Showing papers in "Geophysical Journal International in 1994"

The Interpretation of Shear‐Wave Splitting Parameters In the Presence of Two Anisotropic Layers

Abstract: SUMMARY We consider the behaviour of shear-wave splitting parameters that are made under the assumption of a single layer of anisotropic material when in fact two layers of differing anisotropic properties are present. It is shown that the resulting apparent splitting parameters are still meaningful quantities and in fact can (at fixed frequency) be written as trigonometric functions of the splitting parameters of the individual layers. These expressions reveal many properties of the apparent splitting parameters. For example, the apparent fast polarization 4, and delay time 6t, exhibit systematic variations as a function of incoming polarization with r/2 periodicity. The derived expressions can be used to invert for the individual properties of the two layers in many circumstances. We show that several stations along the San Andreas fault system display the properties of two layers, with the top layer being parallel to the local strike of the San Andreas fault. Finally, the derived expressions for the apparent splitting parameters are shown to be easily generalized to multiple layers.

The fracture criticality of crustal rocks

Abstract: SUMMARY The shear-wave splitting observed along almost all shear-wave ray paths in the Earth’s crust is interpreted as the effects of stress-aligned fluid-filled cracks, microcracks, and preferentially oriented pore space. Once away from the free surface, where open joints and fractures may lead to strong anisotropy of 10 per cent or greater, intact ostensibly unfractured crustal rock exhibits a limited range of shear-wave splitting from about 1.5 to 4.5 per cent differential shear-wave velocity anisotropy. Interpreting this velocity anisotropy as normalized crack densities, a factor of less than two in crack radius covers the range from the minimum 1.5 per cent anisotropy observed in intact rock to the 10 per cent observed in heavily cracked almost disaggregated near-surface rocks. This narrow range of crack dimensions and the pronounced effect on rock cohesion suggests that there is a state of fracture criticality at some level of anisotropy between 4.5 and 10 per cent marking the boundary between essentially intact, and heavily fractured rock. When the level of fracture criticality is exceeded, cracking is so severe that there is a breakdown in shear strength, the likelihood of progressive fracturing and the dispersal of pore fluids through enhanced permeability. The range of normalized crack dimensions below fracture criticality is so small in intact rock, that any modification to the crack geometry by even minor changes of conditions or minor deformation (particularly in the presence of high pore-fluid pressures) may change rock from being essentially intact (below fracture criticality) to heavily fractured (above fracture criticality). This recognition of the essential compliance of most crustal rocks, and its effect on shear-wave splitting, has implications for monitoring changes in any conditions affecting the rock mass. These include monitoring changes in reservoir evolution during hydrocarbon production and enhanced oil recovery, and in monitoring changes before and after earthquakes, amongst others.

A technique for the measurement of thermal changes of magnetic susceptibility of weakly magnetic rocks by the CS-2 apparatus and KLY-2 Kappabridge

Abstract: SUMMARY Techniques for the correction of the effect of the furnace signal on the data measured in the investigation of thermal changes of magnetic susceptibility of weakly magnetic rocks using the CS-2 apparatus and KLY-2 Kappabridge are described. A new method is developed for separating the ferromagnetic and paramagnetic room temperature susceptibility components even for the case in which new magnetite forms during heating.

Modified frequency-wavenumber algorithm for regional seismograms using Filon's quadrature: modelling of Lg waves in eastern North America

Abstract: SUMMARY The main focus of this study is to develop an efficient frequency-wavenumber code to synthesize high-frequency (say, 10 Hz) regional seismograms (up to a distance of more than 1000 km) in a medium consisting of a large number of crustal layers, and investigate effects of regional waveguides on Lg waves. In this newly developed code, the frequency-wavenumber response is evaluated by using the Filon's quadrature at high frequency and the polynomial approximation of Bessei's functions (implementing trapezoidal integration) at low frequency. This has reduced the computation time by several folds compared to the time needed for the trapezoidal integration. Using this algorithm, both broad-band seismograms at Harvard and short-period ECTN (Eastern Canadian Telemetered Network) seismograms from 1988 November 25 Saguenay earthquake were successfully modelled. A moment magnitude of 5.9 was used. Initially an average waveguide was calibrated to successfully excite the regional Pnl and Snl waves at HRV, its near-surface structure constrained by matching the recorded frequency content. The crust-mantle transition zone was constrained by modelling the strength of the broad-band Snl waves, consisting of Sn, sSmSnSmSSmS, and sSmSSmS phases, relative to the initial Pnl waves. The model was modified to consist of thin layers with an alternating high-and low-velocity distribution to match the high-frequency ECTN Lg waves observed from 300 to 600 km away. In addition, the effect of a near-surface irregular receiver structure on the duration of Lg waves was also investigated by convolving its response with the regional Lg seismograms. Initial study suggests this method as viable for modelling the entire Lg waves including the coda. Numerical studies suggest that the shapes and peak amplitudes on Lg seismograms depend more strongly on the source depth, anelasticity and crustal waveguide than do the Pnl seismograms.

