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1.
Larry J. Ruff 《Pure and Applied Geophysics》1989,129(1-2):263-282
Seismic energy release is dominated by the underthrusting earthquakes in subduction zones, and this energy release is further concentrated in a few subduction zones. While some subduction zones are characterized by the occurrence of great earthquakes, others are relatively aseismic. This variation in maximum earthquake size between subduction zones is one of the most important features of global seismicity. Previous work has shown that the variation in maximum earthquake size is correlated with the variation in two other subduction zone properties: age of the subducting lithosphere and convergence rate. These two properties do not explain all the variance in maximum earthquake size. I propose that a third subduction zone property, trench sediments, explains part of the remaining variance in maximum earthquake size. Subduction zones are divided into two groups: (1) those with excess trench sediments, and (2) those with horst and graben structure at the trench. Thirteen of the 19 largest subduction zone events, including the three largest, occur in zones with excess trench sediments. About half the zones with excess trench sediments are characterized by great earthquake occurrence. Most of the other zones with excess trench sediments but without great earthquakes are predicted to have small earthquakes by the age-rate correlation. Two notable exceptions are the Oregon-Washington and Middle America zones. Overall, the presence of excess trench sediments appears to enhance great earthquake occurrence. One speculative physical mechanism that connects trench sediments and earthquake size is that excess trench sediments are associated with the subduction of a coherent sedimentary layer, which at elevated temperature and pressure, forms a homogeneous and strong contact zone between the plates. 相似文献
2.
Subduction zone seismicity and the thermo-mechanical evolution of downgoing lithosphere 总被引：1，自引：0，他引：1
In this paper we discuss characteristic features of subduction zone seismicity at depths between about 100 km and 700 km, with emphasis on the role of temperature and rheology in controlling the deformation of, and the seismic energy release in downgoing lithosphere. This is done in two steps. After a brief review of earlier developments, we first show that the depth distribution of hypocentres at depths between 100 km and 700 km in subducted lithosphere can be explained by a model in which seismic activity is confined to those parts of the slab which have temperatures below a depth-dependent critical valueT
cr.Second, the variation of seismic energy release (frequency of events, magnitude) with depth is addressed by inferring a rheological evolution from the slab's thermal evolution and by combining this with models for the system of forces acting on the subducting lithosphere. It is found that considerable stress concentration occurs in a reheating slab in the depth range of 400 to 650–700 km: the slab weakens, but the stress level strongly increases. On the basis of this stress concentration a model is formulated for earthquake generation within subducting slabs. The model predicts a maximum depth of seismic activity in the depth range of 635 to 760 km and, for deep earthquake zones, a relative maximum in seismic energy release near the maximum depth of earthquakes. From our modelling it follows that, whereas such a maximum is indeed likely to develop in deep earthquake zones, zones with a maximum depth around 300 km (such as the Aleutians) are expected to exhibit a smooth decay in seismic energy release with depth. This is in excellent agreement with observational data. In conclusion, the incoroporation of both depth-dependent forces and depth-dependent rheology provides new insight into the generation of intermediate and deep earthquakes and into the variation of seismic activity with depth.Our results imply that no barrier to slab penetration at a depth of 650–700 km is required to explain the maximum depth of seismic activity and the pattern of seismic energy release in deep earthquake zones. 相似文献
3.
Effects of relative plate motion on the deep structure and penetration depth of slabs below the Izu-Bonin and Mariana island arcs 总被引：2，自引：0，他引：2
An increasing number of seismological studies indicate that slabs of subducted lithosphere penetrate the Earth's lower mantle below some island arcs but are deflected, or, rather, laid down, in the transition zone below others. Recent numerical simulations of mantle flow also advocate a hybrid form of mantle convection, with intermittent layering. We present a multi-disciplinary analysis of slab morphology and mantle dynamics in which we account explicitly for the history of subduction below specific island arcs in an attempt to understand what controls lateral variations in slab morphology and penetration depth. Central in our discussion are the Izu-Bonin and Mariana subduction zones. We argue that the differences in the tectonic evolution of these subduction zones—in particular the amount and rate of trench migration—can explain why the slab of subducted oceanic lithosphere seems to be (at least temporarily) stagnant in the Earth's transition zone below the Izu-Bonin arc but penetrates into the lower mantle below the Mariana arc. We briefly speculate on the applicability of our model of the temporal and spatial evolution of slab morphology to other subduction zones. Although further investigation is necessary, our tentative model shows the potential for interpreting seismic images of slab structure by accounting for the plate-tectonic history of the subduction zones involved. We therefore hope that the ideas outlined here will stimulate and direct new research initiatives. 相似文献
4.
