Seismic anisotropy within the uppermost mantle of southern Germany

This paper presents an updated interpretation of seismic anisotropy within the uppermost mantle of southern Germany. The dense network of reversed and crossing refraction profiles in this area made it possible to observe almost 900 traveltimes of the P_n phase that could be effectively used in a time-term analysis to determine horizontal velocity distribution immediately below the Moho. For 12 crossing profiles, amplitude ratios of the P_n phase compared to the dominant crustal phase were utilized to resolve azimuthally dependent velocity gradients with depth. A P -wave anisotropy of 3–4 per cent in a horizontal plane immediately below the Moho at a depth of 30 km, increasing to 11 per cent at a depth of 40 km, was determined. For the axis of the highest velocity of about 8.03 km s⁻¹ at a depth of 30 km a direction of N31°F was obtained. The azimuthal dependence of the observed P_n amplitude is explained by an azimuth-dependent sub-Moho velocity gradient decreasing from 0.06 s⁻¹ in the fast direction to 0 s⁻¹ in the slow direction of horizontal P -wave velocity. From the seismic results in this study a petrological model suggesting a change of modal composition and percentage of oriented olivine with depth was derived.     

Transient and impulse responses of a one-dimensional linearly attenuating medium—II. A parametric study

Summary. We investigate one-dimensional waves in a standard linear solid for geophysically relevant ranges of the parameters. The critical parameters are shown to be T*= t_u/Q_m where t _u is the travel time and Q_m the quality factor in the absorption band, and τ⁻¹ _m , the high-frequency cut-off of the relaxation spectrum. The visual onset time, rise time, peak time, and peak amplitude are studied as functions of T* and τ _m. For very small τ _m , this model is shown to be very similar to previously proposed attenuation models. As τ _m grows past a critical value which depends on T* , the character of the attenuated pulse changes. Seismological implications of this model may be inferred by comparing body wave travel times with a'one second'earth model derived from long-period observations and corrected for attenuation effects assuming a frequency independent Q over the seismic band. From such a comparison we speculate that there may be a gap in the relaxation spectrum of the Earth's mantle for relaxation times shorter than about one second. However, observational constraints from the attenuation of body waves suggest that such a gap might in fact occur at higher frequencies. Such a hypothesis would imply a frequency dependence of Q in the Earth's mantle for short-period body waves.     

Shear-wave anisotropy: spatial and temporal variations in time delays at Parkfield, Central California

Shear-wave splitting is analysed on data recorded by the High Resolution Seismic Network (HRSN) at Parkfield on the San Andreas fault, Central California, during the three-year period 1988-1990. Shear-wave polarizations either side of the fault are generally aligned in directions consistent with the regional horizontal maximum compressive stress, at some 70° to the fault strike, whereas at station MM in the immediate fault zone, shear-wave polarizations are aligned approximately parallel to the fault. Normalized time delays at this station are found to be about twice as large as those in the rock mass either side. This suggests that fluid-filled cracks and fractures within the fault zone are elastically or seismically different from those in the surrounding rocks, and that the alignment of fault-parallel shear-wave polarizations are associated with some fault-specific phenomenon.
Temporal variations in time delays between the two split shear-waves before and after a M_L = 4 earthquake can be identified at two stations with sufficient data: MM within the fault zone and VC outside the immediate fault zone. Time delays between faster and slower split shear waves increase before the M_L = 4 earthquake and decrease near the time of the event. The temporal variations are statistically significant at 68 per cent confidence levels. Earthquake doublets and multiplets also show similar temporal variations, consistent with those predicted by anisotropic poroelasticity theory for stress modifications to the microcrack geometry pervading the rock mass. This study is broadly consistent with the behaviour observed before three other earthquakes, suggesting that the build-up of stress before earthquakes may be monitored and interpreted by the analysis of shear-wave splitting.     

