Some effects of lateral heterogeneities in the upper mantle on postglacial land uplift close to continental margins

We investigate the effects of lateral heterogeneities in the upper mantle on the calculation of postglacial land uplift. For the model calculations we use a commercial finite-element code, which enables us to solve the equations governing a layered, isotropic. incompressible, Maxwell-viscoelastic half-space with laterally varying layer thicknesses and physical properties. Following previous investigations performed by Sabadini. Yuen & Portney (1986) and Gasperini & Sabadini (1989), we extend their results using a more realistic loading history and different earth models. We then focus our attention on the question whether lateral heterogeneities in the upper mantle can be modelled correctly using a set of homogeneous earth models. To this end, a comparison of model calculations using both laterally homogeneous and heterogeneous earth models is performed.
We find that lateral heterogeneities in the upper mantle significantly influence the calculated postglacial land uplift. The resolving power of relative sea-level observations for the prescribed lateral heterogeneities used in this study is mainly focused on observations around the load margin and outside the glaciated areas, where differences in predicted land uplift between individual models are large enough to be resolved by observations.
We can qualitatively determine lateral heterogeneities in the upper mantle using a set of laterally homogeneous earth models, if the geological structure, for example a continental margin, is known. However, in order to infer the correct values of lithospheric thickness and asthenospheric viscosity, we need to use laterally heterogeneous models.     

Surface-wave propagation in a cracked poroelastic half-space lying under a uniform layer of fluid

The effect of cracks on the elastic properties of an isotropic elastic solid is studied when the cracks are saturated with a soft fluid. A polynomial equation in effective Poisson's ratio is obtained, whose coefficients are functions of Poisson's ratio of the uncracked solid, crack density and saturating fluid parameter. Elastic and dynamical constants used in Blot's theory of wave propagation in poroelastic solids are modified for the introduction of cracks. The effects of cracks on the velocities of three types of waves are observed numerically. The frequency equation is derived for the propagation of Rayleigh-type surface waves in a saturated poroelastic half-space lying under a uniform layer of liquid. Dispersion curves for a particular model of oceanic crust containing cracks are plotted. The effects of variations in crack density and saturation on the phase and group velocity are also analysed.     

Mars landscape evolution: influence of stratigraphy on geomorphology in the north polar region

Lithology and physical properties of strata exposed at the Earth's surface have direct influence on the erosion and geomorphic expression of landforms. While this is well known on our planet, examples on Mars are just coming to light among the tens of thousands of airphoto-quality images (resolutions 1.5–12 m/pixel) acquired since 1997 by the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC). Specific examples occur among martian north polar layered materials, which MOC images reveal are divided into two distinct stratigraphic units: a lower, dark-toned layered unit and a younger, upper, lighter-toned layered unit. The lower unit is less resistant to wind erosion than the upper unit. The upper unit most likely consists of stratified dust and ice, while the lower unit contains abundant, poorly cemented sand. Sand is more easily mobilized by wind than dust; the lower resistance to erosion of the lower unit results from the presence of sand. Where wind erosion in polar troughs has penetrated to the lower unit, geomorphic change has proceeded more rapidly: sand has been liberated from the lower unit, and arcuate scarps have formed as the upper unit has been undermined. Wind erosion of the lower unit thus influences the geomorphology of the north polar region; this result likely explains the genesis of the large polar trough, Chasma Boreale, and the relations between dunes and arcuate scarps that have puzzled investigators for nearly three decades. The properties of the stratigraphic units suggest that the upper limit for the amount of water contained in the north polar layered materials may be 30–50% less than previously estimated.     

Induction in a thin sheet of variable conductance at the surface of a stratified earth - I. Two-dimensional theory

Summary. An existing 2-D integral equation method for modelling electromagnetic induction in a thin sheet at the surface of a uniform half-space can be generalized to deal with a layered half-space by the inclusion of an extra term in the integral equation. The results obtained are shown to be in excellent agreement with finite difference solutions to the same modelling problem.     

