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1.
Koki Idehara 《Physics of the Earth and Planetary Interiors》2011,184(1-2):80-90
An ScP phase reflected and converted at the core–mantle boundary (CMB) beneath the region east of the Philippine Islands shows clear pre- and postcursors, recorded on short-period seismic networks in Japan. These waveform variations can be explained by interaction of the ScP wavefield with thin layers at the CMB. The results of forward modeling of double-array stacks reveal two different structural heterogeneities in the lowermost mantle beneath the region east of the Philippine Islands. One of the structures represents a decreased velocity, and increased density across the reflector at the lowermost ～10 km of the mantle, with P- and S-wave velocity reductions of 5–10% and ～30%, respectively, and an increase in density of 5–10%. Another structure consists of a pair of reflectors at ～10 km and ～5 km above the CMB, both of which are characterized by reduced P- and S-wave velocities. The upper reflector is the interface of a low-velocity zone in which P- and S-wave velocities decrease of 10% and 30%, respectively, accompanied by an extremely large increase in density (20–25%). The lower reflector is characterized by a 25% reduction in S-wave velocity relative to the above low-velocity layer, as well as a 5% decrease in P-wave velocity and no change in density. The nature of the low-velocity zone detected locally at the CMB is comparable with that of ultra-low-velocity zones (ULVZs) observed by various seismic probes in the South Pacific and Central America. Extensive observations of the ULVZ beneath the region east of the Philippine Islands indicate massive partial melting at the bottom of the mantle. Low-S-velocity basal layer partly detected within the ULVZ may be resulting from core–mantle chemical interactions, driven by massive partial melting. 相似文献
2.
We compare lateral variations at the base of the mantle as inferred from a global dataset of PcP-P travel time residuals, measured on broadband records, and existing P and S tomographic velocity models, as well as ScS-S travel time data in some selected regions. In many regions, the PcP-P dataset implies short scale lateral variations that are not resolved by global tomographic models, except under eastern Eurasia, where data and models describe a broad region of fast velocity anomalies across which variations appear to be of thermal origin. In other regions, such as central America and southeastern Africa, correlated short scale lateral variations (several hundred kilometers) are observed in PcP and ScS, implying large but not excessive values for the ratio R=∂ ln Vs/∂ ln Vp (∼2.5). On the other hand, in at least two instances, in the heart of the African Plume and on the edge of the Pacific Plume, variations in P and S velocities appear to be incompatible, implying strong lateral gradients across compositionally different domains, possibly also involving topography on the core-mantle boundary. One should be cautious in estimating R at the base of the mantle from global datasets, as different smoothing and sampling of P and S datasets may result in strong biases and meaningless results. 相似文献
3.
郯庐断裂带是一条纵贯我国大陆东部NNE走向的巨型深断裂,其中南段及邻区(115°E—122°E,29°N—38°N)跨越了华北断块区、扬子断块区和华南褶皱系三大一级构造单元,由于其重要性和复杂性,长期以来一直是地学家们研究的热点.本文从国际地震中心(ISC)、中国地震台网及区域地震台网的地震观测报告中精心挑选出6381个Pn震相数据,用Pn波时间项层析成像法反演得到了郯庐断裂带中南段及邻区上地幔顶部Pn波速度结构和各向异性.结果显示,研究区上地幔顶部具有显著的横向非均匀性,相对于7.95km·s-1的平均速度而言,Pn波速度值在7.68~8.24km·s-1范围内变化.Pn波速度分布在郯庐断裂带中段和南段具有分段性:沿中段及周边存在一NE向低速异常带,低速可能是由于岩石圈的减薄和软流圈的高温物质沿郯庐带上涌导致;沿南段表现为一NNE向弱高波速异常带,作为高低速的边界带清晰地勾勒出了华北与扬子这两个不同块体,该边界在江苏域向华北地块NW方向凹进.Pn波速度各向异性的强弱与速度分布存在一定的相关性.总体上,如鲁西隆起及以南等低速区、茅山断裂附近的高低速过渡带,其速度各向异性较为强烈;而在具有高速异常的苏北盆地、合肥盆地等稳定区域下方其各向异性较弱.本文通过Pn波震相基本未能探测到郯庐断裂带中段的方位各向异性,推测是上地幔顶部被"冻结"下来的各向异性痕迹被软流圈热物质上涌这一强烈构造运动削弱所导致.南段具有与断裂伸展方向近乎平行的快波速方向.Pn波速度横向变化和强震活动存在一定关联.强震主要发生在Pn波低速异常区或高低速过渡带上.郯城8.5级地震震中位于中段和南段高低速过渡带,该区域也是速度横向变化最大的地方,最容易集中应力和产生应力差. 相似文献
4.
