Evolution and diversity of subduction zones controlled by slab width

Subducting slabs provide the main driving force for plate motion and flow in the Earth's mantle, and geodynamic, seismic and geochemical studies offer insight into slab dynamics and subduction-induced flow. Most previous geodynamic studies treat subduction zones as either infinite in trench-parallel extent (that is, two-dimensional) or finite in width but fixed in space. Subduction zones and their associated slabs are, however, limited in lateral extent (250-7,400 km) and their three-dimensional geometry evolves over time. Here we show that slab width controls two first-order features of plate tectonics-the curvature of subduction zones and their tendency to retreat backwards with time. Using three-dimensional numerical simulations of free subduction, we show that trench migration rate is inversely related to slab width and depends on proximity to a lateral slab edge. These results are consistent with retreat velocities observed globally, with maximum velocities (6-16 cm yr(-1)) only observed close to slab edges (<1,200 km), whereas far from edges (>2,000 km) retreat velocities are always slow (<2.0 cm yr(-1)). Models with narrow slabs (< or =1,500 km) retreat fast and develop a curved geometry, concave towards the mantle wedge side. Models with slabs intermediate in width ( approximately 2,000-3,000 km) are sublinear and retreat more slowly. Models with wide slabs (> or =4,000 km) are nearly stationary in the centre and develop a convex geometry, whereas trench retreat increases towards concave-shaped edges. Additionally, we identify periods (5-10 Myr) of slow trench advance at the centre of wide slabs. Such wide-slab behaviour may explain mountain building in the central Andes, as being a consequence of its tectonic setting, far from slab edges.     

Evidence for fault weakness and fluid flow within an active low-angle normal fault.

Determining the composition and physical properties of shallow-dipping, active normal faults (dips < 35 degrees with respect to the horizontal) is important for understanding how such faults slip under low resolved shear stress and accommodate significant extension of the crust and lithosphere. Seismic reflection images and earthquake source parameters show that a magnitude 6.2 earthquake occurred at about 5 km depth on or close to a normal fault with a dip of 25-30 degrees located ahead of a propagating spreading centre in the Woodlark basin. Here we present results from a genetic algorithm inversion of seismic reflection data, which shows that the fault at 4-5 km depth contains a 33-m-thick layer with seismic velocities of about 4.3 km s(-1), which we interpret to be composed of serpentinite fault gouge. Isolated zones exhibit velocities as low as approximately 1.7 km s(-1) with high porosities, which we suggest are maintained by high fluid pressures. We propose that hydrothermal fluid flow, possibly driven by a deep magmatic heat source, and high extensional stresses ahead of the ridge tip have created conditions for fault weakness and strain localization on the low-angle normal fault.     

高速碰撞下二次破片光滑粒子流体力学法数值模拟

采用AUTODYN-2D无网格光滑粒子流体力学方法(SPH)数值技术,对遭遇速度为1.5～2.5 km/s下圆柱钨弹丸垂直碰撞装甲靶板产生的二次破片云分布特征进行了数值模拟,获得了不同碰撞速度下靶板穿孔直径及破片云前端运动速度、侧向膨胀速度和最大飞散角等分布特征.在此基础上,建立了二次破片云质量及空间分布预测模型.结果表明,数值模拟和模型预测均与实验结果相吻合.     

Solar chromospheric spicules from the leakage of photospheric oscillations and flows

Spicules are dynamic jets propelled upwards (at speeds of approximately 20 km s(-1)) from the solar 'surface' (photosphere) into the magnetized low atmosphere of the Sun. They carry a mass flux of 100 times that of the solar wind into the low solar corona. With diameters close to observational limits (< 500 km), spicules have been largely unexplained since their discovery in 1877: none of the existing models can account simultaneously for their ubiquity, evolution, energetics and recently discovered periodicity. Here we report a synthesis of modelling and high-spatial-resolution observations in which numerical simulations driven by observed photospheric velocities directly reproduce the observed occurrence and properties of individual spicules. Photospheric velocities are dominated by convective granulation (which has been considered before for spicule formation) and by p-modes (which are solar global resonant acoustic oscillations visible in the photosphere as quasi-sinusoidal velocity and intensity pulsations). We show that the previously ignored p-modes are crucial: on inclined magnetic flux tubes, the p-modes leak sufficient energy from the global resonant cavity into the chromosphere to power shocks that drive upward flows and form spicules.     

