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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
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晋冀鲁豫交界地区震源位置及震源区速度结构的联合反演 总被引：1，自引：0，他引：1
利用邯郸数字台网记录到的2001—2008年间460次ML≥1.0地震的1861条P波到时数据, 采用震源位置和速度结构联合反演方法确定晋冀鲁豫交界地区(35.0°~38.0°N, 113.0°~116.0°E)地震的震源位置分布和该区域的速度结构。 结果表明: ① 经过重新定位后, P波走时的均方根残差(RMS)由反演前的1.35 s降到反演后的0.45 s。 定位偏差在EW方向上平均为0.031 km, 在NS方向上平均为0.029 km, 在垂直方向上平均为0.060 km。 ② 邢台震区的中小地震明显呈NEE向分布, 深度主要集中分布在7~14 km范围内; 磁县震区中小震分布相对复杂, 具有NEE和NWW两个展布方向, 震源深度主要集中在8~18 km范围内, 总体上晋冀鲁豫交界地区中小地震深度呈现北部浅南部深的趋势。 ③ 反演得到了晋冀鲁豫交界地区的速度结构, 在邢台地震极震区下方7~14 km处存在低速层, 与1966年邢台7.2级地震的震源深度一致;在磁县地震极震区下方13~18 km处也存在低速层与1831年磁县7.5级地震震源深度一致, 且磁县震区下方的速度结构比邢台震区更为复杂。 相似文献
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汤加的俯冲板块结构复杂,板块形态的起因和形成机制不是很清晰.为了探索其演化,我们借助于板块运动学、海底年龄和关键的大地构造特征,用地球动力学模型模拟了本区域40 Ma之后的俯冲历史.通过研究我们发现,沿着海沟的分布位置,不同的黏度、流体的吸力与浮力比特征控制着板块的力学强度,从而影响着板块的俯冲角度和形态.我们的模型显示在俯冲板块形成前,研究区表面西侧的黏度剖面形成的弱化带为水平低黏度区,它对俯冲的发生有一定的引导作用,高黏度的上覆板块的存在和俯冲导致的地幔流体的吸力降低了俯冲板块的倾角,15°S附近的俯冲板块的分离发生在16-8 Ma之间.最后我们通过匹配实际的构造特征,包括贝尼奥夫带和波速结构剖面,用所得的相关性最佳的模型对本区的地幔结构随时间演化提供探索. 相似文献
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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
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根据准动力学计算方案，通过采用等效热源和等效热传导系数的方法，用有限元法计算了不同俯冲角度，而俯冲速度为８ｃｍ／ａ、年龄为１００Ｍａ的俯冲带在稳定俯冲状态的热结构．计算结果表明俯冲带在接近６７０ｋｍ间断面的最低温度可达到１１００℃．全地幔对流模式热结构的计算结果表明６７０ｋｍ间断面以下可存在最低温度达１０００℃的低温区，相应于有０．７％-３．０％的Ｐ波低速异常存在．双层地幔对流模式表明，在６７０ｋｍ间断面以上可有与周围地幔相差约４００℃的水平舌状低温区存在，相应于０．５％-１．４％的Ｐ波低速异常． 相似文献
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西南太平洋板块与澳大利亚板块之间的汤加—克马德克俯冲带,是研究地球动力学最重要的区域之一.本文研究根据MIT-P08地震数据,结合板块构造边界、地震活动分布、海岸地形数据等,基于GOCAD软件平台建立三维地震层析成像,对西南太平洋板块的汤加—克马德克俯冲板片进行三维解释.地震层析成像显示汤加—斐济地区地幔至少存在三个"高速"异常体.早期汤加—克马德克俯冲板片穿过地幔转换带,并进入下地幔,最大深度达1600 km.三维构造模型揭示了汤加—克马德克板片在深度600~800 km处存在断折形变,该俯冲板片去褶皱恢复后,测量其俯冲的最大位移达2600 km.汤加—克马德克板片开始快速俯冲的时间至少在30 Ma之前,平均移动速率约为68~104 mm /a.俯冲板片三维构造重建和恢复,可以有效揭示俯冲板片几何学、运动学,为研究深源地震成因、地球深部变化过程和动力学机制提供约束. 相似文献
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《Earth and Planetary Science Letters》2006,241(3-4):387-397
Thermal models of subduction zones often base their slab–wedge geometry from seismicity at mantle depths and, consequently, cannot be used to evaluate the relationship between seismicity and structure. Here, high-resolution seismic observations from the recent Broadband Experiment Across the Alaska Range (BEAAR) constrain, in a rare instance, the subducting slab geometry and mantle wedge temperature independent of seismicity. Receiver functions reveal that the subducting crust descends less steeply than the Wadati-Benioff Zone. Attenuation tomography of the mantle wedge reveals a high Q and presumably cold region where the slab is