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
5.
Slip partitioning along major convergent plate boundaries 总被引：3，自引：0，他引：3
Along plate boundaries characterized by oblique convergence, earthquake slip vectors are commonly rotated toward the normal of the trench with respect to predicted plate motion vectors. Consequently, relative plate motion along such convergent margins must be partitioned between displacements along the thrust plate interface and deformation within the forearc and back-arc regions. The deformation behind the trench may take the form of strike-slip motion, back-arc extension, or some combination of both. We observe from our analysis of the Harvard Moment Tensor Catalog that convergent arcs characterized by back-arc spreading, specifically the Marianas and New Hebrides, are characterized by a large degree of slip partitioning. However, the observed rates, directions, and location of back-arc spreading are not sufficient to account for degree of partitioning observed along the respective arcs, implying that the oblique component of subduction is also accommodated in part by shearing of the overriding plate. In the case of the Sumatran arc, where partitioning is accommodated by strike-slip faulting in the overriding plate, the degree of partitioning is similar to that observed along the Marianas, but the result is viewed with caution because it is based on a predicted plate motion vector that is based on locally derived earthquake slip vectors. In the case of the Alaskan-Aleutian arc, where back-arc spreading is also absent, the degree of partitioning is less and rotation of slip vectors toward the trench normal appears to increase linearly as a function of the obliquity of convergence. If partitioning in the Alaskan-Aleutian arc is accommodated by strike-slip faulting within the upper plate, the positive relationship between obliquity of convergence and the rotation of earthquake slip vectors to the trench normal may reflect that either (1) the ratio of the depth extent of strike-slip faults behind the trenchZ
s
to the subduction thrustZ
t
increases westward along the arc, (2) the dip of the subduction thrust increases westward along the arc, or (3) the strength of the subduction thrust decreases westward along the arc. 相似文献
6.
The Hikurangi Margin is a region of oblique subduction with northwest-dipping intermediate depth seismicity extending southwest from the Kermadec system to about 42°S. The current episode of subduction is at least 16–20 Ma old. The plate convergence rate varies along the margin from about 60 mm/a at the south end of the Kermadec Trench to about 45 mm/a at 42°S. The age of the Pacific lithosphere adjacent to the Hikurangi Trench is not known.The margin divides at about latitude 39°S into two quite dissimilar parts. The northern part has experienced andesitic volcanism for about 18 Ma, and back-arc extension in the last 4 Ma that has produced a back-arc basin onshore with high heaflow, thin crust and low upper-mantle seismic velocities. The extension appears to have arisen from a seawards migration of the Hikurangi Trench north of 39°S. Here the plate interface is thought to be currently uncoupled, as geodetic data indicate extension of the fore-arc basin, and historic earthquakes have not exceededM
s=7.South of 39°S there is no volcanism and a back-arc basin has been produced by downward flexure of the lithosphere due to strong coupling with the subducting plate. Heatflow in the basin is normal. Evidence for strong coupling comes from historic earthquakes of up to aboutM
s=8 and high rates of uplift on the southeast coast of the North Island.The reason for this division of the margin is not known but may be related to an inferred increase, from northeast to southwest, in the buoyancy of the Pacific lithosphere. 相似文献
7.
Robert McCaffrey 《Pure and Applied Geophysics》1994,142(1):173-224
Deviations of slip vector azimuths of interplate thrust earthquakes from expected plate convergence directions at oblique subduction zones provide kinematic information about the deformation of forearcs and indirect evidence on the dynamics of the plate boundary. A global survey of slip vectors at major trenches of the world reveals a large variability in the kinematic response of forearcs to shear produced by oblique convergence. The variability in forearc deformation inferred from slip vector deflections is suggested to be caused by variations in forearc rheology rather than in the stresses acting on subduction zone thrust faults. Estimated apparent macroscopic rheologies range from elastic to perfectly plastic (or viscous). Forearc rheologies inferred from slip vectors do not correlate with age of the subducting lithosphere, but continental forearcs or old arcs appear to deform less than oceanic or young arcs. The inferred absence of forearc deformation at continental arcs from this study is counter to inferences drawn from compiled geologic information on forearc faults. Correlations of the apparent forearc rheology with backarc spreading, convergence rate, slab dip, arc curvature, and downdip length of the thrust contact are poor. However, great subduction zone earthquakes occur where forearcs are apparently more elastic (i.e., less deformed by oblique convergence), which suggests that the mechanical properties of forearcs rather than stress magnitude on thrust faults control both the kinematic behavior of forearcs and where great subduction zone earthquakes occur. 相似文献
8.