The Somalia plate and the East African Rift System: present-day kinematics

Abstract: SUMMARY The motion of the Somalia plate relative to the Nubia (Africa), Arabia and Antarctica platcs is re-evaluated using a new inversion method based on a Monte Carlo technique and a least absolute value misfit criterion. A subset of the NUVEL 1 data set, with additional data along the Levant Fault and in the Red Sea is used. The results confirm that the motion of Arabia with respect to Africa is significantly different from the motion relative to Somalia. It is further shown that the data along the SW Indian Ridge are compatible with a pole of relative motion between Africa and Somalia located close to the hypothetical diffuse triple junction between the ridge and thc East African Rift. The resulting Africa-Somalia motion is then compatible with the geological structures and seismological data along the East African Rift system. Assuming a separate Somalia plate thus solves kinematic and geological problems around the Afar triple junction and along the East African Rift.

Pn Velocities Beneath Continental Collision Zones: the Turkish-Iranian Plateau

Abstract: SUMMARY Tomographic imaging of the uppermost mantle velocity in the Middle East shows normal Pn velocities (8.0-8.2 kms-’) beneath the Black Sea and the southern Caspian Sea. In contrast, low Pn velocities (<7.9 km s-I) are found beneath the Aegean Sea and much of the Turkish-Iranian Plateau. A region of exceptionally low Pn velocities (c7.6 km s-’) beneath the Lesser Caucasus Mountains and along the Turkish-Iranian border coincides with a region of high Sn attenuation and extensive Neogene volcanism. All these features suggest that near solidus conditions exist within the uppermost mantle beneath the Turkish-Iranian Plateau. Such conditions may result either from the decompression melting associated with upwelling convection cells or by the infiltration of water released from subducted lithosphere into the mantle above. The presence of a partially melted uppermost mantle weakens the lithosphere beneath the Turkish-Iranian Plateau, thus allowing it to become the locus of deformation in the Arabian-Eurasian collision zone.

Implications of earthquake focal mechanism data for the active tectonics of the south Caspian Basin and surrounding regions

Abstract: SUMMARY The south Caspian Basin is a relatively aseismic block within the Alpine-Himalayan Belt, but is surrounded by zones of high seismicity. We used the focal mechanisms of 16 earthquakes whose source parameters we determined from inversion of body waves and the mechanisms of 15 other earthquakes to determine the style of faulting in the seismic belts surrounding the south Caspian Basin. Earthquakes beneath the Talesh Mountains of north-west Iran and immediately off-shore in the south-west Caspian Sea have shallow thrust mechanisms, showing that the continental crust of north-west Iran is overthrusting the ‘oceanic-like’ crust of the south Caspian Basin. Earthquakes south of the Caspian Sea in northern Iran show a mixture of focal mechanisms. Both high-angle reverse faulting and left-lateral strike-slip faulting mechanisms are observed in the high Alborz Mountains. Farther south, on the edge of the central Iran plateau, oblique left-lateral reverse mechanisms are observed. It appears that the NE direction of shortening between central Iran and the south Caspian Basin is partitioned into pure left-lateral strike-slip and thrusting in the WNW-trending high Alborz, but is accommodated by oblique faulting in lower elevations. Earthquakes in the Kopet Dag Mountains east of the Caspian Sea also show a mixture of high-angle reverse and strike-slip faulting mechanisms and may be another example of the partitioning of oblique slip into strike slip and thrust motion. Normal faulting mechanisms at centroid depths of 35–50 km dominate in the belt of seismicity that extends across the central Caspian Sea. The significance of the normal faulting earthquakes is enigmatic. It is improbable that these events represent the motion between the southern Caspian Basin and Eurasia for they imply a sense of motion that is incompatible with the observed topography and folding in the sediments. Two shallow earthquakes at about 12 km depth in this belt, one a small event and the other a large second subevent of a multiple earthquake, have thrusting mechanisms suggesting that shortening occurs as the continental crust of the northern Caspian Sea is thrust over the ‘oceanic-like’ crust of the southern Caspian Basin. Shortening is also suggested by the orientations of folds in the sedimentary cover south of the central Caspian Sea seismic belt. We suggest that this shortening does indeed represent a NNE motion of the Caspian Sea relative to Eurasia, but that the motion is slow and has not produced many earthquakes. The deeper, normal-faulting events may be related to bending or down-dip extension of the incipient subducted slab. If the motion of the Caspian Sea relative to Eurasia is indeed slow, then the motion in the Alborz between central Iran and the south Caspian Basin will be almost the same as that between Iran and Eurasia, as has previously been assumed. The combined effect of the overthrusting of the south Caspian Basin by the Talesh-Alborz Mountains in the south, and by the continental crust of the northern Caspian Sea in the north will lead to the eventual destruction of the south Caspian Basin and the possible formation of an intermediate-depth, dipping seismic zone within the continental interior, similar to that presently observed in the Hindu Kush.