less than 80 km deep. To understand these two observations, we generate thermal models that use the improved wedge geometry from receiver functions and that incorporate temperature- and strain-rate-dependent olivine rheology. These calculations show that seismicity within the subducting crust falls in a narrow belt of pressure–temperature conditions, illuminating an effective Clapeyron slope of 0.1 K/MPa at temperatures of 450–750 °C. These conditions typify the breakdown of high-pressure hydrous minerals such as lawsonite and suggest that a single set of dehydration reactions may trigger intermediate-depth seismicity. The models also require that the upper, cold nose of the mantle wedge be isolated from the main flow in the mantle wedge in order to sustain the cold temperatures inferred from the Q tomography. Possibly, sufficient mechanical decoupling occurs at the top of the downgoing slab along a localized shear zone to 80 km depth, considerably deeper than inferred from thrust zone seismicity. 相似文献
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洋中脊速度结构是揭示大洋岩石圈演化过程的重要约束.为探讨不同扩张速率下洋中脊的洋壳速度结构特征,挑选了全球152处快速(全扩张速率 90mm·a-1)、慢速(全扩张速率20～50mm·a-1)和超慢速(全扩张速率20mm·a-1)扩张洋中脊和非洋中脊的洋壳1-D地震波速度结构剖面,通过筛选统计、求取平均值等方法对分类的洋壳1-D速度结构进行对比研究,获得了不同扩张速率下洋中脊洋壳速度结构差异以及洋中脊与非洋中脊洋壳速度结构差异的新认识:(1)快速、慢速和超慢速扩张洋中脊的平均正常洋壳厚度分别为6.4km、7.2km和5.3km,其中洋壳层2的厚度基本相似,洋壳厚度差异主要源自洋壳层3;其洋壳厚度变化范围分别为4.9～8.1km、4.6～8.7km和4.2～10.2km,随着洋中脊扩张速率减小,洋壳厚度的变化范围逐渐增大;(2)快速扩张洋中脊的洋壳速度大于慢速和超慢速,可能与快速扩张脊洋壳生成过程中深部高密度岩浆上涌比较充足有关;(3)非洋中脊(10Ma)的洋壳比洋中脊(10Ma)的洋壳厚～0.3km,表明洋壳厚度与洋壳年龄有一定的正相关性. 相似文献
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河砂岩屑热年代学被广泛应用于揭示造山带和流域范围内热演化历史.由于受到地貌特征、剥蚀速率的空间分布、年龄与高程关系等多种因素的影响,河砂岩屑热年代学年龄所代表的意义存在多解性.本文提出了一种利用地貌形态特征和实测河砂热年代学数据模拟流域热史的计算模型.该模型首先利用DEM数据计算流域高程分布特征,通过数据中各象元对应的坡度角大小定量计算剥蚀速率的空间分布,以确定不同高程区域对河砂岩屑样品组分的贡献量.然后根据区域地质特征建立多种可能的热史年龄-高程关系,并模拟计算出与设定的年龄-高程关系相对应的河砂年龄概率分布曲线.最后,通过对模拟河砂年龄概率分布曲线与实测分布曲线的匹配度进行卡方检验,选取最可能形成实测河砂年龄分布的年龄-高程关系,即代表了流域真实的热史演化.通过河砂岩屑磷灰石裂变径迹方法将该模型应用于藏东南地区察隅河两条支流桑曲和贡日嘎布曲流域,模拟计算结果表明两个地区的热史演化均具有多阶段的特征,桑曲流域在38~7Ma之间均匀冷却,对应的剥露速率约为0.14km/Ma,7 Ma以来剥露速率加快,达到1.62km/Ma;贡日嘎布曲的热史年龄记录比桑曲新,18~14 Ma的隆升速率为0.32km/Ma,14~8 Ma比较稳定;8 Ma以来隆升速率逐渐加快,8~5 Ma对应的隆升速率为0.21km/Ma,5~3 Ma为0.43km/Ma,3~1.1 Ma为0.83km/Ma.桑曲的模拟计算结果与前人利用该区域基岩年龄数据所揭示的热史演化特征及剥露速率基本吻合,表明该方法可以准确模拟河砂岩屑年龄所代表的流域热史特征.因此,在地形险峻或者冰川覆盖而无法获取基岩样品的野外地区,可以通过采集河砂样品替代基岩剖面模拟地质体热史特征. 相似文献
11.
BURIAL AND EXHUMATION OF THE XIGAZE FORE-ARC BASIN FROM LOW TEMPERATURE THERMOCHRONOLOGICAL EVIDENCE
