Slow differential rotation of the Earth's inner core indicated by temporal changes in scattering

The finding that the Earth's inner core might be rotating faster than the mantle has important implications for our understanding of core processes, including the generation of the Earth's magnetic field. But the reported signal is subtle--a change of about 0.01 s per year in the separation of two seismic waves with differing paths through the core. Subsequent studies of such data have generally supported the conclusion that differential rotation exists, but the difficulty of accurately locating historic earthquakes and possible biases induced by strong lateral variations in structure near the core-mantle boundary have raised doubt regarding the proposed inner-core motion. Also, a study of free oscillations constrained the motion to be relatively small compared to previous estimates and it has been proposed that the interaction of inner-core boundary topography and mantle heterogeneity might lock the inner core to the mantle. The recent detection of seismic waves scattered in the inner core suggests a simple test of inner-core motion. Here we compare scattered waves recorded in Montana, USA, from two closely located nuclear tests at Novaya Zemlya, USSR, in 1971 and 1974. The data show small but coherent changes in scattering which point toward an inner-core differential rotation rate of 0.15 degrees per year--consistent with constraints imposed by the free-oscillation data.     

Hemispherical variations in seismic velocity at the top of the Earth's inner core

Knowledge of the seismic velocity structure at the top of the Earth's inner core is important for deciphering the physical processes responsible for inner-core growth. Previous global seismic studies have focused on structures found 100 km or deeper within the inner core, with results for the uppermost 100 km available for only isolated regions. Here we present constraints on seismic velocity variations just beneath the inner-core boundary, determined from the difference in travel time between waves reflected at the inner-core boundary and those transmitted through the inner core. We found that these travel-time residuals-observed on both global seismograph stations and several regional seismic networks-are systematically larger, by about 0.8 s, for waves that sample the 'eastern hemisphere' of the inner core (40 degrees E to 180 degrees E) compared to those that sample the 'western hemisphere' (180 degrees W to 40 degrees E). These residuals show no correlation with the angle at which the waves traverse the inner core; this indicates that seismic anisotropy is not strong in this region and that the isotropic seismic velocity of the eastern hemisphere is about 0.8% higher than that of the western hemisphere.     

Inner-core shear-wave anisotropy and texture from an observation of PKJKP waves

Since the discovery of the Earth's core a century ago, and the subsequent discovery of a solid inner core (postulated to have formed by the freezing of iron) seismologists have striven to understand this most remote part of the deep Earth. The most direct evidence for a solid inner core would be the observation of shear-mode body waves that traverse it, but these phases are extremely difficult to observe. Two reported observations in short-period data have proved controversial. Arguably more successful have been studies of longer-period data, but such averaging limits the usefulness of the observations to reported sightings. We present two observations of an inner-core shear-wave phase at higher frequencies in stacked data from the Japanese High-Sensitivity Array, Hi-Net. From an analysis of timing, amplitude and waveform of the 'PKJKP' phase we derive constraints on inner-core compressional-wave velocity and shear attenuation at about 0.3 Hz which differ from standard isotropic core models. We can explain waveform features and can partially reconcile the otherwise large differences between core wavespeed and attenuation models that our observations apparently suggest if we invoke shear-wave anisotropy in the inner core. A simple model of an inner core composed of hexagonal close-packed iron with its c axis aligned perpendicular to the rotation axis yields anisotropy that is compatible with both the shear-wave anisotropy that we observe and the well-established 3 per cent compressional-wave anisotropy.     

Melting of the Earth's inner core

The Earth's magnetic field is generated by a dynamo in the liquid iron core, which convects in response to cooling of the overlying rocky mantle. The core freezes from the innermost surface outward, growing the solid inner core and releasing light elements that drive compositional convection. Mantle convection extracts heat from the core at a rate that has enormous lateral variations. Here we use geodynamo simulations to show that these variations are transferred to the inner-core boundary and can be large enough to cause heat to flow into the inner core. If this were to occur in the Earth, it would cause localized melting. Melting releases heavy liquid that could form the variable-composition layer suggested by an anomaly in seismic velocity in the 150 kilometres immediately above the inner-core boundary. This provides a very simple explanation of the existence of this layer, which otherwise requires additional assumptions such as locking of the inner core to the mantle, translation from its geopotential centre or convection with temperature equal to the solidus but with composition varying from the outer to the inner core. The predominantly narrow downwellings associated with freezing and broad upwellings associated with melting mean that the area of melting could be quite large despite the average dominance of freezing necessary to keep the dynamo going. Localized melting and freezing also provides a strong mechanism for creating seismic anomalies in the inner core itself, much stronger than the effects of variations in heat flow so far considered.     

