映了俯冲带地区的区域构造形态,在此基础上基于艾利模式计算了均衡异常以反映地壳均衡特征.利用震源分布资料,分别从垂直俯冲带走向与沿俯冲带走向划定了横截剖面(cross-sections)进行了地震提取,讨论了俯冲带地区的Wadati-Benioff带形态特征,并借助于俯冲带地震等深线图直观描述了俯冲带的俯冲形态.在日本俯冲带与伊豆—小笠原俯冲带各选取了一条典型剖面进行了重力2.5D反演,研究了俯冲带地区的壳幔结构特征.研究结果表明,九州—帕劳海脊与IBM岛弧在均衡异常上存在差异,前者已逐渐趋向于地壳均衡.IBM的Wadati-Benioff带存在明显的南北差异,反映出伊豆—小笠原俯冲板片停留在了660km转换带中,而马里亚纳俯冲板片很可能垂直穿过了这一转换带,造成这种南北差异的原因与板块相对运动、岩石圈黏性和年龄差异以及俯冲板片的重力效应等因素有关.在IBM的中部和南部存在板片撕裂现象.日本俯冲带的俯冲洋壳密度随俯冲深度变化较小,洋幔存在一定程度的蛇纹岩化,地幔楔蛇纹岩化作用不典型,海沟处有一范围较小的含水畸变带;伊豆—小笠原俯冲带俯冲洋壳密度随深度增大而明显增大,洋幔蛇纹岩化程度较日本俯冲带低,地幔楔蛇纹岩化作用强烈,板块交汇处存在明显的蛇纹岩底辟.日本俯冲带与IBM俯冲带一线自北向南板片俯冲变陡,两侧板块耦合度降低,与俯冲带两侧的板块运动速率差异有关. 相似文献
5.
6.
Estimation of crustal structure in central and west Japan from waveform modeling of regionalPL waves
An inverse method of modeling the regionalPL waveform with the predominant period of about 20 s was developed to estimate the averageS-velocity structure of the upper crust. Applicability of the waveform modeling was confirmed by the results of the numerical experiments: thePL waveform is most sensitive to theS velocity in the upper crust, whereas it is not affected significantly by errors involved in the focal mechanism solution and focal depth determination when thePL wave is well developed. The method was applied to the observed seismograms recorded in central Japan from the earthquakes with epicentral distances 300–500 km. As a result, distinct regional differences were found in the upper crustalS velocity; in particular, between the southern Shikoku district, west Japan, and the southern Chubu district, central Japan, and between the mountainous and the coastal areas in the southern Chubu district. These differences are in agreement with the general features ofP-velocity structures obtained by explosion experiments and by analyses of natural earthquakes. Our method is effective to the extent that the crustal structure along the propagation path can be assumed a horizontally layered structure; it is not applicable when the sensitivity of thePL waveform to the error in the focal mechanism solution is exceptionally high. 相似文献
7.
L. P. Vinnik I. M. Aleshin M. K. Kaban S. G. Kiselev G. L. Kosarev S. I. Oreshin Ch. Reigber 《Izvestiya Physics of the Solid Earth》2006,42(8):639-651
A 3-D velocity model of the Tien Shan crust and upper mantle is constructed through the inversion of the receiver functions of P and S waves together with teleseismic traveltime anomalies at nearly 40 local seismic stations. It is found that in the vast central region, where no strong earthquakes have been known over the past century, the S wave velocity at depths of 10–35 km is lower than in adjacent regions by up to 10%. These data are evidence for mechanical weakness of the crust preventing the accumulation of elastic energy. Apparently, the lower velocity and the weakness of the crust are due to the presence of water. The weakness of the crust is one of the possible reasons for the strain localization responsible for the formation of the present Tien Shan but can also be due in part to the young orogenesis. The crustal thickness is largest (about 60 km) in the Tarim-Tien Shan junction zone. The crust-mantle boundary in this region descends by a jump as a result of an increase in the lower crust thickness. This is probably due to the underthrusting of the Tien Shan by the Tarim lithosphere. This causes the mechanically weak lower crust of the Tarim to delaminate and accumulate in nearly the same way as an accretionary prism during the subduction of oceanic lithosphere. In the upper mantle, the analysis has revealed a low velocity anomaly, apparently related to basaltic outflows of the Upper Cretaceous-Early Paleogene. The Cenozoic Bachu uplift in the northern Tarim depression is also associated with the low velocity anomaly. The Naryn depression is characterized by a high velocity in the upper mantle and can be interpreted as a fragment of an ancient platform. 相似文献
8.