Shear-wave polarizations on a curved wavefront at an isotropic free surface

Summary. We present polarization diagrams of the particle motions at the free surface of an isotropic half-space generated by incident shear waves from a local buried point source. The reflectivity technique is used to calculate synthetic seismograms from which the particle motions are plotted. The particle motions are examined over a range of epicentral distances in a uniform isotropic half-space for different source frequencies and polarization angles, and for different Poisson's ratios. The particle motions due to a curved wavefront possess different characteristics from those generated by plane wavefronts at corresponding incidence angles. A curved wavefront generates a local SP -phase: a P -headwave which propagates along the free surface, and arrives shortly before the direct S -wave. These two arrivals give rise to cruciform particle motions in the sagittal and horizontal planes, which could be misinterpreted as anisotropy-induced shear-wave splitting. An examination of the particle motion in the transverse plane, mutually orthogonal to the sagittal and horizontal planes, can be used to discriminate between isotropic and anisotropic interpretations. The amplitude of the SP -phase is enhanced when it propagates in a low-velocity surface layer overlying the source layer, and may then become the dominant phase on radial-component seismograms. The presence of even a single surface layer may introduce considerable complexity into the seismogram, and we examine the effects of layer thickness, velocity contrast, and source depth on the corresponding polarization diagrams. Reliable information on the source and propagation path characteristics of shear waves from a buried local point source can only be obtained from free-surface records if they are recorded within a very limited epicentral distance range.     

Multimode surface waveform tomography of the Pacific Ocean: a closer look at the lithospheric cooling signature

We present a regional surface waveform tomography of the Pacific upper mantle, obtained using an automated multimode surface waveform inversion technique on fundamental and higher mode Rayleigh waves, to constrain the   V_SV   structure down to ∼400 km depth. We have improved on previous implementations of this technique by robustly accounting for the effects of uncertainties in earthquake source parameters in the tomographic inversion. We have furthermore improved path coverage in the South Pacific region by including Rayleigh wave observations from the French Polynesian Pacific Lithosphere and Upper Mantle Experiment deployment. This improvement has led to imaging of vertical low-velocity structures associated with hotspots within the South Pacific Super-Swell region. We have produced an age-dependent average cross-section for the Pacific Ocean lithosphere and found that the increase in   V_SV   with age is broadly compatible with a half-space cooling model of oceanic lithosphere formation. We cannot confirm evidence for a Pacific-wide reheating event. Our synthetic tests show that detailed interpretation of average   V_SV   trends across the Pacific Ocean may be misleading unless lateral resolution and amplitude recovery are uniform across the region, a condition that is difficult to achieve in such a large oceanic basin with current seismic stations.     

Finite-frequency sensitivity kernels for head waves

Head waves are extremely important in determining the structure of the predominantly layered Earth. While several recent studies have shown the diffractive nature and the 3-D Fréchet kernels of finite-frequency turning waves, analogues of head waves in a continuous velocity structure, the finite-frequency effects and sensitivity kernels of head waves are yet to be carefully examined. We present the results of a numerical study focusing on the finite-frequency effects of head waves. Our model has a low-velocity layer over a high-velocity half-space and a cylindrical-shaped velocity perturbation placed beneath the interface at different locations. A 3-D finite-difference method is used to calculate synthetic waveforms. Traveltime and amplitude anomalies are measured by the cross-correlation of synthetic seismograms from models with and without the velocity perturbation and are compared to the 3-D sensitivity kernels constructed from full waveform simulations. The results show that the head wave arrival-time and amplitude are influenced by the velocity structure surrounding the ray path in a pattern that is consistent with the Fresnel zones. Unlike the 'banana–doughnut' traveltime sensitivity kernels of turning waves, the traveltime sensitivity of the head wave along the ray path below the interface is weak, but non-zero. Below the ray path, the traveltime sensitivity reaches the maximum (absolute value) at a depth that depends on the wavelength and propagation distance. The sensitivity kernels vary with the vertical velocity gradient in the lower layer, but the variation is relatively small at short propagation distances when the vertical velocity gradient is within the range of the commonly accepted values. Finally, the depression or shoaling of the interface results in increased or decreased sensitivities, respectively, beneath the interface topography.     