Global anisotropy and the thickness of continents

For decades there has been a vigorous debate about the depth extent of continental roots. The analysis of heat-flow, mantle-xenolith and electrical-conductivity data all indicate that the coherent, conductive part of continental roots (the 'tectosphere') is at most 200-250 km thick. Some global seismic tomographic models agree with this estimate, but others suggest that a much thicker zone of high velocities lies beneath continental shields, reaching a depth of at least 400 km. Here we show that this disagreement can be reconciled by taking into account seismic anisotropy. We show that significant radial anisotropy, with horizontally polarized shear waves travelling faster than those that are vertically polarized, is present under most cratons in the depth range 250-400 km--similar to that found under ocean basins at shallower depths of 80-250 km. We propose that, in both cases, the anisotropy is related to shear in a low-viscosity asthenospheric channel, located at different depths under continents and oceans. The seismically defined 'tectosphere' is then at most 200-250 km thick under old continents. The 'Lehmann discontinuity', observed mostly under continents at about 200-250 km, and the 'Gutenberg discontinuity', observed under oceans at depths of about 60-80 km, may both be associated with the bottom of the lithosphere, marking a transition to flow-induced asthenospheric anisotropy.     

高速电动列车受电弓的主动控制

在高速(可至400km·h~(-1))电动列车的设计中,为了确保动力采集装置正常运行,若采用传统的的被动受电弓,需配置复式链形悬挂系统;而若配置简单链形悬挂系统,则需采用主动控制的受电弓。由于接触网系统是固定设备投资的主要部分,因此,有必要研究受电弓的主动控制,以使列车能运行于现行简单链形悬挂系统从而降低成本。文中受电弓弓头高度与接触力的模拟结果以计算程序FAST(Fahrleitung-Stromabnehmer)在时速直至360km·h~(-1)的情况下得到。与采用传统的被动受电弓所得的结果比较,主动控制受电弓显示了优越性。     

Mega-impact formation of the Mars hemispheric dichotomy

The Mars hemispheric dichotomy is expressed as a dramatic difference in elevation, crustal thickness and crater density between the southern highlands and northern lowlands (which cover approximately 42% of the surface). Despite the prominence of the dichotomy, its origin has remained enigmatic and models for its formation largely untested. Endogenic degree-1 convection models with north-south asymmetry are incomplete in that they are restricted to simulating only mantle dynamics and they neglect crustal evolution, whereas exogenic multiple impact events are statistically unlikely to concentrate in one hemisphere. A single mega-impact of the requisite size has not previously been modelled. However, it has been hypothesized that such an event could obliterate the evidence of its occurrence by completely covering the surface with melt or catastrophically disrupting the planet. Here we present a set of single-impact initial conditions by which a large impactor can produce features consistent with the observed dichotomy's crustal structure and persistence. Using three-dimensional hydrodynamic simulations, large variations are predicted in post-impact states depending on impact energy, velocity and, importantly, impact angle, with trends more pronounced or unseen in commonly studied smaller impacts. For impact energies of approximately (3-6) x 10(29) J, at low impact velocities (6-10 km s(-1)) and oblique impact angles (30-60 degrees ), the resulting crustal removal boundary is similar in size and ellipticity to the observed characteristics of the lowlands basin. Under these conditions, the melt distribution is largely contained within the area of impact and thus does not erase the evidence of the impact's occurrence. The antiquity of the dichotomy is consistent with the contemporaneous presence of impactors of diameter 1,600-2,700 km in Mars-crossing orbits, and the impact angle is consistent with the expected distribution.     