Applied geophysical inversion

Abstract: SUMMARY Using the 2-D DC-resistivity tomography experiment as an example, we examine some of the difficulties inherently associated with constructing a single maximally smooth model as a solution to a geophysical inverse problem. We argue that this conventional approach yields at best only a single model from a myriad, of possible models and at worst produces a model which, although having minimum structure, frequently has little useful relation to the earth that gave rise to the observed data. In fact in applied geophysics it is usual to have significant prior information which is to be supplemented by further geophysical experiments. With, this perspective we suggest an alternate approach to geophysical inverse problems which emphasizes the prior information and includes the data from the geophysical experiment as a supplementary constraint. To this end we take all available prior information and construct an inversion algorithm which, given an arbitrary starting model and the absence of any data, will produce a preconceived earth model and then introduce the observed data into the inversion to determine how the prior earth model is influenced by the supplementary geophysical data.

Shear‐wave velocity variations in the upper mantle beneath central Europe

Abstract: SUMMARY A new model, EUR-S91, of shear-wave velocity variations in the upper mantle beneath central Europe and surrounding regions, down to a depth of 670km, is presented. The model is derived from the inversion of the waveforms of 217 seismograms, using the partitioned waveform-inversion method for the seismogram in the time window from the S-wave arrival to the fundamental mode of the Rayleigh wave. The seismograms were mostly assembled from digitally recording long-period and broad-band stations in Europe. The resulting 3-D model accurately predicts most of the observed waveforms, in a wide band of frequencies. Body waves were fit for frequencies up to 60mHz. The fundamental Rayleigh mode, which at high frequencies is more prone to scattering and multipathing, was generally low passed at 25 mHz. The resolving power of the data set depends strongly on the density of available wave paths and varies as a function of geographical position. Small-scale heterogeneities like the subducted lithosphere in the Hellenic collision zone were imaged in the region with the highest density of wave paths, which indicates an optimum resolution of better than 200 km. The main new results of this study pertain to the transition between east and central Europe. We present new information about the structure below the Tornquist-Teisseyre Zone (TTZ). The TTZ is generally regarded as the boundary between the Precambrian crust of the Baltic Shield/Russian Platform and the younger crust of central Europe. Our 3-D S-velocity model reveals that below this line a sharp lateral boundary extends down to at least 140 km depth, with high velocities beneath the Baltic Shield and Russian Platform contrasting to the low velocities beneath the younger regions of Europe. The observed velocity contrast across the TTZ is largest between the Pannonian Basin and the Russian Platform, where it is equal to 12 per cent at 80 km depth. At depths of 300–400 km, the TTZ is underlain by a zone of low S velocity, which indicates a local thinning of the deeply rooted high-velocity structure or tectosphere that underlies most of eastern Europe. In addition to these new results on the transition region between east and central Europe, a number of features that are present in other tomographic results have been confirmed: the Precambrian provinces of Europe are characterized by high S velocities; low velocities are found beneath the Pannonian Basin, the western Mediterranean, northern Aegean Sea/Turkey and a small region south-west of the Massif Central. The lithosphere beneath the Paris Basin has a positive S-velocity anomaly. The resolution of all these anomalies has been established with sensitivity tests.