消除近地表地震散射噪音的方法

假设近地表垂直点力产生的地震波场由入射波场和散射波场组成。散射波场由深层散射和浅层散射两个部分组成 ,若只考虑消除近地表区域的面波散射对地震资料的影响 ,则通过地震散射模型来估算近地表散射波分布。由波动理论建立近地表传播和散射模型 ,并在平行地表的一个网格面的各个节点上设置波阻抗差函数来近似地表的非均匀性。散射波场是入射波场与波阻抗差函数的函数。在这样的模式下 ,由炮集记录提取入射波场 ,进而通过最小二乘法 ,求解波阻抗差函数 ,最终得到散射波场的估计。从炮集记录中减去散射波场 ,实现地震散射噪音的衰减     

The plastic deformation of iron at pressures of the Earth's inner core

Soon after the discovery of seismic anisotropy in the Earth's inner core, it was suggested that crystal alignment attained during deformation might be responsible. Since then, several other mechanisms have been proposed to account for the observed anisotropy, but the lack of deformation experiments performed at the extreme pressure conditions corresponding to the solid inner core has limited our ability to determine which deformation mechanism applies to this region of the Earth. Here we determine directly the elastic and plastic deformation mechanism of iron at pressures of the Earth's core, from synchrotron X-ray diffraction measurements of iron, under imposed axial stress, in diamond-anvil cells. The epsilon-iron (hexagonally close packed) crystals display strong preferred orientation, with c-axes parallel to the axis of the diamond-anvil cell. Polycrystal plasticity theory predicts an alignment of c-axes parallel to the compression direction as a result of basal slip, if basal slip is either the primary or a secondary slip system. The experiments provide direct observations of deformation mechanisms that occur in the Earth's inner core, and introduce a method for investigating, within the laboratory, the rheology of materials at extreme pressures.     

Elasticity of iron at the temperature of the Earth's inner core.

Seismological body-wave and free-oscillation studies of the Earth's solid inner core have revealed that compressional waves traverse the inner core faster along near-polar paths than in the equatorial plane. Studies have also documented local deviations from this first-order pattern of anisotropy on length scales ranging from 1 to 1,000 km (refs 3, 4). These observations, together with reports of the differential rotation of the inner core, have generated considerable interest in the physical state and dynamics of the inner core, and in the structure and elasticity of its main constituent, iron, at appropriate conditions of pressure and temperature. Here we report first-principles calculations of the structure and elasticity of dense hexagonal close-packed (h.c.p.) iron at high temperatures. We find that the axial ratio c/a of h.c.p. iron increases substantially with increasing temperature, reaching a value of nearly 1.7 at a temperature of 5,700 K, where aggregate bulk and shear moduli match those of the inner core. As a consequence of the increasing c/a ratio, we have found that the single-crystal longitudinal anisotropy of h.c.p. iron at high temperature has the opposite sense from that at low temperature. By combining our results with a simple model of polycrystalline texture in the inner core, in which basal planes are partially aligned with the rotation axis, we can account for seismological observations of inner-core anisotropy.     

Texturing of the Earth's inner core by Maxwell stresses.

Elastic anisotropy in the Earth's inner core has been attributed to a preferred lattice orientation, which may be acquired during solidification of the inner core or developed subsequent to solidification as a result of plastic deformation. But solidification texturing alone cannot explain the observed depth dependence of anisotropy, and previous suggestions for possible deformation processes have all relied on radial flow, which is inhibited by thermal and chemical stratification. Here we investigate the development of anisotropy as the inner core deforms plastically under the influence of electromagnetic (Maxwell) shear stresses. We estimate the flow caused by a representative magnetic field using polycrystal plasticity simulations for epsilon-iron, where the imposed deformation is accommodated by basal and prismatic slip. We find that individual grains in an initially random polycrystal become preferentially oriented with their c axes parallel to the equatorial plane. This pattern is accentuated if deformation is accompanied by recrystallization. Using the single-crystal elastic properties of epsilon-iron at core pressure and temperature, we average over the simulated orientation distribution to obtain a pattern of elastic anisotropy which is similar to that observed seismologically.     

用柱面波模型探讨地震电磁信息

本文从理论上探讨了地壳波导中传播的柱面电磁波,发现地震电磁波没有TM型波,这一特点可以用来识别地震电磁信号,引入衰减因子后,得到kr<1的区域,高导电层的衰减随频率的升高而减小,从而可以定性地解释观测到高频地震电磁信息的现象。     

Thermochemical flows couple the Earth's inner core growth to mantle heterogeneity