5.
通过分析全球巨大地震活动周期、活跃时段和活动空间分布特征,探讨未来全球发生巨大地震的活跃时段和主体区域.结果表明:① 全球地震活动存在着置信度远超过95%的50年左右显著活动周期,此外还存在80——100年左右显著性不强的活动周期;② 通过对全球地震MSge;8.3时间序列图和MSge;8.0应变能时间序列图分析,得到活跃期10——14年,平静期39——41年,2004年后又进入了巨大地震的强活跃期,可能会一直持续到2018年;③ 强活跃时段内发生的MSge;8.5巨大地震存在着一定的相关性,太平洋板块北边界（美国阿拉斯加附近区域——阿留申群岛——千岛群岛）是发生8.5级以上地震最关注的地区,其次是太平洋板块的南美洲边界(尤其是秘鲁及其周边区域),应对这两个地区进行重点关注和监测．本文从地震活动性角度研究巨大地震的强活跃期问题,并推测全球发生8.5级以上地震的活跃时段和主体地区,而对于其机理问题值得进一步深入研究． 相似文献
6.
Anomalous high frequency PKKPBC signals (displaying a large amount of energy around 2.5 Hz), recorded globally for deep and intermediate depth earthquakes,
are compared to PKKPAB signals. The attenuation difference
t\textAB* - t\textBC* t_{\text{AB}}^{*} - t_{\text{BC}}^{*} is evaluated from spectral amplitudes in the range 96–111°, being approximately twice the results provided by full-wave theory
and PREM (with no low Qμ zone in the lowermost mantle and a nearly infinite QK in the outer core). Most ray paths for such recordings are piercing the D″ region in the proximity of regions where ultra-low
velocity zones (ULVZ) have been previously reported beneath the North Atlantic Ocean, the Southwest Pacific and the southwestern
part of South America. If BC amplitudes around 2.5 Hz and at low frequencies (0.5–1.5 Hz) are comparable, the observed attenuation
difference (in the frequency range 0.2–2.5 Hz) is small (around 0.25 s) and close to the PREM value. The particle motion of
the high-frequency PKKPBC at 2.5 Hz is quite similar to that of the raw recording, suggesting a deep source. An explanation for this might be scattering
of the BC branch in some very restricted areas of the lowermost mantle. Alternately, the presence of a thin layer with high
attenuation in the D″ region would most likely be associated with either the ultra-low velocity zone (ULVZ) or light sediments
on the underside of the core-mantle boundary (CMB). Correlated to other methods to investigate the lowermost mantle, the high-frequency
PKKPBC can be used to map lateral variations of attenuation above the CMB, possibly associated with the boundary of the superplumes,
especially when PKKPAB is observed. 相似文献
7.