The Xigaze fore-arc basin is adjacent to the Indian plate and Eurasia collision zone. Understanding the erosion history of the Xigaze fore-arc basin is significant for realizing the impact of the orogenic belt due to the collision between the Indian plate and the Eurasian plate. The different uplift patterns of the plateau will form different denudation characteristics. If all part of Tibet Plateau uplifted at the same time, the erosion rate of exterior Tibet Plateau will be much larger than the interior plateau due to the active tectonic action, relief, and outflow system at the edge. If the plateau grows from the inside to the outside or from the north to south sides, the strong erosion zone will gradually change along the tectonic active zone that expands to the outward, north, or south sides. Therefore, the different uplift patterns are likely to retain corresponding evidence on the erosion information. The Xigaze fore-arc basin is adjacent to the Yarlung Zangbo suture zone. Its burial, deformation and erosion history during or after the collision between the Indian plate and Eurasia are very important to understand the influence of plateau uplift on erosion.
In this study, we use the apatite fission track(AFT)ages and zircon and apatite(U-Th)/He(ZHe and AHe)ages, combined with the published low-temperature thermochronological age to explore the thermal evolution process of the Xigaze fore-arc basin. The samples' elevation is in the range of 3 860~4 070m. All zircon and apatite samples were dated by the external detector method, using low~U mica sheets as external detectors for fission track ages. A Zeiss Axioskop microscope(1 250×, dry)and FT Stage 4.04 system at the Fission Track Laboratory of the University of Waikato in New Zealand were used to carry out fission track counting. We crushed our samples finely, and then used standard heavy liquid and magnetic separation with additional handpicking methods to select zircon and apatite grains.
The new results show that the ZHe age of the sample M7-01 is(27.06±2.55)Ma(Table 2), and the corresponding AHe age is(9.25±0.76)Ma. The ZHe and AHe ages are significantly smaller than the stratigraphic age, indicating suffering from annealing reset(Table 3). The fission apatite fission track ages are between(74.1±7.8)Ma and(18.7±2.9)Ma, which are less than the corresponding stratigraphic age. The maximum AFT age is(74.1±7.8)Ma, and the minimum AFT age is(18.7±2.9)Ma. There is a significant north~south difference in the apatite fission track ages of the Xigaze fore-arc basin. The apatite fission track ages of the south part are 74~44Ma, the corresponding exhumation rate is 0.03~0.1km/Ma, and the denudation is less than 2km; the apatite fission track ages of the north part range from 27 to 15Ma and the ablation rate is 0.09~0.29km/Ma, but it lacks the exhumation information of the early Cenozoic. The apatite(U-Th)/He age indicates that the north~south Xigaze fore-arc basin has a consistent exhumation history after 15Ma.