A present-day tectonic stress map for eastern Asia region 总被引：15，自引：1，他引：15
许忠淮 《地震学报(英文版)》2001,14(5):524-533
Introduction Tectonically the eastern Asia refers to the region bounded by the following three active tec-tonic zones: in the east the western Pacific subduction zone, including Japan trench, Ryukyu trench and Philippine trench; in the southwest the Himalaya continental collision zone and the Burma-arc-Java-trench subduction zone; in the northwest the Tianshan-Baikal continental defor-mation zone (Figure 6). In the world the eastern Asia is one of the regions with the strongest pre-sent-da… 相似文献
9.
New advances of seismic tomography and its applications to subduction zones and earthquake fault zones: A review 总被引：9，自引：0，他引：9
Dapeng Zhao 《Island Arc》2001,10(1):68-84
Abstract There have been significant advances in the theory and applications of seismic tomography in the last decade. These include the refinements in the model parameterization, 3-D ray tracing, inversion algorithm, resolution and error analyses, joint use of local, regional and teleseismic data, and the addition of converted and reflected waves in the tomographic inversion. Applications of the new generation tomographic methods to subduction zones have resulted in unprecedentedly clear images of the subducting oceanic lithosphere and magma chambers in the mantle wedge beneath active arc volcanoes, indicating that geodynamic systems associated with the arc magmatism and back-arc spreading are related to deep processes, such as the convective circulation in the mantle wedge and deep dehydration reactions in the subducting slab. High-resolution tomographic imagings of earthquake fault zones in Japan and California show that rupture nucleation and earthquake generating processes are closely related to the heterogeneities of crustal materials and inelastic processes in the fault zones, such as the migration of fluids. Evidence also shows that arc magmatism and slab dehydration may also contribute to the generation of large crustal earthquakes in subduction regions. 相似文献
10.
西太平洋地壳年龄较老,因而岩石层较冷和比重较大,俯冲带的角度也较大,活动和成熟的弧后盆地则较多;条件与之相反的东太平洋弧后盆地则较少.本文探讨这种相关关系的力学成因,计算了俯冲板块诱生的弧后上涌地幔流动.计算表明,俯冲角度大及存在后撤俯冲时,有利于在弧后地区产生明显的上涌地幔流,这种深部热物质的上涌会导致弧后扩张.反之,年龄较轻的海洋地块较热和较轻,俯冲角度一般也较小,不易诱生上涌地幔物质流动和弧后扩张.大陆地壳密度小于地幔物质,大陆碰撞区就更不具备弧后扩张的条件. 相似文献
11.
The seismically active Macquarie Ridge complex forms the Pacific-India plate boundary between New Zealand and the Pacific-Antarctic spreading center. The Late Cenozoic deformation of New Zealand and focal mechanisms of recent large earthquakes in the Macquarie Ridge complex appear consistent with the current plate tectonic models. These models predict a combination of strike-slip and convergent motion in the northern Macquarie Ridge, and strike-slip motion in the southern part. The Hjort trench is the southernmost expression of the Macquarie Ridge complex. Regional considerations of the magnetic lineations imply that some oceanic crust may have been consumed at the Hjort trench. Although this arcuate trench seems inconsistent with the predicted strike-slip setting, a deep trough also occurs in the Romanche fracture zone.Geoid anomalies observed over spreading ridges, subduction zones, and fracture zones are different. Therefore, geoid anomalies may be diagnostic of plate boundary type. We use SEASAT data to examine the Macquarie Ridge complex and find that the geoid anomalies for the northern Hjort trench region are different from the geoid anomalies for the Romanche trough. The Hjort trench region is characterized by an oblique subduction zone geoid anomaly, e.g., the Aleutian-Komandorski region. Also, limited first-motion data for the large 1924 earthquake that occurred in the northern Hjort trench suggest a thrust focal mechanism. We conclude that subduction is occurring at the Hjort trench. The existence of active subduction in this area implies that young oceanic lithosphere can subduct beneath older oceanic lithosphere. 相似文献
12.