Clement G. Chase 《Earth and Planetary Science Letters》1978,37(3):355-368
Euler vectors (relative angular velocity vectors) have been determined for twelve major plates by global inversion of carefully selected sea-floor spreading rates, transform fault trends, and earthquake slip vectors. The rate information comes from marine magnetic anomalies less than 5 m.y. old, so the motions are valid for post-Miocene times. Plate motions in a mean hotspot frame of reference have also been determined, and statistical confidence limits for all the Euler vectors estimated. Among the consequences of the global motion model is the conclusion that fast-spreading ridges (separation rates greater than 3 cm/yr) have plate motion nearly perpendicular to the strike of the ridge and magnetic anomalies. Four more slowly separating ridges have an average obliquity of spreading of almost 20°.For several plate boundaries, results that differ from previous studies are in agreement with geological evidence. The North and South American plates converge slowly about a pole east of the Antilles and near the Mid-Atlantic Ridge. The results for Africa versus Somalia imply slow east-west extension on the East African Rift Valleys. The pole for motion of Eurasia relative to North America is located near Sakhalin, in accordance with evidence from Siberia and Sakhalin. 相似文献
8.
ABSTRACT Hydrological drought is currently underrepresented in global monitoring systems, mainly due the shortage of near real-time estimates of river discharge at the global scale. In this study, the outputs of the Lisflood model are used to define a low-flow drought index, which shows a good correspondence with long-term records of the Global Runoff Data Centre in the period 1980–2014, as well as with verified information from the literature on six major drought events (covering different regions and watershed sizes). In contrast, the near real-time simulation (from 2015 onward) provides temporally inconsistent estimates over about 20% of the modelled cells (mostly over South America and Central Africa), even if reasonable results are obtained over other regions, as confirmed by intercomparison with the operational outcomes of the European Drought Observatory for the 2018 drought. In spite of the highlighted limitations, valuable information for operational drought monitoring can be retrieved from these simulations. 相似文献
9.
Based on the GPS velocity field data of 1999-2007 and 2011-2013,we used the least squares configuration method and GPS velocity profile results to synthetically analyze the dynamic evolution characteristics of crustal deformation in the Yunnan area before and after the Wenchuan earthquake. The dynamic evolution of GPS velocity field shows that the direction is gradually changed from the south in the southern part of the Sichuan-Yunnan block to the south-west in the southern Yunnan block and there is a clear relative motion characteristic near the block boundary fault zone. Compared with the GPS velocity of 1999-2007, the results of 2011-2013 also reflect segmental deformation characteristics of the block boundary fault zone. Southeast movement shows a significant increase, which may be related to crustal deformation adjustment after the Wenchuan earthquake. The dynamic evolution of strain parameters shows a pattern of "extension in the middle and compression at both ends" in the whole area and the distribution of deformation (shear, extension or compression) is closely related to the background motion and deformation characteristics of the main fault zone. Compared with the results of the period of 1999-2007, the extensional deformation zone of 2011-2013 is expanded eastward and southward. The compressional deformation of the eastern boundary (the Xiaojiang fault zone) of the Sichuan-Yunnan block is no longer significant, which is mainly concentrated in the northern section of the Xiaojiang fault zone and may be related to the post-seismic deformation adjustment of the Wenchuan earthquake. The GPS velocity profile results show that the left-lateral slip velocity of the Xiaojiang fault zone reduced gradually from north to south (10mm/a-5mm/a), and the width of the northern section is wider. The right-lateral slip rate of the Honghe fault zone is about 4mm/a, and the deformation width is wider. The dynamic results show that the Wenchuan earthquake has little effect on the deformation modes of these two fault zones. 相似文献
10.