The results of low temperature thermochronology show that exhumation histories are different between the northern and southern Xigaze fore-arc basin. From 70 to 60Ma, the southern Xigaze fore-arc basin has been maintained in the depth of 0~6km in the near surface, and has not been eroded or buried beyond this depth. The denudation is less than the north. The low-temperature thermochronological data of the northern part only record the exhumation history after 30Ma because of the young low-temperature thermochronological data. During early Early Miocene, the rapid erosion in the northern part of Xigaze fore-arc basin may be related to the river incision of the paleo-Yarlungzangbo River. The impact of Great Count Thrust on regional erosion is limited. The AHe data shows that the exhumation history of the north-south Xigaze fore-arc basin are consistent after 15Ma. In addition, the low-temperature thermochronological data of the northern Xigaze fore-arc basin constrains geographic range of the Kailas conglomerate during the late Oligocene~Miocene along the Yarlung Zangbo suture zone. The Kailas Basin only develops in the narrow, elongated zone between the fore-arc basin and the Gangdese orogenic belt.
The southern part of the Xigaze fore-arc basin has been uplifted from the sea level to the plateau at an altitude of 4.2km, despite the collision of the Indian plate with the Eurasian continent and the late fault activity, but the plateau has been slowly denuded since the early Cenozoic. The rise did not directly contribute to the accelerated erosion in the area, which is inconsistent with the assumption that rapid erosion means that the orogenic belt begins to rise. 相似文献
12.
We propose a thermal model of the subducting Ionian microplate. The slab sinks in an isothermal mantle, and for the boundary conditions we take into account the relation between the maximum depth of seismicity and the thermal parameter Lth of the slab, which is a product of the age of the subducted lithosphere and the vertical component of the convergence rate. The surface heat-flux dataset of the Ionian Sea is reviewed, and a convective geotherm is calculated in its undeformed part for a surface heat flux of 42 mW m–2, an adiabatic gradient of 0.6 mK m–1, a mantle kinematic viscosity of 1017 m2 s–1 and an asthenosphere potential temperature of 1300°C. The calculated temperature-depth distribution compared to the mantle melting temperature indicates the decoupling limit between lithosphere and asthenosphere occurs at a depth of 105 km and a temperature of 1260°C. A 70–km thick mechanical boundary layer is found. By considering that the maximum depth of the seismic events within the slab is 600 km, a Lth of 4725 km is inferred. For a subduction rate equal to the spreading rate, the corresponding assimilation and cooling times of the microplate are about 7 and 90 Myr, respectively. The thermal model assumes that the mantle flow above the slab is parallel and equal to the subducting plate velocity of 6 cm yr–1, and ignores the heat conduction down the slab dip. The critical temperature, above which the subduced lithosphere cannot sustain the stress necessary to produce seismicity, is determined from the thermal conditions governing the rheology of the plate. The minimum potential temperature at the depth of the deepest earthquake in the slab is 730°C. 相似文献
13.
受俯冲残留体影响的410km间断面起伏形态的研究对于确定地球内部物质构成及地球动力学过程具有重要作用.帕米尔—兴都库什俯冲区域拥有全球少有的中、深源地震,为研究410km间断面起伏提供了良好的资源.利用日本Hi-net地震台网和美国TA台阵记录的帕米尔—兴都库什俯冲区域的6个震源深度为154.0~220.9km、震级为Mb5.6~6.4的中、深源地震的短周期/宽频带波形资料,经过4次根倾斜叠加处理,获得了36组Hi-net子台网和TA记录资料的倾斜叠加灰度图,从中提取了与410km间断面相关的次生转换震相SdP,发现受俯冲残留体影响下的410km间断面的深度位于372~398km.较之持续俯冲的西太平洋地区海洋岩石圈,研究区域俯冲滞留体对于410km间断面的相变线的影响要小得多. 相似文献
14.