First imaging observation of the gravity waves in the mesopause region in China

The first imaging observation experiment of the atmospheric gravity waves in the mesopause region with the all-sky OH Meinel band airglow CCD imager is made during 02:00 to 06:00 on January 5,2009,at Hancun (39.4°N,116.6°E),Langfang,Hebei in China.Quasi-monochromatic gravity waves (QMGWs) are found clearly in the observation data.Case studies show that their horizontal wavelengths,horizontal phase velocities,azimuth angles of horizontal propagation directions and periods of three QMGW examples are about 12....     

The velocity structure of the Yinggehai Basin and its hydrocarbon implication

Two profiles of refraction and wide_angle reflection were completed in the Yinggehai Basin using ocean bottom hydrophones (OBHs) as receiver. These profiles reveal a low velocity layer (LVL) to the west of No. 1 Fault in the basin, with velocity of 3.0-3.5 km/s. Its upper interface lies at 3.5 km below the sea bottom at the NE end of the profiles, and deepens southwestward to 4 6 km near the axis of the basin. It coincides with the top of a gas_bearing, high_pressure layer defined by well logging. This suggests that the LVL is a layer of less_compacted and gas_charged sediments. The lower interface of the LVL is about 9 km in depth. Sediments above the LVL have velocity varying from 2 0 to 4 1 km/s. The acoustic basement is found only east of No. 1 Fault, with velocity of 5 8-6 2 km/s. A NNW_SSE_running OBH seismic profile across the Xisha Trough was completed on the same cruise. Moho and other deep interfaces are clearly visible in the profile.     

Analysis of characteristic of aurora substorm observed at Antarctic Zhongshan station

The characteristics of a strong aurora substorm observed at Antarctic Zhongshan station (magnetic latitude=74.5°) on 8 April, 1999, were discussed and analyzed. The developing steps of the aurora substorm that happened in dusk time were almost the same with that of midnight aurora substorm. The averaged moving speed of the aurora arc toward pole area during the substorm expansion phase was about 3.0 km/s, westward-traveling surge speed was about 2.0 km/s. The extension from south to north in the substorm can cover 1100 km in distance. Foundation item: Supported by the National Natural Science Foundation of China (49634160) Biography: Ai Young (1958-), male, Associate professor, research direction: upper atmosphere physics.     

North China sub-craton lithospheric structure elucidated through coal mine blasting

We present a super-range seismic observation along the >1300-km-long profile passing through the Yinchuan basin and the Ordos block from the blasting point towards the southeast triggered by a large-(dynamite) scale coal blast in the Ningxia Hui Autonomous Region’s Helan Mountain (yielded a profile). The seismic wave information from the uppermost mantle reflecting different depths was obtained by the China continental seismic survey. Pn refracted waves from the uppermost mantle were effectively traced up t...     

Lower-crustal intrusion on the North Atlantic continental margin

When continents break apart, the rifting is sometimes accompanied by the production of large volumes of molten rock. The total melt volume, however, is uncertain, because only part of it has erupted at the surface. Furthermore, the cause of the magmatism is still disputed-specifically, whether or not it is due to increased mantle temperatures. We recorded deep-penetration normal-incidence and wide-angle seismic profiles across the Faroe and Hatton Bank volcanic margins in the northeast Atlantic. Here we show that near the Faroe Islands, for every 1 km along strike, 360-400 km(3) of basalt is extruded, while 540-600 km(3) is intruded into the continent-ocean transition. We find that lower-crustal intrusions are focused mainly into a narrow zone approximately 50 km wide on the transition, although extruded basalts flow more than 100 km from the rift. Seismic profiles show that the melt is intruded into the lower crust as sills, which cross-cut the continental fabric, rather than as an 'underplate' of 100 per cent melt, as has often been assumed. Evidence from the measured seismic velocities and from igneous thicknesses are consistent with the dominant control on melt production being increased mantle temperatures, with no requirement for either significant active small-scale mantle convection under the rift or the presence of fertile mantle at the time of continental break-up, as has previously been suggested for the North Atlantic Ocean.     