本文分析了青藏高原及其邻区大量近期地震的震源深度分布资料,发现中源地震不仅分布在众所周知的兴都库什和印缅山弧一带,而且在印度洋板块与欧亚板块汇聚带印度河-雅鲁藏布江以南,以及欧亚板块内部的帕米尔、西昆仑、柴达木和天山南缘一带也有中源地震分布,它们构成了这一地区三条向南倾斜的震源带。 这些中源地震震源带的存在表明,向北运动的印度次大陆与亚洲大陆碰撞以后,印度次大陆北缘本身并没有消减,而是迫使亚洲大陆通过三条向南倾斜的岩石层消减带产生了大规模的消减作用。 中源地震在平面上分布的不连续性,揭示了这一地区的许多条走滑断层的现代活动。这些走滑断层的巨大位移显示了青藏高原内部各块体之间的横向运动也是很可观的。 最后,提出了亚洲大陆多条南倾消减带的形成和发展模式。 相似文献
13.
汕头－吕宋岛岩石圈速度结构与震源构造──台湾浅滩7．3级地震构造之一 总被引：1，自引：0，他引：1
汕头－吕宋岛岩石圈速度结构剖面，划分出华南陆缘古生代陆壳、陆架区晚古生代－中生代陆壳、陆坡带中生代－早第三纪过渡壳、新生代南海海盆洋壳及吕宋岛中生代－新生代岛弧陆壳与东吕宋海槽洋壳等地壳构造组分，并确定了上述地壳构造之间的边界断裂构造及其性质。结合地震震源分布及机制，初步确定了华南陆架盆岭构造带北、南两侧地震构造的控震构造与发震构造性质及其震源力学特征；１）指出１９９４年９月１６日台湾浅滩７．３级地震属于板缘壳幔地震及造成一千公里有感范围的原因；２）马尼拉海沟的海底地堑构造与南海海盆岩石圈地幔上隆是马尼拉海沟俯冲带震源显示正断层性质的原因，且为被动的或转换俯冲带；３）东吕宋海槽仍属于菲律宾海俯冲带性质；吕宋岛东西两侧俯冲带岩石圈板片震源深度的准三层分布，可能表明俯冲带岩石圈板片存在相应的低速滑移层。 相似文献
14.
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
15.
Sean C. Solomon 《Physics of the Earth and Planetary Interiors》1975,11(2):97-108
The top of the olivine-spinel phase change in subducted oceanic lithosphere can be located by the travel times of seismic waves which have propagated through the slab. P-wave travel-time residuals from deep earthquakes in the Tonga island are observed at Australian seismic stations are grouped according to the depth of the earthquake. The change in mean residual with a change in earthquake depth is related to the velocity contrast between slab and normal mantle at that depth. The curve mean residual versus earthquake depth displays a region of markedly increased slope between earthquake depths of about 250 and 350 km. The most probable explanation of this observation is an elevation by 100 km of the olivine-spinel phase change within the relatively cooler slab. No evidence was found for vertical displacements within the slab of any deeper phase changes.A temperature contrast between slab and normal mantle of about 1,000°C at 250 km depth is implied. This finding confirms current thermal models for subducted lithosphere but is inconsistent with the global intraplate stress field unless only a few percent of the negative buoyancy force at subduction zones is transmitted to the surface plates. 相似文献
16.
On the initiation of subduction zones 总被引：1，自引：0，他引：1
Analysis of the relation between intraplate stress fields and lithospheric rheology leads to greater insight into the role that initiation of subduction plays in the tectonic evolution of the lithosphere. Numerical model studies show that if after a short evolution of a passive margin (time span a few tens of million years) subduction has not yet started, continued aging of the passive margin alone does not result in conditions more favorable for transformation into an active margin.Although much geological evidence is available in supporting the key role small ocean basins play in orogeny and ophiolite emplacement, evolutionary frameworks of the Wilson cycle usually are cast in terms of opening and closing of wide ocean basins. We propose a more limited role for large oceans in the Wilson cycle concept. In general, initiation of subduction at passive margins requires the action of external plate-tectonic forces, which will be most effective for young passive margins prestressed by thick sedimentary loads. It is not clear how major subduction zones (such as those presently ringing the Pacific Basin) form but it is unlikely they form merely by aging of oceanic lithosphere. Conditions likely to exist in very young oceanic regions are quite favorable for the development of subduction zones, which might explain the lack of preservation of back-arc basins and marginal seas.Plate reorganizations probably occur predominantly by the formation of new spreading ridges, because stress relaxation in the lithosphere takes place much more efficiently through this process than through the formation of new subduction zones. 相似文献
17.