《Journal of Geodynamics》1999,27(4-5):567-583
Upper mantle P and S wave velocities in the western South America region are obtained at depths of foci from an analysis of travel time data of deep earthquakes. The inferred velocity models for the Chile–Peru–Ecuador region reveal an increase of P velocity from 8.04 km/s at 40 km to 8.28 km/s at 250 km depth, while the S velocity remains almost constant at 4.62 km/s from 40 to 210 km depth. A velocity discontinuity (probably corresponding to the L discontinuity in the continental upper mantle) at 220–250 km depth for P and 200–220 km depth for S waves, with a 3–4% velocity increase, is inferred from the velocity–depth data. Below this discontinuity, P velocity increases from 8.54 km/s at 250 km to 8.62 km/s at 320 km depth and S velocity increases from 4.81 km/s at 210 km to 4.99 km/s at 290 km depth. Travel time data from deep earthquakes at depths greater than 500 km in the Bolivia–Peru region, reveal P velocities of about 9.65 km/s from 500 to 570 km depth. P velocity–depth data further reveal a velocity discontinuity, either as a sharp boundary at 570 km depth with 8–10% velocity increase or as a broad transition zone with velocity rapidly increasing from 560 to 610 km depth. P velocity increases to 10.75 km/s at 650 km depth. A comparison with the latest global average depth estimates of the 660 km discontinuity reveals that this discontinuity is at a relatively shallow depth in the study region. Further, a velocity discontinuity at about 400 km depth with a 10% velocity increase seems to be consistent with travel time observations from deep earthquakes in this region. 相似文献
11.
Global aridification in the second half of the 20th century and its relationship to large-scale climate background 总被引：5，自引：0，他引：5
MA ZhuGuo & FU CongBin Key Laboratory of Regional Climate-Environment Research for Temperate East Asia Institute of Atmospheric Physics Chinese Academy of Sciences Beijing China 《中国科学D辑(英文版)》2007,50(5):776-788
The variation in surface wetness index (SWI), which was derived from global gridded monthly precipi- tation and monthly mean surface air temperature datasets of Climatic Research Unit (CRU), from 1951― 2002 over global land was analyzed in this paper. The characteristics of the SWI variation in global continents, such as North America, South America, Eurasia, Africa, and Australia, were compared. In addition, the correlation between the SWI variation of each continent (or across the globe) and the large-scale background closely related to SST variations, which affects climate change, was analyzed. The results indicate that the SWI variation shows distinct regional characteristics in the second half of the 20th century under global warming. A drying trend in the last 52 years occurred in Africa, Eurasia, Australia and South America, most obviously in Africa and Eurasia. North America shows a wetting trend after 1976. A 30-year period of dry-wet oscillation is found in South America and Australia; the latest is in a drying period in two regions. The results also revealed that global warming has changed the dry-wet pattern of the global land. South America and Australia have a drying trend despite in- creases in precipitation. This indicates that increases in surface air temperature cannot be ignored in aridification studies. Global dry-wet variation is closely related to large-scale SST variations: the drying trend in Africa and Eurasia and the wetting trend in North America are correlated with Pacific Decadal Oscillation (PDO); the interdecadal oscillation of SWI in South America and Australia is consistent with the interdecadal variation in Southern Oscillation Index (SOI). 相似文献
12.
岩石圈-软流圈边界(lithosphere-asthenosphere boundary)是上地幔内具有负速度梯度的地震波速度界面.对俯冲带区域LAB开展地震学探测有助于进一步认识岩石圈和软流圈的相互作用以及与板块俯冲有关的地球动力学过程.本文收集了2006-2012年发生于南美中部地区4个深源地震的垂向宽频带波形资料,利用线性倾斜叠加处理得到了相对走时-慢度域的灰度图,并成功提取了sP在LAB底反射的前驱震相S_(LAB)P.基于改进的一维速度模型IASP91-SA计算得出了6个S_(LAB)P震相反射点的水平分布,并划分为西部(Ⅰ)和东部(Ⅱ)分区:分区工内LAB深度位于60~63 km,平均深度为61 km,起伏为3 km;分区Ⅱ内LAB深度位于78~82 km,平均深度为80 km,起伏为4 km.研究结果显示南美中部地区LAB深度自西向东呈变大的趋势,这可能反映了大陆岩石圈受改造程度的差异.我们推测在靠近海沟的地区,软流圈内部分熔融程度较高且熔体较为富集,对大陆岩石圈的侵蚀作用较强;在远离海沟的地区,软流圈内部分熔融程度降低且熔体分布减少,对大陆岩石圈的侵蚀作用减弱. 相似文献
13.