基于遥感的合肥市建成区时空变化分析

以合肥市为研究区,以6期Landsat卫星影像为数据源,采用目视解译法和支持向量机法对合肥市遥感影像进行监督分类与精度评价,在此基础上结合紧凑度指数、分形维数、重心转移指数等指标分析合肥市建成区的时空变化.研究结果表明:(1)1995—2013年合肥市建成区扩展以边缘式扩张形式为主,由主城区边缘向外以发射状延伸;201...     

Sound velocities of majorite garnet and the composition of the mantle transition region

The composition of the mantle transition region, characterized by anomalous seismic-wave velocity and density changes at depths of approximately 400 to 700 km, has remained controversial. Some have proposed that the mantle transition region has an olivine-rich 'pyrolite' composition, whereas others have inferred that it is characterized by pyroxene- and garnet-rich compositions ('piclogite'), because the sound velocities in pyrolite estimated from laboratory data are substantially higher than those seismologically observed. Although the velocities of the olivine polymorphs at these pressures (wadsleyite and ringwoodite) have been well documented, those of majorite (another significant high-pressure phase in the mantle transition region) with realistic mantle compositions have never been measured. Here we use combined in situ X-ray and ultrasonic measurements under the pressure and temperature conditions of the mantle transition region to show that majorite in a pyrolite composition has sound velocities substantially lower than those of earlier estimates, owing to strong nonlinear decreases at high temperature, particularly for shear-wave velocity. We found that pyrolite yields seismic velocities more consistent with typical seismological models than those of piclogite in the upper to middle parts of the region, except for the potentially larger velocity jumps in pyrolite relative to those observed at a depth of 410 km. In contrast, both of these compositions lead to significantly low shear-wave velocities in the lower part of the region, suggesting possible subadiabatic temperatures or the existence of a layer of harzburgite-rich material supplied by the subducted slabs stagnant at these depths.     

Possible seismic reflector in the lower crust： Evidence from fabrics and experiments of seismic velocity on layered gabbro at high temperature and high pressure

Lattice preferred orientations (LPO) of plagioclase and augite are measured on layered gabbro from the Panxi region, Sichuan Province. The LPO concentration [010] of plagioclase and [100] of augite are perpendicular to the foliation, which indicates a kind of growth fabric associated with crystallizing habits of minerals when the magma is solidifying under the compaction. Calculated seismic velocities based on LPO data of minerals give rise to rather strong anisotropy 5.81% and 5.54% for compressional seismic wave (Vp) and shear seismic wave (Vs), respectively. The experiments at high temperature and high pressure show that the P-wave velocity of layered gabbro is 6.4-6.97 km/s with the maximum Vp anisotropy 5.22% and the Poisson‘s ratio is between 0.28-0.31. According to the comparison of fabrics with seismic velocities of layered gabbro, it is suggested that the large-scale layered intrusive body or the similar layered geological body may exist in the lower crust of this area. Such a layered intrusive body which has strong seismic anisotropy may be the seismic reflector in the lower crust.     