在对温度场计算所需的初始条件、边界条件、热源条件和介质热参数进行讨论和计算的基础上,利用二维热传导问题的有限单元法,计算了日本海俯冲带热结构的演化.发现400℃等温线在板块俯冲7Ma后趋于稳定,最大深度约60km;800℃等温线在板块俯冲11Ma后趋于稳定,最大深度约280km;1200℃等温线在板块俯冲50Ma后趋于稳定,最大深度约530km.通过计算不同热源组合情况下日本海俯冲带的热结构,讨论了热源的因素对俯冲带热结构的影响.结果表明,剪切生热和脱水热只对俯冲带浅部热结构有很大影响,绝热压缩生热对热结构的影响范围最大,而橄榄石→尖晶石相变生热是400km深度以下热结构的控制性热源因素. 相似文献
15.
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
16.
A linear relationship between the sea floor depth and the square root of age has been found for ocean lithosphere spreading from mid-ocean ridges. The asymptotic solution of depth as a function of age for the thermally contracting lithosphere predicts a linear dependence of depth ontwith a proportionality involving the initial lithosphere temperature, the thermal diffusivity, and the isostatic expansion coefficient averaged to include any temperature dependent phase changes. Empirical depth observations, when plotted as a function of the square root of age, bear out this prediction well, but there is a variation in the gradient,ht, along the ridge on a fine scale (up to 20% over 200 km). This implies a fundamental variation of the contraction parameter over the same scale, most probably of compositional origin. Details of a more complete cooling model near the ridge crest, including a crust of different thermal parameters than those of the mantle, predict a crestal height about 0.2 km below that of the simplified model. Individual profiles from the southeast Pacific show no such crestal deviation, and it is concluded that by quickly cooling the new crust, hydrothermal circulation may remove any effects of the crust which would be seen in the topography of a lithosphere cooled totally by conduction. The straightness of depth versust for older ocean data (to 80 m.y.) precludes any basal isothermal boundary shallower than 100 km. 相似文献
17.
《Physics of the Earth and Planetary Interiors》1986,43(1):34-46
The behaviour of the magnetic field variations over the Juan de Fuca Plate region is studied using a scaled laboratory analogue model. The model includes a simulation of the complex Juan de Fuca Plate subducting the Vancouver Island region. The subducting plate is modelled with a profile of increasing inclination from east to west; horizontal offshore, dipping at 10° under Vancouver Island, and bending further under Georgia Strait to subduct the continent at 30° for the B.C. region and 45° for the Washington-Oregon region. The strike of the bending plate follows the general strike of the continental coastline with an abrupt change in direction (42°) in the Puget Sound area. The model substructure simulates a subducting plate, overplated by a sediment layer several kilometres thick, and underlain by a 30 km thick highly conducting upper asthenosphere. The model source frequencies used simulate periods 5–120 min in the geophysical scale. In-phase and quadrature Hx, Hy, and Hz magnetic field measurements for the modelled region are presented for an approximately uniform overhead horizontal source field for E- and H-polarizations (electric field of the source approximately parallel and perpendicular, respectively, to the west coast of Vancouver Island). The fields for three regions of the model; over Vancouver Island, over the Olympic Peninsula and over a linear portion of the U.S. coastline, are examined in detail. The general conclusion is that the effect of the dipping subducting plates is to significantly attenuate, at short periods, the maxima in the anomalies at the coastlines underlain by the 10° dipping plate, while leading to anomalous vertical and horizontal fields over ranges as large as 500 km inland over a wide period range. Anomalous fields are observed over the offshore and inland knee-bends of the subducting plates at all periods for both E- and H-polarizations. For locations on land, the in-phase induction arrows point seaward and perpendicular to the strikes of the dipping plates for all periods, while the quadrature arrows at short periods point landward and rotate to point seaward for periods greater than 20 min. 相似文献
18.