Statistical properties of small-scale inhomogeneities (wavelengths between 20 and 70 km) near the core-mantle boundary are inferred from scattered core waves. Observations of scattered core waves at large seismic arrays and worldwide networks indicate that the inhomogeneities have a global nature with similar characteristics. However, there may exist a few regions having markedly stronger or weaker strengths. Scattering by volumetric inhomogeneities of about 1% inP-wave velocity in the lower mantle or by about 300 m of topographic relief of the core-mantle boundary can explain the observations. At present it is not possible to rule out either of these two alternatives, or a combination of both. 相似文献
14.
Current state of knowledge regarding South America wetlands and their future under global climate change 总被引：1，自引：0，他引：1
Wolfgang J. Junk 《Aquatic Sciences - Research Across Boundaries》2013,75(1):113-131
The exact size of the wetland area of South America is not known but may comprise as much as 20% of the sub-continent, with river floodplains and intermittent interfluvial wetlands as the most prominent types. A few wetland areas have been well studied, whereas little is known about others, including some that are very large. Despite the fact that most South American countries have signed the Ramsar convention, efforts to elaborate basic data have been insufficient, thereby hindering the formulation of a wetland-friendly policy allowing the sustainable management of these areas. Until now, the low population density in many wetland areas has provided a high level of protection; however, the pressure on wetland integrity is increasing, mainly as a result of land reclamation for agriculture and animal ranching, infrastructure building, pollution, mining activities, and the construction of hydroelectric power plants. The Intergovernmental Panel on Climate Change has predicted increasing temperatures, accelerated melting of the glaciers in Patagonia and the Andes, a rise in sea level of 20–60 cm, and an increase in extreme multiannual and short-term climate events (El Niño and La Niña, heavy rains and droughts, heat waves). Precipitation may decrease slightly near the Caribbean coast as well as over large parts of Brazil, Chile, and Patagonia, but increase in Colombia, Ecuador, and Peru, around the equator, and in southeastern South America. Of even greater impact may be a change in rainfall distribution, with precipitation increasing during the rainy season and decreasing during the dry season. There is no doubt that the predicted changes in global climate will strongly affect South American wetlands, mainly those with a low hydrologic buffer capacity. However, for the coming decades, wetland destruction by wetland-unfriendly development planning will by far outweigh the negative impacts of global climate change. South American governments must bear in mind that there are many benefits that wetlands bring about for the landscape and biodiversity as well as for humans. While water availability will be the key problem for the continent’s cities and agroindustries, intact wetlands can play a major role in storing water, buffering river and stream discharges, and recharging subterranean aquifers. 相似文献
15.
Fang Shen Xueshang Feng Changqing Xiang Wenbin Song 《Journal of Atmospheric and Solar》2010,72(13):1008-1018
The basic characteristics of the global distribution for the corona plasma and magnetic field near 2.5 Rs are analyzed with the statistical and numerical methods for 136 Carrington Rotations (CRs) covering four different phases of solar activity. By using the observational data and the velocity distribution model in the corona, the statistical average distribution of the magnetic field, density and the coronal mass outputs are analyzed for the four different phases. Then, a numerical study of the global distribution near 2.5 Rs has been made by solving a self-consistent MHD system. Finally, the solar wind speed at 1 AU is given by mapping the speed at 2.5 Rs to that near 1 AU, and the comparison of the numerical results with the observational measurements and the simulation result of the Wang–Sheeley–Arge (WSA) model are made during more than 5 years. The numerical results indicate that the global distributions on the source surface of 2.5 Rs at different phases of solar activity could be used to predict the change of the solar wind in interplanetary space. 相似文献
16.