利用山东及周边区域地震台网1975—2014年1月期间记录到的1369个地震的13781个P波到时数据对山东地区地壳结构进行了层析成像研究.结果表明,山东地区地壳速度结构存在明显的不均匀性.沂沭断裂带介质速度结构复杂,呈现明显的分段特征,两侧块体速度存在差异,具有块体边界的构造特征.鲁西断块20km以下深度处存在大规模的低速异常区,这与该地区始新世发生的大规模滑脱拆离构造有关,可能产生于太平洋板块的西向俯冲导致地幔热物质沿沂沭断裂带向上并向西涌动.历史大震及ML4.0以上中强震大部分为走滑型地震,主要发生于高低速异常过渡带且有深大断裂穿过的地区.震群主要发生于低速体上部或周边,且震源深度优势分布在中上地壳,这与地下介质富含流体并导致应力集中有关. 相似文献
9.
William Power Laura Wallace Xiaoming Wang Martin Reyners 《Pure and Applied Geophysics》2012,169(1-2):1-36
We assess the tsunami hazard posed to New Zealand by the Kermadec and southern New Hebrides subduction margins. Neither of these subduction zones has produced tsunami large enough to cause significant damage in New Zealand over the past 150?years of well-recorded history. However, as this time frame is short compared to the recurrence interval for major tsunamigenic earthquakes on many of the Earth’s subduction zones, it should not be assumed that what has been observed so far is representative of the long term. For each of these two subduction zones we present plate kinematic and fault-locking results from block modelling of earthquake slip vector data and GPS velocities. The results are used to estimate the current rates of strain accumulation on the plate interfaces where large tsunamigenic earthquakes typically occur. We also review data on the larger historical earthquakes that have occurred on these margins, as well as the Global CMT catalogue of events since 1976. Using this information we have developed a set of scenarios for large earthquakes which have been used as initial conditions for the COMCOT tsunami code to estimate the subsequent tsunami propagation in the southwest Pacific, and from these the potential impact on New Zealand has been evaluated. Our results demonstrate that there is a significant threat posed to the Northland and Coromandel regions of New Zealand should a large earthquake (M w ?8.5) occur on the southern or middle regions of the Kermadec Trench, and that a similarly large earthquake on the southern New Hebrides Trench has the potential to strongly impact on the far northern parts of New Zealand close to the southern end of the submarine Three Kings Ridge. We propose logic trees for the magnitude–frequency parameters of large earthquakes originating on each trench, which are intended to form the basis for future probabilistic studies. 相似文献
10.
基于简化的波速结构,利用二维伪谱法研究了俯冲带不同位置震源激发的地震波的传播特征.发现当俯冲板块中存在6%的低速异常时,直达P波和直达S波之间存在波阵面近平面的易于识别的P-S转换震相,其传播方向不因震源位置偏离低速层的中心而改变.对于低速层的边界存在±2 km的随机起伏、低速层为楔形、周围介质为水平分层模型这三种情况,这组震相仍然清晰可辨.对于150 km长度的橄榄石楔,模拟结果显示仍然能在一定震中距范围内接收到这一震相.因此,可以尝试用这组震相来探测俯冲带亚稳态橄榄石的存在状态. 相似文献
11.
根据中国地震台网和ISC台站提供的P波走时资料,使用差异演化全局优化算法(DE算法)和移动窗方法反演了琉球-台湾-吕宋地区岩石层尺度的P波速度结构.在台站和地震分布较为密集的地区,反演窗口为2°×2°,移动步长为1°;在台站和地震较少的地区,反演窗口为4°×4°左右,移动步长为2°.反演结果揭示出琉球-台湾-吕宋地区壳幔结构的横向差异:琉球岛弧西侧受冲绳海槽地幔热扰动的地壳减薄,东侧由于菲律宾海板决的俯冲挤压地壳略有增厚;欧亚大陆与菲律宾海板块的相互碰撞导致台湾地区地壳及岩石层明显增厚;吕宋及菲律宾北部岩石层受岛弧火山下方热流影响较大.结果表明,非线性全局优化算法和移动窗方法能够用于反演较大尺度速度结构的横向变化. 相似文献
12.