《Earth and Planetary Science Letters》2006,241(3-4):789-803
We present a combined method, using sP depth-phase data and double-difference arrival times, to determine the precise hypocenter locations of earthquakes that occur under the Pacific Ocean outside of the area covered by the land-based seismic network. We assess the effectiveness of the combined method using a data set of P- and S-wave arrival times and sP depth phase from suboceanic earthquakes recorded by both land-based seismic stations and offshore seismic stations (OFS). The hypocenters of the offshore earthquakes relocated using the combined method are consistent with those determined using the standard location method and OFS data. The differences in the hypocenters relocated by the two methods are less than 4 km. We applied the method to the subduction region that underlies the Kanto district, central Japan, and located a large number of earthquakes that occurred beneath the Pacific Ocean. We then determined the detailed 3D seismic velocity structure by inverting a large number of arrival times of P- and S-waves and sP depth phase from the relocated earthquakes in the study region. High-velocity anomalies related to the cold subducting Pacific slab and low-velocity anomalies related to the hot mantle wedge are clearly imaged. Beneath active volcanoes, low-velocity zones are visible from the surface to a depth of 100 km, reflecting fluids released by dehydration of the subducting Pacific slab. Strong lateral heterogeneities are revealed on the upper boundary of the Pacific slab beneath the forearc region. The low-velocity areas under the offshore region are associated with low seismicity and weak interplate coupling. A low-velocity layer is imaged along the upper boundary of the Philippine Sea slab in the northern part of Kanto district, which may reflect dehydration of the slab. Our tomographic images indicate that the overlaying Philippine Sea plate has effects on the spatial distribution of active volcanoes related to the subducting Pacific slab in the study region. 相似文献
19.
We have determined the elastic thicknessTe of the oceanic lithosphere along two volcanic chains of the South Central Pacific: Cook-Austral and Society islands. We used a three-dimensional spatial method to model the lithospheric flexure assuming a continuous elastic plate. The model was constrained by geoid height data from the SEASAT satellite.Along the Cook-Austral chain the elastic thickness increases westward, from 2–4 km at McDonald hot spot to 14 km at Rarotonga. At McDonald seamount, however, the data are better explained by a local compensation model. The observed trend shows an increase ofTe with age of plate at loading time. However, the elastic layer under the Cook-Austral appears systematically thinner by several kilometers than expected for “normal” seafloor, suggesting that substantial thermal thinning has taken place in this region. Considering the apparent thermal age of the plate instead of crustal age improves noticeably the results. Along the Society chainTe varies from 20 km under Tahiti to 13 km under Maupiti which is located 500 km westward. When plotting together the Society and Cook-AustralTe results versus age of load, we notice that within the first five million years after loading,Te decreases significantly while tending rapidly to an equilibrium value. This may be interpreted as the effect of initial stress relaxation which occurs just after loading inside the lower lithosphere and suggests that the presently measured elastic thickness under the very young Tahiti load ( 0.8 Ma) is not yet the equilibrium thickness. 相似文献
20.
Koichiro Obana Shuichi Kodaira Yoshiyuki Kaneda 《Earth and Planetary Science Letters》2004,222(2):561-572
Along the Nankai trough, southwestern Japan, the Philippine Sea plate (PSP) is subducting beneath the Eurasian plate, and large interplate earthquakes have occurred repeatedly with a recurrence interval of about 100-200 years. The most recent large thrust event in the eastern Nankai trough off Kii Peninsula was the 1944 Tonankai earthquake. In this region, current seismicity is very low and hypocenters are not determined accurately by the land seismic network. We conducted microseismicity observations around the rupture area of the 1944 Tonankai earthquake using ocean bottom seismographs (OBSs). Hypocenters were determined using a 2-D seismic velocity structure model based on an airgun-OBS seismic survey. Results obtained show that the seismicity was relatively active near the trough axis. These earthquakes may relate to deformation of the subducting Philippine Sea plate. On the other hand, microseismicity in the rupture area of the 1944 Tonankai earthquake was very low. This low-level seismic activity in the co-seismic rupture area of the 1944 Tonankai earthquake likely relates to a single large asperity off Kii Peninsula. 相似文献