Comments on the Chandler wobble Q

Summary. The Chandler wobble Q _w, as obtained from the astronomical data cannot be equated with the Q _m of the source of damping, as an examination of Chandler wobble energetics reveals. We find that if dissipation occurs in the mantle then Q _w≃ 9 Q _m, implying that either the mantle Q is frequency dependent or the wobble Q is much larger than 100. If the dissipation is in the oceans then Q _w≃ 20 Q _o, and the pole tide must be far from equilibrium.     

Anomalies of some tidal waves of UT1

Summary. The M _f and M _m waves of UT1 have been analysed from the BIH data during the period 1967.0 to 1984.0 in order to derive the Love number k .
These analyses performed during successive intervals of this period show some anomalies in the values of the Love number k derived from the M _f wave. Large variations with time of the amplitude and the phase appear for this wave while the values derived from the M _m wave present a good stability during the whole period.
The spectrum of the UT1-residuals (as obtained by removing the theoretical zonal tidal UT1 terms) shows the existence of a perturbation wave near the M _f period. This wave lies in the range 13.5–13.9 day according to the analysed interval; it could be related with the perturbation wave noticed by some authors near the M _f gravimetric wave.     

Palaeomagnetic results from the Cretaceous Bagh Group in the Narmada Basin, central India: evidence of pervasive Deccan remagnetization and its implications for Deccan volcanism

A palaeomagnetic study of 115 samples (328 specimens) from 22 sites of the Mid- to Upper Cretaceous Bagh Group underlying the Deccan Traps in the Man valley (22°  20'N, 75°  5'E) of the Narmada Basin is reported. A characteristic magnetization of dominantly reverse polarity has been isolated from the entire rock succession, whose depositional age is constrained within the Cretaceous Normal Superchron. Only a few samples in the uppermost strata have yielded either normal or mixed polarity directions. The overall mean of reverse magnetization is D _m=144°, I _m=47° ( α ₉₅=2.8°, k =152, N =18 sites) with the corresponding S-pole position 28.7°S, 111.2°E ( A ₉₅=3.1°) and a palaeolatitude of 28°S±3°. The characteristic remanence is carried dominantly by magnetite. Similar magnetizations of reverse polarity are also exhibited by Deccan basalt samples and a mafic dyke in the study area. This pole position falls near the Late Cretaceous segment of the Indian APWP and is concordant with poles reported from the Deccan basalt flows and dated DSDP cores (75–65  Ma) of the Indian Ocean. It is therefore concluded that the Bagh Group in the eastern part of the Narmada Basin has been pervasively remagnetized by the igneous activity of Deccan basalt effusion. This overprinted palaeomagnetic signature in the Bagh Group indicates a counter-clockwise rotation by 13°±3° and a latitudinal drift northwards by 3°±3° of the Indian subcontinent during Deccan volcanism.     

A three-dimensional radially anisotropic model of shear velocity in the whole mantle

We present a 3-D radially anisotropic S velocity model of the whole mantle (SAW642AN), obtained using a large three component surface and body waveform data set and an iterative inversion for structure and source parameters based on Non-linear Asymptotic Coupling Theory (NACT). The model is parametrized in level 4 spherical splines, which have a spacing of ∼ 8°. The model shows a link between mantle flow and anisotropy in a variety of depth ranges. In the uppermost mantle, we confirm observations of regions with   V_SH > V_SV   starting at ∼80 km under oceanic regions and ∼200 km under stable continental lithosphere, suggesting horizontal flow beneath the lithosphere. We also observe a   V_SV > V_SH   signature at ∼150–300 km depth beneath major ridge systems with amplitude correlated with spreading rate for fast-spreading segments. In the transition zone (400–700 km depth), regions of subducted slab material are associated with   V_SV > V_SH   , while the ridge signal decreases. While the mid-mantle has lower amplitude anisotropy (<1 per cent), we also confirm the observation of radially symmetric   V_SH > V_SV   in the lowermost 300 km, which appears to be a robust conclusion, despite an error in our previous paper which has been corrected here. The 3-D deviations from this signature are associated with the large-scale low-velocity superplumes under the central Pacific and Africa, suggesting that   V_SH > V_SV   is generated in the predominant horizontal flow of a mechanical boundary layer, with a change in signature related to transition to upwelling at the superplumes.     