强震震前(preseismic)动力学过程的研究对于地震预测具有十分重要的意义,但由于观测资料的限制,目前对强震前孕震区力学状态及其演化过程的认识还非常有限.2011年日本东北9.0特大地震(Tohoku-Oki)发生在GPS观测台站最为密集的地区,为研究特大地震震间(interseismic)与震前的变形状态提供了难得的机会.文中将利用日本东北大地震之前连续的GPS观测资料,分别计算震间与震前的速度场与变形场.通过对比分析发现,日本东北地区(Tohoku)震前的应变状态与震间的有很大的不同,震间的变形主要受到太平洋板块向日本海沟北西西向的俯冲挤压作用所控制,其主压应变以近东西向压缩为主,日本东北地区的运动方向与太平洋板块的运动方向大体一致.但是,临近地震前(震前)日本东北地区的运动方向发生了很大变化,震前30天的连续GPS观测结果显示,速度场的优势方向经常变换,间歇性地出现与太平洋板块运动方向相反的情况.这意味着震前孕震区的力学状态发生了很大的改变.这种变化可能与震前破裂成核或慢滑移及慢地震等过程有关,这些过程将加速或促进大地震的发生,从而为大地震的发生准备了力学条件.值得特别强调的是,这些现象都是可以通过直接观测能够发现的大地震之前的异常现象.由此可见,加密GPS站点进行连续观测,寻找震前变形异常区以及探索异常的物理机制对于地震预测预报有重要的科学意义. 相似文献
13.
本文通过面波层析成像得到了中国东部海域及邻近地区的地壳上地幔S波速度图像,给出了主要构造单元的区划及其结构特征,并讨论了速度结构与现今构造活动及构造演化历史的关系.研究区内中下地壳的平均速度与地震活动存在比较显著的关系,强震基本都发生在低速区内或高低速过渡区.太行山以东地壳内存在几条北西向低速带,其中张家口—渤海地震带下方的低速带最为显著.东部海域划分成北黄海、南黄海、东海、和冲绳海槽等4个构造块体.北黄海具有较薄较高速的岩石圈,与南华北盆地类似,推测是中生代特提斯洋向北俯冲造成岩石圈减薄的遗迹.北华北地区具有低速的地壳和较厚的岩石圈,岩石圈地幔速度偏低且上下比较均匀,可能反映中生代沿北方缝合带持续碰撞作用的特点.南黄海具有相对较厚的岩石圈,较多地保存了下扬子克拉通的特征.在下扬子与华北地块的拼合过程中,洋壳俯冲可能是北黄海和苏皖地区上地幔低速特征的成因.在125°E以东的朝鲜半岛地区未发现这一拼合过程的遗迹.有可能整个朝鲜半岛都是华北地块的一部分;但也有可能是太平洋俯冲和日本海张开的作用完全改造了朝鲜半岛的岩石圈上地幔,抹去了以往构造运动的痕迹.东海地区的地壳厚度,特别是岩石圈厚度向冲绳海槽方向减小,反映出菲律宾海板块俯冲在弧后广大地区都有影响.冲绳海槽地区可见俯冲的菲律宾海板片以及板片上方显著低速的地壳和上地幔,为冲绳海槽的弧后扩张机制提供了证据. 相似文献
14.
In this paper, we present seismological evidence for the influence of fluids and magma on the generation of large earthquakes in the crust and the subducting oceanic slabs under the Japan Islands. The relationship between seismic tomography and large crustal earthquakes (M=5.7-8.0) in Japan during a period of 116 years from 1885 to 2000 is investigated and it is found that most of the large crustal earthquakes occurred in or around the areas of low seismic velocity. The low-velocity zones represent weak sections of the seismogenic crust. The crustal weakening is closely related to the subduction process in this region. Along the volcanic front and in back-arc areas, the crustal weakening is caused by active volcanoes and arc magma resulting from the convective circulation process in the mantle wedge and dehydration reactions in the subducting slab. In the forearc region of southwest Japan, fluids are suggested in the 1995 Kobe earthquake source zone, which have contributed to the rupture nucleation. The fluids originate from the dehydration of the subducting Philippine Sea slab. The recent 2001 Geiyo earthquake (M=6.8) occurred at 50 km depth within the subducting Philippine Sea slab, and it was also related to the slab dehydration process. A detailed 3D velocity structure is determined for the northeast Japan forearc region using data from 598 earthquakes that occurred under the Pacific Ocean with hypocenters well located with SP depth phases. The results show that strong lateral heterogeneities exist along the slab boundary, which represent asperities and results of slab dehydration and affect the degree and extent of the interplate seismic coupling. These results indicate that large earthquakes do not strike anywhere, but only anomalous areas which can be detected with geophysical methods. The generation of a large earthquake is not a pure mechanical process, but is closely related to physical and chemical properties of materials in the crust and upper mantle, such as magma, fluids, etc. 相似文献
15.