Architecture,formation and implication of active structure-controlled intraslope channel system southeast offshore Sendai,Tohoku, Japan

Located off the Pacific coast of central Tohoku (NE Japan), the Ishinomaki slope channel (ISC) provides an excellent opportunity to study a structure-controlled intraslope channel and downslope sedimentation along the active margin. The seismic reflection data across ISC show an extensive basal surface and overlying channel complexes between the basement structures of the Abukuma ridge to the south and Kitakami massif to the north, indicating that the formation of the intraslope basin, channelization of ISC and sedimentation of the downstream channel-lobe transition zone (CLTZ) are very likely to be structure-controlled. The oblique channel stacking pattern, faulting of the seafloor and subsurface Abukuma ridge in the upper and lower domains of ISC, collectively suggest that ISC has migrated northward and is currently under the influence of active compression. Differences in styles of accommodation space between the upper and lower domains of ISC suggest that differential subsidence occurred along the strike-slip tectonic line. Based on the regional strike-slip tectonic line, we propose that a Kitakami-Abukuma ridge existed before the formation of ISC. The strike-slip faulting divided the Kitakami-Abukuma ridge into the Kitakami massif to the north and the Abukuma ridge to the south, and an intervening fault trough as the precursor of the intraslope basin and ISC. As the subduction of the Pacific Plate and associated compressional events continued, the Abukuma ridge was reactivated to narrow the intraslope basin into a confined channel. Located near the epicentre of the devastating 2011 Tohoku earthquake event, the ISC, downstream CLTZ and underlying intraslope basin provide information on active basement structure and the evolving sediment routing system on the tectonically active margin.     

Implications of stress concentration on a strike-slip fault in an elastic plate subject to basal shear stress

Summary We consider a long strike-slip fault in a lithosphere modelled as an elastic slab. To the base of the slab a shear stress distribution is applied which simulates the viscous drag exerted by the asthenosphere. The resulant stress on the fault plane may directly fracture the lithosphere in its brittle upper portion; alternatively it may give rise at first to a stable aseismic sliding in the lower portion. In the latter case, stress concentration due to the deep aseismic slip is the relevant feature of the pre-seismic stress acting on the upper section of the lithosphere. The two cases are examined by use of dislocation theory and their observable effects compared. Different depths of the aseismic slip zone and the presence or absence of a uniform friction on the seismic fault are allowed for. If the model is applied to the San Andreas fault region, where a steady sliding condition actually seems to be present at shallow depth, it turns out that the slip amplitudes commonly associated with large earthquakes are consistent with average basal stress values which can be substantially lower than a few bars, a value often quoted as the steady state basal stress due to a velocity gradient in the upper asthenosphere.     

A hybrid solution for wave propagation problems in regular media with bounded irregular inclusions

Summary. This paper describes a method that enables one to calculate the effects of localized heterogeneities on the wavefield in an otherwise regular medium. It does so by connecting a finite element solution for a heterogeneous inclusion to any type of solution for the regular medium, e.g. a reflectivity solution for a layered medium or an analytical solution for a simple half-space. Once the Green's functions for the regular medium are determined, the method reduces to a coupled set of algebraic equations for the wavefield, with the incident field and/or body forces as known variables. An efficient numerical scheme is derived for the solution of these equations.     

Upper Mantle Velocity Structure Estimated From Ps-Converted Wave Beneath the North-Eastern Japan Arc

Summary. The upper boundary of the descending oceanic plate is located by using PS -waves (converted from P to S at the boundary) in the Tohoku District, the north-eastern part of Honshu, Japan. the observed PS-P time data are well explained by a two-layered oceanic plate model composed of a thin low-velocity upper layer whose thickness is less than 10 km and a thick high-velocity lower layer; the upper and lower layers respectively have 6 per cent lower and 6 per cent higher velocity than the overriding mantle. the estimated location of the upper boundary is just above the upper seismic plane of the double-planed deep seismic zone. This result indicates that events in the upper seismic plane, at least in the depth range from 60 to 150 km, occur within the thin low-velocity layer on the surface of the oceanic plate.     