Mikhail K. Kaban Alexey G. Petrunin Harro Schmeling Meysam Shahraki 《Surveys in Geophysics》2014,35(6):1361-1373
A joint effect of weak zones, dividing lithospheric plates, and lateral viscosity variations (LVV) in the whole mantle on the observed geoid is investigated by a new numerical approach. This technique is based on the substantially revised method introduced by Zhang and Christensen (Geophys J Int 114:531–547, 1993) for solving the Navier–Stokes–Poisson equations in the spectral domain with strong LVV. Weak plate boundaries (WPB) are introduced based on an integrated global model of plate boundary deformations GSRM (Kreemer et al. in Geophys J Int 154:8–34, 2003). The effect of WPB on the geoid is significant and reaches ?40 to 70 m with RMS ~20 m. The peaks are observed over large subduction zones in South America and the southwestern Pacific in agreement with previous studies. The positive geoid anomaly in South America could be explained largely by a dynamic effect of decoupling of the Nazca and South American plates. The negative changes of the geoid mostly relate to mid-oceanic ridges. The amplitude of the effect depends on the viscosity contrasts at WPB compared with the plate viscosity until its value reaches the limit of 2.5–3 orders of magnitude. This value might be considered as a level at which the plates are effectively decoupled. The effect of WPB exceeds the effect of LVV in the whole mantle and generally does not correlate with it. However, inclusion of LVV reduces the geoid perturbations due to WPB by about 10 m. Therefore, it is important to consider all factors together. The geoid changes mainly result from changes of the dynamic topography, which are about ?300 to +500 m. The obtained results show that including WPB may significantly improve the reliability of instantaneous global dynamic models. 相似文献
17.
选取太平洋板块南部边界的板块相对运动速度不同的两个洋脊－洋脊－转换断层（RRF）型三联点,即麦夸里（Macquarie）三联点和南太平洋三联点,为研究对象,通过数值模拟的方法,研究该类型三联点走滑断层边界两侧的板块相对运动速度对三联点附近地区地幔流动场和温度结构的影响。模拟结果表明:太平洋南部边界RRF三联点走滑断层边界两侧的板块相对运动速度控制着三联点附近的温度分布和地幔流动;随着走滑断层边界两侧板块相对运动速度的增加,转换断层相对滑动速度增加,温度上升,距洋脊边界100 km范围内的地幔流体速度变大;麦夸里三联点和南太平洋三联点处3个板块的相对运动,使得三联点的转换断层边界浅部产生剪应力集中,导致震源深度集中在15—25 km;同时相对运动产生的地幔流动引起温度结构变化,该变化控制着地形变化。 相似文献
18.
用高分辨率地震体波速度成像以及相关的地球物理资料,计算地幔垂直流动形式及流动速度,得到全球地幔流垂直运动模式.从全球尺度来看,地幔流基本可划分为以下几个区域：欧亚大陆—澳大利亚、北美洲—南美洲为两个大规模下降流区域,西印度洋—非洲及大西洋、中南太平洋及东太平洋为两个大规模地幔上升流区域.地幔上升流起源于核幔边界,主要表现在地幔中部和上地幔下部.地幔垂直流动速度约每年1～4cm.地幔流动对地表板块运动、海洋中脊和中隆、俯冲带和碰撞带的分布起着控制作用.地幔上升流与地表现代热点有密切关系.从东亚尺度看,地幔流大体分为三个区域：东亚边缘裂谷系和西太平洋边缘海为上升流、西伯利亚地幔深度表现为物质下降流、青藏高原—缅甸—印度尼西亚特提斯俯冲带地幔下降流,这三个区域地幔流动与地表的西太平洋构造域、亚洲构造域和特提斯构造域相吻合.勾勒出南海地区构造特征：从上到下的大体结构是上部呈“工"字型、中间为圆柱型、底部呈盾形的地幔上升流. 相似文献
19.