Review of the low-temperature magnetic properties of magnetite from a rock magnetic perspective

The low-temperature magnetic properties of magnetite are reviewed, and implications for rock magnetism considered. The behaviour of fundamental properties of magnetite at low temperatures near the Verwey transition ( T _v ) are documented, and attention is given to various Verwey transition theories. The low-temperature behaviour of the magnetic energies that control domain structure is reviewed in detail. For the first time in rock magnetic literature, the low-temperature anomaly in spontaneous magnetization ( M _s ) is documented and the differences between the saturation magnetization and M _s near the Verwey transition are discussed. It is argued that the low-temperature behaviour of the magnetocrystalline anisotropy, and in particular the anomaly at T _v , is most likely to affect multidomain remanence during low-temperature cycling. For multidomain crystals it is calculated that the large increase in magnetocrystalline anisotropy intensity and reduction in symmetry on cooling through T _v is likely to reduce the stability of closure domains.     

Palaeomagnetism of Dhar traps and drift of the subcontinent during the Deccan volcanism

Summary. Palaeomagnetic investigations were carried out on nine lava flows around the Dhar region, which constitute the northern part of the Deccan traps. The stability of remanent magnetism of these specimens was analysed by an alternating magnetic field, thermal demagnetization and memory tests. Six flows exhibited characteristic components of magnetization, with a mean direction of D =143°, I =+46° ( K = 107.1, α₉₅=5.5°). This gives a VGP located at 29° N, 67° W (δ_p=4.5°, δ_m= 7.0°). The lower site with normal magnetization and the upper five sites with reverse magnetization indicate a geomagnetic field reversal during the initial phases of Deccan volcanism in the Early Tertiary period. A rapid northward migration of about 18° in latitude and a simultaneous anticlockwise rotation of 37° is calculated for the subcontinent.     

Significance of the sign changing of the imaginary arrows in geomagnetic induction investigation

Summary. In this investigation, we carry out a two-dimensional study of the dependence of the imaginary Parkinson arrows on the frequency of the inducing geomagnetic field. Our results demonstrate that the imaginary arrows reverse direction as the inducing period varies. Therefore, we consider that there is no way to fix a consistent sign convention for the imaginary arrows even when the time factor is taken into account. We find that in the twodimensional case there exists a characteristic period T _c at which the phase difference between the vertical and horizontal magnetic components is zero. It is anticipated that T _c is related to the parameters of the conductivity anomaly and the status of the half-space host.     

Calculating horizontal stress orientations with full or partial knowledge of the tectonic stress tensor

Earthquakes potentially serve as abundant and cost-effective gauges of tectonic stress provided that reliable means exist of extracting robust stress parameters. Several algorithms have been developed for this task, each of which typically provides information on the orientations of the three principal stresses and a single stress magnitude parameter. A convenient way of displaying tectonic stress results is to map the azimuth of maximum horizontal compressive stress, which is usually approximated using the azimuth of the larger subhorizontal principal stress. This approximation introduces avoidable errors that depend not only on the principal stress axes' plunges but also on the value of the stress magnitude parameter. Here we outline a method of computing the true direction of maximum horizontal compressive stress ( S _H) and show that this computation can be performed using only the four stress parameters obtained in routine focal mechanism stress estimation. Using theoretical examples and new stress inversion results obtained with focal mechanism data from the central Grímsey lineament, northern Iceland, we show that the S _H axis may differ by tens of degrees from its commonly adopted proxy. In order to most appropriately compare tectonic stress estimates with other geophysical parameters, such as seismic fast directions or geodetically measured strain rate tensors, or to investigate spatiotemporal variations in stress, we recommend that full use be made of the routinely estimated stress parameters and that a formal axis of maximum horizontal compression be calculated.     