观山湖区城镇用地拓展及驱动力分析

以LandsatTM/ETM卫星影像数据为基础,采用面向对象分类技术提取观山湖区2000年、2005年和2010年3期土地覆被数据。观山湖区城镇用地面积2000年为12.02km2,2010年为51.13km2,净增长了39.11km2,增长率为325.37%,年均增长率为32.54%,城镇用地2000年到2010年总体呈快速拓展。观山湖区城镇用地的快速拓展是受自然驱动因素、社会经济因素以及贵阳市城市规划的辐射带动综合作用的结果。     

Tracking the rupture of the Mw = 9.3 Sumatra earthquake over 1,150 km at teleseismic distance

On 26 December 2004, a moment magnitude Mw = 9.3 earthquake occurred along Northern Sumatra, the Nicobar and Andaman islands, resulting in a devastating tsunami in the Indian Ocean region. The rapid and accurate estimation of the rupture length and direction of such tsunami-generating earthquakes is crucial for constraining both tsunami wave-height models as well as the seismic moment of the events. Compressional seismic waves generated at the hypocentre of the Sumatra earthquake arrived after about 12 min at the broadband seismic stations of the German Regional Seismic Network (GRSN), located approximately 9,000 km from the event. Here we present a modification of a standard array-seismological approach and show that it is possible to track the propagating rupture front of the Sumatra earthquake over a total rupture length of 1,150 km. We estimate the average rupture speed to be 2.3-2.7 km s(-1) and the total duration of rupture to be at least 430 s, and probably between 480 and 500 s.     

IS-20003x反向链路仿真研究

IS－－2000是美国向ITU提出的第三代移动通信空中接口,其中IS－－2000－3x能够满足IMT－2000的性能要求,但目前在技术实现方面尚未成熟,本文着重对车载环境下IS－－2000－3x反向链路的性能进行仿真研究,提出了相应的RAKE接收机模型,并针对100km/h,300km/h和500km/h三种车辆行驶速度分别进行76．8kbit/s和153.6kbit／s两种数据传输速率的性能仿真,结果表明,只要RAKE接收机的信道估计长度选取能够自适应于数据传输速率和终端移动速率,即可获得良好的数据输性能,满足IMT－－2000的要求。     

No meridional plasma flow in the heliosheath transition region

Over a two-year period, Voyager 1 observed a gradual slowing-down of radial plasma flow in the heliosheath to near-zero velocity after April 2010 at a distance of 113.5 astronomical units from the Sun (1 astronomical unit equals 1.5?×?10(8) kilometres). Voyager 1 was then about 20 astronomical units beyond the shock that terminates the free expansion of the solar wind and was immersed in the heated non-thermal plasma region called the heliosheath. The expectation from contemporary simulations was that the heliosheath plasma would be deflected from radial flow to meridional flow (in solar heliospheric coordinates), which at Voyager?1 would lie mainly on the (locally spherical) surface called the heliopause. This surface is supposed to separate the heliosheath plasma, which is of solar origin, from the interstellar plasma, which is of local Galactic origin. In 2011, the Voyager project began occasional temporary re-orientations of the spacecraft (totalling about 10-25 hours every 2 months) to re-align the Low-Energy Charged Particle instrument on board Voyager?1 so that it could measure meridional flow. Here we report that, contrary to expectations, these observations yielded a meridional flow velocity of +3?±?11?km?s(-1), that is, one consistent with zero within statistical uncertainties.     

台阵网格自动定位方法在鄂尔多斯地块南部和秦岭‒大别造山带的应用

利用新近发展起来的台阵网格自动事件定位方法, 对鄂尔多斯地块及秦岭-大别造山带区域136个宽频带流动台站记录的连续波形进行扫描和定位。通过与地震目录的对比发现, 不仅在常规地震带上分布着大量的事件, 而且在地震目录中极少发生地震的区域出现大量地震事件集中分布的现象。这些事件主要分布于鄂尔多斯地块南部以及秦岭-大别造山带附近的断层带。经过对定位台站的抽样分析和对定位事件地震波形的仔细人工确认, 验证了这些被扫描并定位的事件的真实性。通过慢度分析, 获得所定位事件的面波传播视速度, 绝大部分事件对应的视速度在2.5~5.2 km/s之间, 与地震面波群速度相符。所定位的地震事件可用于补充秦岭-大别造山带的地震目录, 同时可以通过进一步的波形拟合来分析其震源机制。