Seismic waves sampling the top 100 km of the Earth's inner core reveal that the eastern hemisphere (40 degrees E-180 degrees E) is seismically faster, more isotropic and more attenuating than the western hemisphere. The origin of this hemispherical dichotomy is a challenging problem for our understanding of the Earth as a system of dynamically coupled layers. Previously, laboratory experiments have established that thermal control from the lower mantle can drastically affect fluid flow in the outer core, which in turn can induce textural heterogeneity on the inner core solidification front. The resulting texture should be consistent with other expected manifestations of thermal mantle control on the geodynamo, specifically magnetic flux concentrations in the time-average palaeomagnetic field over the past 5 Myr, and preferred eddy locations in flows imaged below the core-mantle boundary by the analysis of historical geomagnetic secular variation. Here we show that a single model of thermochemical convection and dynamo action can account for all these effects by producing a large-scale, long-term outer core flow that couples the heterogeneity of the inner core with that of the lower mantle. The main feature of this thermochemical 'wind' is a cyclonic circulation below Asia, which concentrates magnetic field on the core-mantle boundary at the observed location and locally agrees with core flow images. This wind also causes anomalously high rates of light element release in the eastern hemisphere of the inner core boundary, suggesting that lateral seismic anomalies at the top of the inner core result from mantle-induced variations in its freezing rate.     

Low-velocity zone atop the 410-km seismic discontinuity in the northwestern United States

The seismic discontinuity at 410 km depth in the Earth's mantle is generally attributed to the phase transition of (Mg,Fe)2SiO4 (refs 1, 2) from the olivine to wadsleyite structure. Variation in the depth of this discontinuity is often taken as a proxy for mantle temperature owing to its response to thermal perturbations. For example, a cold anomaly would elevate the 410-km discontinuity, because of its positive Clapeyron slope, whereas a warm anomaly would depress the discontinuity. But trade-offs between seismic wave-speed heterogeneity and discontinuity topography often inhibit detailed analysis of these discontinuities, and structure often appears very complicated. Here we simultaneously model seismic refracted waves and scattered waves from the 410-km discontinuity in the western United States to constrain structure in the region. We find a low-velocity zone, with a shear-wave velocity drop of 5%, on top of the 410-km discontinuity beneath the northwestern United States, extending from southwestern Oregon to the northern Basin and Range province. This low-velocity zone has a thickness that varies from 20 to 90 km with rapid lateral variations. Its spatial extent coincides with both an anomalous composition of overlying volcanism and seismic 'receiver-function' observations observed above the region. We interpret the low-velocity zone as a compositional anomaly, possibly due to a dense partial-melt layer, which may be linked to prior subduction of the Farallon plate and back-arc extension. The existence of such a layer could be indicative of high water content in the Earth's transition zone.     

Inner core’s seismic anisotropy is related to its rotation

The inner core has a differential rotation relative to the crust and mantle, the relative linear velocity between the solid inner core and the molten outer core is the biggest at the equator and zero at pole area. As a result, the inner core grows faster at the equator than at the pole area. The gravitational force drives the material flow from the equator to the pole area and makes the inner core remain quasi-orbicular. The corresponding axial symmetric stress field makesc-axes of hexagonal close packed (hcp) iron align with inner core’s rotation axis, resulting in observed seismic anisotropy.     

Outer-core compositional stratification from observed core wave speed profiles

Light elements must be present in the nearly pure iron core of the Earth to match the remotely observed properties of the outer and inner cores. Crystallization of the inner core excludes light elements from the solid, concentrating them in liquid near the inner-core boundary that potentially rises and collects at the top of the core, and this may have a seismically observable signal. Here we present array-based observations of seismic waves sensitive to this part of the core whose wave speeds require there to be radial compositional variation in the topmost 300?km of the outer core. The velocity profile significantly departs from that of compression of a homogeneous liquid. Total light-element enrichment is up to five weight per cent at the top of the core if modelled in the Fe-O-S system. The stratification suggests the existence of a subadiabatic temperature gradient at the top of the outer core.     

基于双孔模型的含流体各向异性介质地震波场数值模拟

裂缝的存在是诱导地下介质产生各向异性的主要原因,而裂缝中的流体则能够对地震波产生十分明显的能量衰减。本文基于双重孔隙介质理论,对含流体各向异性介质的地震响应特征进行了数值模拟,发现双孔介质的各向异性与介质的渗透率、裂缝密度、有效压力和频率均有重要联系。基于衰减各向异性理论,推导了含流体各向异性介质二维三分量应力-应变关系,结合旋转交错网格有限差分法,通过数值模拟给出了双孔介质中地震波的传播规律,结果表明裂缝密度的增大能够显著地增强介质的各向异性,使得q P波和快慢q S波垂直或平行于裂缝面的波前面均更接近椭圆,且流体的存在能够对q P和q S波相速度造成较为显著的衰减。     