—Spatial variations in mechanical properties of the interplate thrust faults along the Japan and Middle America subduction zones are examined using teleseismic broadband earthquake recordings. Moment-normalized source duration is used to probe rigidity variations along the interface. We invert body waves to estimate source depth and source duration for 40 events in the Japan subduction zone and 38 events in the Middle America subduction zone. For both areas, there is a systematic decrease in source duration with increasing depth along the subduction zone interface. This is most likely a result of variation in properties of sediments on the plate contact. Variations in source duration are greatly reduced at depths greater than 18 km in both regions. Enhanced spatial heterogeneity at shallow depth may reflect variations in plate roughness, sediment distribution, permeability of the fault zone, and stress. 相似文献
18.
A recently established relation between the penetration depth of oceanic lithosphere and the lithospheric age appears to be of special interest to the understanding of the South American subduction zone. The main characteristics of this complicated zone, such as the absence of deep-focus earthquakes south of 30°S, the variations in the dip angle of the descending Nazca plate and the gap in seismic activity between depths of approximately 300 and 525 km, can be understood if the spatial and temporal variations in the age of the descending oceanic lithosphere are taken into account. In view of the significance of local aspects of the subduction process the South American-Nazca plate interaction cannot simply be considered as a type-example of the interaction between a continental and an oceanic plate. 相似文献
19.
采用有限元方法模拟了俯冲带耦合作用对巽他弧及其邻区的影响.根据模拟结果,对比GPS、地震和地质学观测数据,定量分析了苏门答腊及其周边地区的应变强度和主应变方向的分布特征,据此探讨了该区构造特征、地震发生模式与耦合面积之间的关系.模型由具有黏弹性性质的岩石圈和软流圈上地幔组成,其中岩石圈包括了大陆岩石圈和大洋岩石圈以及俯冲至上地幔中的俯冲板片.研究结果如下:(1) 通过对不同俯冲带耦合面积模拟,发现苏门答腊前弧伴随耦合面积的增加应变强度逐渐增大,而增大的应变强度又影响了其周边地区的应变分布特征,因此整个苏门答腊前弧呈现出明显的分段性,这与该区地震破裂模式有较好的对应.(2)苏门答腊北部地区主应变方向与南部相比存在一定的差异,该差异是俯冲带的俯冲方向、俯冲速度、俯冲形态以及不同区域间耦合面积共同作用的结果.(3)虽然苏门答腊2004年地震主震区处于弱耦合状态,但从本文模拟的结果中可以看到,在俯冲作用下该区依然存在垂直向下的位移,这为地震激发海啸提供了有利的构造环境. 相似文献
20.
Lattice-Preferred orientations of olivine in subducting oceanic lithosphere derived from the observed seismic anisotropies in double seismic zones 
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下载免费PDF全文 Subduction zones can generally be classified into Mariana type and Chilean type depending on plate ages,plate thicknesses, subduction angles, back-arc deformation patterns, etc. The double seismic zones(DSZs) in subduction zones are mainly divided into type I and type II which, respectively, correspond to the Mariana type and Chilean type in most cases. Seismic anisotropy is an important parameter characterizing the geophysical features of the lithosphere, including the subduction zones,and can be described by the two parameters of delay time dt and fast wave polarization direction /. We totally collected 524 seismic anisotropy data records from 24 DSZs and analyzed the statistical correlations between seismic anisotropy and the related physical parameters of DSZs.Our statistical analysis demonstrated that the fast wave polarization directions are parallel to the trench strike with no more than 30° for most type I DSZs, while being nearlyperpendicular to the trench strike for type II DSZs. We also calculated roughly linear correlations that the delay time dt increases with dip angles but decreases with subduction rates. A linear equation was summarized to describe the strong correlation between DSZ's subduction angle aDSZ and seismic anisotropy in subduction zones. These results suggest that the anisotropic structure of the subducting lithosphere can be described as a possible equivalent crystal similar to the olivine crystal with three mutually orthogonal polarization axes, of which the longest and the second axes are nearly along the trench-perpendicular and trench-parallel directions, respectively. 相似文献