俯冲带上覆板片密度特征是认识俯冲及其引发深部岩浆过程的一个窗口.本文以苏门答腊俯冲带空间重力异常数据为基础,在2.5D密度结构剖面约束下,通过3D密度反演,获得了研究区3D密度结构分布.反演结果表明,俯冲板片角度和下倾极限深度均从研究区西北向东南方向逐渐增加;研究区上覆板片下地壳存在低密度异常,主要集中在东南部,分布范围也随深度而增加.分析认为这有可能是俯冲引发的洋壳脱水对上覆板片下地壳侵染所造成.此外,研究区东南部存在的地幔板片撕裂可能是造成该区下地壳低密度异常最为显著的另一个原因.3D密度切片揭示出下地壳密度异常具有沿NE方向延伸迹象,推断应与印澳板块在苏门答腊地区的斜向俯冲作用有关.本文还对研究区的地震分布特征进行了讨论.大部分的浅源地震集中在下倾极限附近,为脆性破裂或摩擦滑移所引发.震源深度大于200 km的地震基本分布在研究区中部和东南部,震源深度从西北向东南方向逐渐加大,这从另一角度为研究区东南部地幔存在板片撕裂的观点提供了佐证. 相似文献
16.
本文以中俄、俄日学者合作所得到的地球物理资料为主,结合其它相关地质-地球物理数据,组构了俄罗斯贝加尔湖-日本仙台(BS)4000 km长断面,用于区域性大尺度地研究东北亚洲地壳结构和一系列地质构造问题.研究BS断面地震波速结果表明:(1)西伯利亚板块和黑龙江板块地壳结构变化较大,并可分为上、中、下部地壳,欧亚板块东部陆缘带地壳结构较简单,基本两分.贝加尔裂谷带下部地壳厚度比松辽盆地的薄约7 km,而上部地壳则相反,前者的比后者的厚约9 km.两个裂谷带在Moho界面之下的波速分布差异也较大.(2)结合前人认识,综合分析认为,贝加尔裂谷带属主动式裂谷,松辽盆地属于混合型裂谷.贝加尔裂谷形成动力主要来自地球构造圈B″层物质上涌所形成的地幔热柱的垂向作用,由BLV带佐证,松辽盆地形成动力主要来自太平洋板块斜向俯冲的中远程效应.(3)日本国所位于的西太平洋岛弧带是多地震带,除了太平洋板块俯冲产生的浅部效应、地壳中断裂与流体的直接作用等因素,本文指出仙台等速块的物性条件是岛弧带的主要不稳定因素.同时指出需要关注日本东海岸深约30~40 km的大级次地震的发生. 相似文献
17.
用三维流变非连续变形与有限元相结合（DDA+FEM）的方法,在青藏高原及邻近地区三维构造块体相互制约的大背景中,考虑了龙门山断裂带东西两侧地势、地壳厚度和分层的明显变化,及断裂带东侧四川盆地及鄂尔多斯块体坚硬地壳阻挡的影响,通过用GPS资料做位移速率边界约束和震源机制约束,计算得到研究区的速度场和应力场与该地区GPS测量结果和震源机制分布结果基本一致.在此基础上,模拟计算现今构造块体边界断层上表征剪应力及法向应力等综合影响的危险度分布.结果表明,上、中地壳层危险度分布中危险度较高的地段多数与近几十年来发生的七级以上大震区域基本一致.包括2008年汶川8.0级等大震的发震断层.通过分别对龙门山断裂带东西两侧的两种不同构造格局进行试算表明,龙门山断裂带东西两侧地势、地壳厚度、分层与物性明显变化对汶川大震的孕育发生均起了关键性作用.计算得到的应变率强度分布图可见,高原东部整个边缘地带均接近应变率强度的陡变带.其中以龙门山断裂带上的陡变最为明显,西侧应变率强度是东侧的近4倍,而且断裂带东侧应变率强度等值线衰减比西侧快.反映了汶川大震逆冲型发震断层地区独特的特征.此外,由计算得到的应变能密度分布图可见,龙门山断裂带在上、中地壳层中均位于宽度相同、其走向与龙门山断裂带走向一致的高应变能密度带中,在上地壳层这个带的东西两侧则是应变能密度较低的地区,而在中地壳层,其强度在断裂带东侧逐渐向东衰减,西侧应变能密度高,而东侧应变能密度较低.表明在印度板块强烈推挤作用和高原各构造块体相互制约及龙门山断裂带东西两侧特殊构造环境中,高原地壳物质向东水平运动,受到龙门山断裂带东侧介质刚性强度较大的四川盆地阻挡,使得汶川大震发震断层在大震前已积累了相当水平的应变能,并同时处于力学上的不稳定状态. 相似文献
18.