Regional variations of heat flow differences with depth in Alberta, Canada

Summary. Differences between estimated average heat flow values for the Mesozoic and Cenozoic formations ( Q ₁) and estimated average heat flow values for the Palaeozoic formations below the erosional unconformity ( Q ₂) are calculated for the Alberta part of the western Canadian sedimentary basin. Significant heat flow differences exist for these two intervals and the map of Δ Q = Q ₁– Q ₂ shows that Q ₂ is generally greater than Q ₁ in the western and south-western part of Alberta, while in the northern part of the province Q ₂ is generally less than Q ₁. The regional variations of Δ Q are large, with standard deviation of 26 mW m⁻² and average value –13.5 mW m⁻². A regional trend of Δ Q correlates with topographic relief and the hydraulic head variations in the basin. It is shown that there is a heat flow increase with depth in water recharge areas and a decrease in heat flow with depth in the low topographic elevation water discharge areas when comparing the average heat flow in Mesozoic + Cenozoic and Palaeozoic formations.     

Aseismic fault movement before the 1995 Kobe earthquake detected by a GPS survey: implication for preseismic stress localization?

By inversion analysis of the baseline changes and horizontal displacements observed with GPS (Global Positioning System) during 1990–1994, a high-angle reverse fault was detected in the Shikoku-Kinki region, southwest Japan. The active blind fault is characterized by reverse dip-slip (0.7±0.2  m yr⁻¹ within a layer 17–26  km deep) with a length of 208±5  km, a (down-dip) width of 9±2  km, a dip-angle of 51°±2° and a strike direction of 40°±2° (NE). Evidence from the geological investigation of subfaults close to the southwestern portion of the fault, two historical earthquakes ( M _L=7.0, 1789 and 6.4, 1955) near the centre of the fault, and an additional inversion analysis of the baseline changes recorded by the nationwide permanent GPS array from 18 January to 31 December 1995 partially demonstrates the existence of the fault, and suggests that it might be a reactivation of a pre-existing fault in this region. The fact that hardly any earthquakes ( M _L>2.0) occurred at depth on the inferred fault plane suggests that the fault activity was largely aseismic. Based on the parameters of the blind fault estimated in this study, we evaluated stress changes in this region. It is found that shear stress concentrated and increased by up to 2.1 bar yr⁻¹ at a depth of about 20  km around the epicentral area of the 1995 January 17  Kobe earthquake ( M _L=7.2, Japan), and that the earthquake hypocentre received a Coulomb failure stress of about 5.6 bar yr⁻¹ during 1990–1994. The results suggest that the 1995  Kobe earthquake could have been induced or triggered by aseismic fault movement.     

Present‐day stress orientation in the Clarence‐Moreton Basin of New South Wales,Australia: a new high density dataset reveals local stress rotations

Early phases of the Australian Stress Map project revealed that plate boundary forces acting on the Indo‐Australian Plate control the long wavelength of the maximum horizontal present‐day stress orientation in the Australian continent. However, all numerical models of the stress field to date are unable to predict the observed orientation of maximum horizontal stress in the northeast of New South Wales, Australia. Recent coal seam gas exploration in the Clarence‐Moreton Basin, eastern Australia, provides an opportunity to better evaluate the state of crustal stress in this part of the continent where only limited information was available prior to this study. Herein, we conduct the first analysis of the present‐day tectonic stress in the Clarence‐Moreton Basin, from drilling‐induced tensile fractures and borehole breakouts interpreted using 11.3 km of acoustic image logs in 27 vertical wells. A total of 2822 drilling‐induced stress indicators suggest a mean orientation of N069°E (±23°) for the maximum horizontal present‐day stress in the basin which is different from that predicted by published geomechanical‐numerical models. In addition, we find significant localised perturbations of borehole breakouts, both spatially and with depth, that are consistent with stress variations near faults, fractures and lithological contrasts, indicating that local structures are an important source of stress in the basin. The observation that structures can have a major control on the stresses in the basin suggests that, while gravity and plate boundary forces have the major role in the long wavelength (first‐order) stress pattern of the continent, local perturbations are significant and can lead to substantial changes in the orientation of the maximum horizontal present‐day stress, particularly at the basin scale. These local perturbations of stress as a result of faults and fractures have important implications in borehole stability and permeability of coal seam gas reservoirs for safe and sustainable extraction of methane in this area.     