Recurrence relations for computing complete P and SV seismograms

Summary. A set of recurrence relations which are computationally more efficient than those of the reflection matrix method of Kennett & Kerry is presented for P - and SV -wave generation in a ( n + 1) layered medium. The recurrence relations contain no growing terms and thus provide a stable algorithm for computing complete P and SV synthetic seismograms. Our algorithm requires a fewer algebraic operations for computing the reflectivity and transmissivity coefficients, ranging from 15 per cent less for a source in the half-space to 30 per cent less for a source in the top layer, than the reflection matrix method.     

广东南部地理景观中土壤重金属垂直分异及其地球化学分层性

本文研究热带亚热带土壤中重金属分布的垂直分异及其分层性特征.土壤剖面中,水溶态、可交换态和有机结合态的重金属浓度自上层向底层降低.水溶态和有机结合态重金属的分布与有机质一致,而与pH值相反.Fe-Mn氧化物结合态的重金属分布与粘粒一致.残渣态分布的差异极小.     

Evidence for different scaling of earthquake source parameters for large earthquakes depending on faulting mechanism

Scaling relationships between seismic moment, rupture length, and rupture width have been examined. For this purpose, the data from several previous studies have been merged into a database containing more than 550 events. For large earthquakes, a dependence of scaling on faulting mechanism has been found. Whereas small and large dip-slip earthquakes scale in the same way, the self-similarity of earthquakes breaks down for large strike-slip events. Furthermore, no significant differences in scaling could be found between normal and reverse earthquakes and between earthquakes from different regions. Since the thickness of the seismogenic layer limits fault widths, most strike-slip earthquakes are limited to rupture widths of between 15 and 30 km while the rupture length is not limited. The aspect ratio of dip-slip earthquakes is similar for all earthquake sizes. Hence, the limitation in rupture width seems to control the maximum possible rupture length for these events. The different behaviour of strike-slip and dip-slip earthquakes can be explained by rupture dynamics and geological fault growth. If faults are segmented, with the thickness of the seismogenic layer controlling the length of each segment, strike-slip earthquakes might rupture connected segments more easily than dip-slip events, and thus could produce longer ruptures than dip-slip events of the same width     

Slab low-velocity layer in the eastern Aleutian subduction zone

Local earthquakes in the vicinity of the Alaskan Peninsula's Shumagin Islands often produce arrivals between the main P and S arrivals not predicted by standard traveltime tables. Based on traveltime and polarization, these anomalous arrivals appear to be from P -to- S conversions at the surface of the subducted Pacific Plate beneath the recording stations. The P -to- S conversion occurs at the top of a low-velocity layer which extends to at least 150 km depth and is 8 ˜ 2 per cent slower than the overlying mantle. The slab is ˜ 7 per cent faster than the mantle. The low-velocity layer contains the foci of the earthquakes in the upper plane of the double seismic zone and confines PS ray paths to lie within it. These observations indicate that layered structures persist to positions well past the surface location of the volcanic front. Reactions forming high-pressure minerals do not yield slab-like velocities until beyond the point that subduction zone magma genesis occurs. If the subducted oceanic crust forms the layer, it is subducted essentially intact.     

Dynamic nucleation process of shallow earthquake faulting in a fault zone

Two distinct phases are commonly observed at the initial part of seismograms of large shallow earthquakes: low-frequency and low-amplitude waves following the onset of a P wave ( P ₁) are interrupted by the arrival of the second impulsive phase P₂ enriched with high-frequency components. This observation suggests that a large shallow earthquake involves two qualitatively different stages of rupture at its nucleation.
We propose a theoretical model that can naturally explain the above nucleation behaviour. The model is 2-D and the deformation is assumed to be anti-plane. A key clement in our model is the assumption of a zone in which numbers of pre-existing cracks are densely distributed; this cracked zone is a model for the fault zone. Dynamic crack growth nucleated in such a zone is intensely affected by the crack interactions, which exert two conflicting effects: one tends to accelerate the crack growth, and the other tends to decelerate it. The accelerating and decelerating effects are generally ascribable to coplanar and non-coplanar crack interactions, respectively. We rigorously treat the multiple interactions among the cracks, using the boundary integral equation method (BIEM), and assume the critical stress fracture criterion for the analysis of spontaneous crack propagation.
Our analysis shows that a dynamic rupture nucleated in the cracked zone begins to grow slowly due to the relative predominance of non-coplanar interactions. This process radiates the P₁ phase. If the crack continues to grow, coalescence with adjacent coplanar cracks occurs after a short time. Then, coplanar interactions suddenly begin to prevail and crack growth is accelerated; the P₂ phase is emitted in this process. It is interpreted that the two distinct phases appear in the process of the transition from non-coplanar to coplanar interaction predominance.     