Connection between earthquakes and deep fluids revealed by magnetotelluric imaging in Songyuan,China
Yu TANG Aihua WENG Yue YANG Shiwen LI Jianjun NIU Yanhui ZHANG Yabin LI Jianping LI 《中国科学:地球科学(英文版)》2021,64(1):161-176
Songyuan is the most earthquake prone area in northeast China.Since 2006,earthquakes have occurred in the area in the form of swarms,with a maximum magnitude of M_L5.8.There is much controversy about the cause of the Songyuan earthquakes.We attempted to determine the cause using a three-dimensional electrical conductivity structure inverted from a regional network of magnetotelluric data in the Songyuan area.The L-BFGS inversion method was applied,with a fullimpedance tensor data set used as the inversion input.Combined with an evaluation of the earthquake locations,the resistivity model revealed a northeast-oriented hidden fault running through the Songyuan earthquake area(SEA),which was speculated to be the preexisting Fuyu-Zhaodong Fault(FZF).Our resistivity model also found an apparent lithospheric low-resistivity anomaly beneath the earthquake area,which breached the high-resistivity lithospheric mantle and stalled at the base of the crust.A petrophysical analysis showed that this lower crustal low-resistivity anomaly was most likely attributed to hydrated partial melting,which could release water into the lower crust during later magma emplacements.While weakening the strength of the FZF,these ascending fluids also increased the pore pressure in the fault,further reducing the shear strength of the fault.Shear stress action(a fault strike component of the east-west regional compress),together with possible near-surface disturbances,may drive the fault to slip and trigger the earthquakes in Songyuan.It is possible that the continuous replenishment of fluids from the deeper mantle forces the Songyuan earthquakes into the form of swarms.We infer that the Songyuan earthquakes could be attributed to a combination of preexisting faults,regional stress,and deep fluids associated with plate subduction,and near surface disturbances might induce the earthquakes in advance.The Songyuan earthquakes are inherently induced earthquakes,fed by deep fluids. 相似文献
19.
The seismogenic zone of subduction thrust faults 总被引：13，自引：0，他引：13
Abstract Subduction thrust faults generate earthquakes over a limited depth range. They are aseismic in their seaward updip portions and landward downdip of a critical point. The seaward shallow aseismic zone, commonly beneath accreted sediments, may be a consequence of unconsolidated sediments, especially stable-sliding smectite clays. Such clays are dehydrated and the fault may become seismogenic where the temperature reaches 100--150°C, that is, at a 5--15 km depth. Two factors may determine the downdip seismogenic limit. For subduction of young hot oceanic lithosphere beneath large accretionary sedimentary prisms and beneath continental crust, the transition to aseismic stable sliding is temperature controlled. The maximum temperature for seismic behavior in crustal rocks is ～ 350°C, regardless of the presence of water. In addition, great earthquake ruptures initiated at less than this temperature may propagate with decreasing slip to where the temperature is ～ 450°C. For subduction beneath thin island arc crust and beneath continental crust in some areas, the forearc mantle is reached by the thrust shallower than the 350°C temperature. The forearc upper mantle probably is aseismic because of stable-sliding serpentinite hydrated by water from the underthrusting oceanic crust and sediments. For many subduction zones the downdip seismogenic width defined by these limits is much less than previously assumed. Within the narrowly defined seismic zone, most of the convergence may occur in earthquakes. Numerical thermal models have been employed to estimate temperatures on the subduction thrust planes of four continental subduction zones. For Cascadia and Southwest Japan where very young and hot plates are subducting, the downdip seismogenic limit on the subduction thrust is thermally controlled and is shallow. For Alaska and most of Chile, the forearc mantle is reached before the critical temperature, and mantle serpentinite provides the limit. In all four regions, the seismogenic zones so defined agree with estimates of the extent of great earthquake rupture, and with the downdip extent of the interseismic locked zone. 相似文献
20.
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