Bending of spherical lithosphere–axisymmetric case

Effects of sphericity are commonly ignored in the lithospheric bending problem. In order to examine its effects, I solve a simple axisymmetric spherical-shell model. The full solution and the asymptotic solution are derived from the basic equations, and their relationship to the flat-plate solution is examined. For displacement, effects of sphericity are small, and use of the flat-plate solution produces results that are numerically indistinguishable from those of the spherical solution. The most significant effect of sphericity appears in the stress, in particular the normal stress along the strike direction of the trench. This stress is approximately given by Eu_r/R , where E is Young's modulus, u_r is the vertical deformation of the shell and R is its radius of curvature. If the shell (lithosphere) is bent downwards and reaches 30 km, this stress can become about 5 kbar in the Earth. While plastic behaviour may set in under such high pressure conditions and analysis beyond elasticity theory may be required, sphericity may be a cause of large compressive stress in the trench strike direction. This stress may play an important role in forming the overall shape of the Earth's subduction zones.     

Seismic waves in rocks with fluids and fractures

Seismic wave propagation through the earth is often strongly affected by the presence of fractures. When these fractures are filled with fluids (oil, gas, water, CO₂, etc.), the type and state of the fluid (liquid or gas) can make a large difference in the response of the seismic waves. This paper summarizes recent work on methods of deconstructing the effects of fractures, and any fluids within these fractures, on seismic wave propagation as observed in reflection seismic data. One method explored here is Thomsen's weak anisotropy approximation for wave moveout (since fractures often induce elastic anisotropy due to non-uniform crack-orientation statistics). Another method makes use of some very convenient crack/fracture parameters introduced previously that permit a relatively simple deconstruction of the elastic and wave propagation behaviour in terms of a small number of crack-influence parameters (whenever this is appropriate, as is certainly the case for small crack densities). Then, the quantitative effects of fluids on these crack-influence parameters are shown to be directly related to Skempton's coefficient B of undrained poroelasticity (where B typically ranges from 0 to 1). In particular, the rigorous result obtained for the low crack density limit is that the crack-influence parameters are multiplied by a factor  (1 − B )  for undrained systems. It is also shown how fracture anisotropy affects Rayleigh wave speed, and how measured Rayleigh wave speeds can be used to infer shear wave speed of the fractured medium in some cases. Higher crack density results are also presented by incorporating recent simulation data on such cracked systems.     

Experimental study of the anisotropy of longitudinal and transverse waves from local earthquake records

Summary. The paper gives the results of a study of the anisotropy of seismic wave velocities within the Ashkhabad test field in Central Asia. The anisotropy was studied by analysing variations in the values of apparent velocities of first arrivals for epicentral distances ranging from 30 to 130 km and by analysing the delays (Δ t_s1-_s2 ) between the arrival times of shear waves with different polarizations.
The velocities of P -waves vary with azimuth from 5.3 to 6.27 km s^-1 and the velocities of S -waves vary from 3.15 to 3.5 km s^-1.
The delay times Δ t_S1 - _S2 depend on the direction of the propagation. The character of the variation of the propagation velocity of the longitudinal wave, the presence of two differently polarized shear waves S 1 and S 2 propagating at different velocities, and the character of the distribution of Δ t_S1 - _S2 on the stereogram suggest that the symmetry of the anisotropic medium is close to hexagonal with a nearly horizontal symmetry axis coinciding with the direction of maximal velocity. The azimuth of the symmetry axis of the medium is 140° and coincides with the direction of geological faults.     