国外应用纵波各向异性技术检测裂缝的研究进展

 裂缝性油气藏作为一种重要的油藏资源,已经成为地震勘探研究的热门课题。相对于国内而言,国外应用各向异性技术来检测裂缝的研究开展较早并逐渐成熟。本文回顾了各向异性技术的由来和发展,介绍了裂缝研究使用的有效介质理论,总结了纵波各向异性应用于裂缝检测的新技术新方法。分析表明,随着关注度的增强和采集处理技术的发展,裂缝检测技术的方法更加多样,实际应用的有效性得到提高。     

Pressure sensitivity of olivine slip systems and seismic anisotropy of Earth's upper mantle

The mineral olivine dominates the composition of the Earth's upper mantle and hence controls its mechanical behaviour and seismic anisotropy. Experiments at high temperature and moderate pressure, and extensive data on naturally deformed mantle rocks, have led to the conclusion that olivine at upper-mantle conditions deforms essentially by dislocation creep with dominant [100] slip. The resulting crystal preferred orientation has been used extensively to explain the strong seismic anisotropy observed down to 250 km depth. The rapid decrease of anisotropy below this depth has been interpreted as marking the transition from dislocation to diffusion creep in the upper mantle. But new high-pressure experiments suggest that dislocation creep also dominates in the lower part of the upper mantle, but with a different slip direction. Here we show that this high-pressure dislocation creep produces crystal preferred orientations resulting in extremely low seismic anisotropy, consistent with seismological observations below 250 km depth. These results raise new questions about the mechanical state of the lower part of the upper mantle and its coupling with layers both above and below.     

Global azimuthal seismic anisotropy and the unique plate-motion deformation of Australia

Differences in the thickness of the high-velocity lid underlying continents as imaged by seismic tomography, have fuelled a long debate on the origin of the 'roots' of continents. Some of these differences may be reconciled by observations of radial anisotropy between 250 and 300 km depth, with horizontally polarized shear waves travelling faster than vertically polarized ones. This azimuthally averaged anisotropy could arise from present-day deformation at the base of the plate, as has been found for shallower depths beneath ocean basins. Such deformation would also produce significant azimuthal variation, owing to the preferred alignment of highly anisotropic minerals. Here we report global observations of surface-wave azimuthal anisotropy, which indicate that only the continental portion of the Australian plate displays significant azimuthal anisotropy and strong correlation with present-day plate motion in the depth range 175-300 km. Beneath other continents, azimuthal anisotropy is only weakly correlated with plate motion and its depth location is similar to that found beneath oceans. We infer that the fast-moving Australian plate contains the only continental region with a sufficiently large deformation at its base to be transformed into azimuthal anisotropy. Simple shear leading to anisotropy with a plunging axis of symmetry may explain the smaller azimuthal anisotropy beneath other continents.     

Melting of iron at the physical conditions of the Earth's core

Seismological data can yield physical properties of the Earth's core, such as its size and seismic anisotropy. A well-constrained iron phase diagram, however, is essential to determine the temperatures at core boundaries and the crystal structure of the solid inner core. To date, the iron phase diagram at high pressure has been investigated experimentally through both laser-heated diamond-anvil cell and shock-compression techniques, as well as through theoretical calculations. Despite these contributions, a consensus on the melt line or the high-pressure, high-temperature phase of iron is lacking. Here we report new and re-analysed sound velocity measurements of shock-compressed iron at Earth-core conditions. We show that melting starts at 225 +/- 3 GPa (5,100 +/- 500 K) and is complete at 260 +/- 3 GPa (6,100 +/- 500 K), both on the Hugoniot curve-the locus of shock-compressed states. This new melting pressure is lower than previously reported, and we find no evidence for a previously reported solid-solid phase transition on the Hugoniot curve near 200 GPa (ref. 16).     

频散AVO反演方法在致密碎屑岩储层烃类检测中的应用研究

近年来实验室模拟和理论研究均表明含流体填充介质常常导致地震波发生不同程度的频散和衰减,利用地震波衰减方法来预测储层的油气分布情况文献较为常见,而利用含烃储层导致地震波的频散方法来进行储层的流体识别较少。本文从原理出发,推导了频散AVO反演公式,印证了频散AVO近似式应用于地震勘探领域的精度是可行的,应用频散AVO反演方法对大牛地气田致密碎屑岩储层的含气性进行了预测,取得的结果与实际钻井结果吻合很好。     

基于应力波传输的随钻测量系统研究

阐述了在钻井过程中使用应力波通过钻柱实时传输井下数据技术,介绍了西部钻探钻井工程技术研究院研制的基于应力波传输的随钻测量系统(CS-MWD)的工作原理和构成,并对CS-MWD系统进行现场试验,指出在浅层井中CS-MWD系统具有传输速度快、测量精度高的优点,而在深层井中因受到应力波钻柱衰减作用和噪音的影响,其应用还受限制.