东北高空湿度变化特征及其与地面气温和降水的关系

利用1971~2005年探空和地面观测资料,详细分析了东北地区高空比湿和相对湿度的时空变化特征,并探讨了比湿和相对湿度与地面气温、降水量的关系。结果表明：东北地区比湿空间分布主要受到水汽来源的影响,地面由东南向西北递减,高空由南向北递减;相对湿度受水汽、海拔高度和纬度的共同影响,地面和对流层下层由南向北先减后增,对流层中层由南向北递增,赤峰向通辽延伸的西南-东北向干舌地面最明显,随高度增加逐渐减弱。1971~2005年,东北地区比湿从地面到高空均为增加趋势,对流层中下层的增加趋势更加显著;相对湿度在地面呈显著减小趋势,对流层中层呈显著增加趋势。大气比湿与地面气温在年、季尺度上存在一致的显著正相关关系,大气相对湿度与地面气温在季节尺度上存在显著负相关关系;对流层中下层相对湿度与降水量相关最显著;地面气温升高对东北气候趋于干旱化起了重要作用,高空相对湿度增加有利于降水增加,气温与比湿的相互消长,影响了气候的干、湿变化。     

Love waves excited by discontinuous propagation of a rupture front

Summary. Analytical results are presented for Love waves generated by sudden changes of the rate of advance of a curved rupture front in an inclined fault plane that is embedded in an elastic half-space. The boundary condition at the surface of the half-space approximates the presence of an overlying layer. The calculation consists of two parts. First, ray theory is used to calculate far-field approximations to the horizontally polarized wavefields which are emitted when the speed of the rupture front suddenly changes. These fields can be expressed as products of emission coefficients (which govern the angular dependence) and propagation terms. Secondly, a representation integral for the Love wave over a surface enclosing the rupture front is constructed, using the emitted signal and an appropriate Green's function. This integral is evaluated asymptotically. The resulting approximate Love-wave spectrum shows an explicit dependence on the nature of the rupture process, on the rupture-front and fault-plane geometry, and on the magnitude of a sudden change in the rate of advance of the rupture front.     

Extended formulation for Rayleigh-wave computations at high frequencies in a spherical layered earth

Summary. The earlier formulation of dispersion function, amplitude response and surface ellipticity for Rayleigh waves in a spherical layered earth has been extended by the reduced-delta matrix to avoid computational instability at high frequencies. We derive a modified formulation, where the layer thickness is kept separated during matrix multiplications. The present formulations are valid for an unlimited frequency range. A numerical experiment with single precision (six decimal digits accuracy) on an IBM 360/44 computer is performed at frequencies from 0.4 to 12.5 Hz on a Shield structure down to a depth of 1090 km below which a complete sphere is assumed. The experiment shows that both reduced-delta matrix and modified formulations generate roots of the dispersion equation without any reduction of layers. However, to avoid unnecessary computation, a layer-reduction procedure has been derived from a modified formulation when the contribution of deep layers becomes insignificant on surface waves.     

Seismic anisotropy — the state of the art: II

Summary. The theory, causes, observations, and possible applications of seismic anisotropy in the Earth have developed considerably since the previous state of the art paper was published in 1977. The behaviour of waves in layered anisotropic media is now much better understood and the evidence for seismic anisotropy indicates that anisotropy is likely to be present throughout much of the crust and upper mantle. The top few hundred kilometres of the mantle appears to be anisotropic with the orientations aligned by the present or palaeo stress-field. The upper part of the crust is frequently anisotropic, probably due to cracks differentially aligned by the non-lithostatic stresses. The possibility of being able to monitor crack geometry by seismic techniques opens a wide range of applications in currently important activities.