Fracturing and fluid‐flow during post‐rift subsidence in carbonates of the Jandaíra Formation,Potiguar Basin,NE Brazil

Pervasive fracture networks are common in many reservoir‐scale carbonate bodies even in the absence of large deformation and exert a major impact on their mechanical and flow behaviour. The Upper Cretaceous Jandaíra Formation is a few hundred meters thick succession of shallow water carbonates deposited during the early post‐rift stage of the Potiguar rift (NE Brazil). The Jandaíra Formation in the present onshore domain experienced <1.5 km thermal subsidence and, following Tertiary exhumation, forms outcrops over an area of >1000 km². The carbonates have a gentle, <5?, dip to the NE and are affected by few regional, low displacement faults or folds. Despite their simple tectonic history, carbonates display ubiquitous open fractures, sub‐vertical veins, and sub‐vertical as well as sub‐horizontal stylolites. Combining structural analysis, drone imaging, isotope studies and mathematical modelling, we reconstruct the fracturing history of the Jandaíra Formation during and following subsidence and analyse the impact fractures had on coeval fluid flow. We find that Jandaíra carbonates, fully cemented after early diagenesis, experienced negligible deformation during the first few hundreds of meters of subsidence but were pervasively fractured when they reached depths >400–500 m. Deformation was accommodated by a dense network of sub‐vertical mode I and hybrid fractures associated with sub‐vertical stylolites developed in a stress field characterised by a sub‐horizontal σ₁ and sub‐vertical σ₂. The development of a network of hybrid fractures, rarely reported in the literature, activated the circulation of waters charged in the mountainous region, flowing along the porous Açu sandstone underlying the Jandaíra carbonates and rising to the surface through the fractured carbonates. With persisting subsidence, carbonates reached depths of 800–900 m entering a depth interval characterised by a sub‐vertical σ₁. At this stage, sub‐horizontal stylolites developed liberating calcite which sealed the sub‐vertical open fractures transforming them in veins and preventing further flow. During Tertiary exhumation, several of the pre‐existing veins and stylolites opened and became longer, and new fractures were created typically with the same directions of the older features. The simplicity of our model suggests that most rocks in passive margin settings might have followed a similar evolution and thus display similar structures.     

TRM deviations in anisotropic assemblages of multidomain magnetite

Anisotropic assemblages of multidomain magnetite particles develop an anisotropy of magnetic susceptibility (AMS), which in turn induces deviations of thermo-remanent magnetization (TRM) from the field direction. From the theories of multidomain TRM acquisition, it is shown that the TRM anisotropy tensor has its eigenvalue ratios ( T _i) related to the principal weak-field susceptibility ratios ( P _i) by the order of magnitude T _i≃ P ²_i. This relation has been experimentally verified on two sets of highly anisotropic rock samples. The exponent has been determined to be 1.94 in the samples from a Peruvian gabbro, and 1.81 in those from the granite of Flamanville (NW France). Accounting for experimental difficulties in determining the TRM anisotropy tensors, these exponents are judged to agree well with the expected one. It is therefore stressed that AMS measurements provide a good means of evaluating the magnetic field direction from deviated TRM directions, providing magnetic carriers are mainly multidomain magnetites.     

Modelling the compliance of crustal rock—I. Response of shear-wave splitting to differential stress

We show that seismic shear waves may be used to monitor the in situ stress state of deep inaccessible rocks in the crust. The most widespread manifestation of the stress-related behaviour of seismic waves is the shear-wave splitting (shear-wave birefringence) observed in almost all rocks, where the polarizations of the leading split shear waves are usually subparallel to the direction of the local maximum horizontal stress. It has been recognized that such shear-wave splitting is typically the result of propagation through distributions of stress-aligned fluid-filled microcracks and pores, known as extensive-dilatancy anisotropy or EDA. This paper provides a quantitative basis for the EDA hypothesis. We model the evolution of anisotropic distributions of microcracks in triaxial differential stress, where the driving mechanism is fluid migration along pressure gradients between neighbouring microcracks and pores at different orientations to the stress field. This leads to a non-linear anisotropic poroelasticity (APE) model for the stress-sensitive behaviour of fluid-saturated microcracked rocks. A companion paper shows that APE modelling matches a range of observed phenomena and is a good approximation to the equation of state of a stressed fluid-saturated rock mass.