Elastodynamic fundamental solutions for anisotropic solids

Abstract: SUMMARY 3-D and 2-D time-domain elastodynamic fundamental solutions (or Green's functions) for linearly elastic anisotropic materials are obtained by the Radon transform. Fundamental solutions in the frequency domain follow directly by a subsequent evaluation of the Fourier transforms of the time-domain solutions. The solutions are in the form of a surface integral over a unit sphere for 3-D cases and in the form of a contour integral over a unit circle for 2-D cases. The integrals have a simple structure that can be interpreted as a superposition of plane waves. The wavefields can be separated into singular and regular parts. The singular parts correspond to the elastostatic fundamental solutions. The regular parts are bounded continuous functions. The integrals have been evaluated numerically for several examples. The results presented in this paper have direct applications to the formulation of boundary-integral equations for bodies of anisotropic materials and for the subsequent solution of these equations by the boundary-element method.

Magnetic enhancement and palaeoclimate: study of a loess/palaeosol couplet across the Loess Plateau of China

Abstract: SUMMARY The origin of the Milankovitch palaeoclimatic signal so dramatically recorded in the Chinese loess remains uncertain. At least five hypotheses have been put forward to explain the well-documented enhancement of magnetic susceptibility in palaeosols which formed when conditions were warmer and wetter. We report here a magnetic study of a prominent loess-soil couplet (L4-S3) at three widely separated sites where present-day mean annual rainfall and temperature range from ∼350mm to ∼700mm and from ∼6°C to ∼13°C. the properties determined are laboratory remanences (isothermal, IRM; anhysteretic, ARM; viscous, VRM, including viscous decay), room-temperature susceptibility, and thermomagnetic behaviour. At fields beyond 100 mT, the IRM spectra are all essentially identical, both in shape and in absolute magnitude, suggesting a uniform magnetic component widespread across the Plateau. Subtracting the signal of this component isolates a second component, the amount of which varies by an order of magnitude between sites. the magnetic properties of this latter material are remarkably similar to those reported for bacterial magnetite in geological sediments and modern soils. Viscous decay of VRM takes place in two distinct stages, one of which falls to zero in < 10min. We attribute magnetic enhancement-which is the source of the palaeoclimatic signal to in situ pedogenetic formation of magnetite, which we characterize in our interpretation as two separate forms of bacterial magnetofossils. the most magnetic sample requires a mass fraction of about half of one per cent of this biogenic material.

Constraints On the Seismotectonics of the Central Mediterranean From Very Long Baseline Interferometry

Abstract: SUMMARY Very Long Baseline Interferometry (VLBI) determined site velocities from seven stations in western Europe reveal a stable continental platform north of the Alps. Deformations between Sweden, Germany and Spain can not exceed 2 mm yr-'. South of the Alps, significant motions are occurring with respect to stable Europe. Two sites east of the Apennine mountains on peninsular Italy have north-easttrending velocities which increase from 2 mm yr-l in the north at Medicina to 6 mm yr-l in the south at Matera. In contrast, the VLBI site in the south-eastern corner of Sicily is moving 7 mm yr-I to the north-north-west. These velocities are largely explained if southern Sicily is attached to a north-westerly moving African Plate and the eastern portion of the Italian peninsula forms part of a hypothesized Adriatic Sea crustal block which is rotating counter-clockwise with respect to Europe about a pole near the Alps. Such an explanation is consistent with the styles of the larger historical earthquakes of the region, which show NE-SW extension across the Apennines, north-south convergence across the Alps, and NE-SW compression in coastal Yugoslavia. The Adria plate model generates 1.5-2.2 X 10l8 Nm yr-l of potential earthquake moment along the northern and central Apennines. Historical records suggest that 30-60 per cent of this moment is released seismically. Based on a direct-strain-rate measurement, recurrence intervals for Italian earthquakes south of Medicina are estimated to be 12-46 yr for M6.5+ quakes and 35-143 years for M7.0+ quakes.

Calculation of sensitivities for the frequency-domain electromagnetic problem

Abstract: SUMMARY A simple derivation is presented for the computation of sensitivities needed to solve parametric inverse problems in electromagnetic induction. It is shown that ~en­ sitivities for any component of an electromagnetic field can be obtained by solving two boundary-value problems which are identical except for the specification of the source terms and (possibly) prescribed boundary conditions. The electric fields from these primal and auxiliary problems are multiplied and integrated to produce a numerical value for the sensitivity. Although the final formulae derived here are equivalent to those developed through the use of formal adjoint or Green's functions approaches, our work does not require explicit derivation of the adjoint operator and boundary conditions and does not formally invoke reciprocity.

Global spherical harmonic analysis by least‐squares and numerical quadrature methods in historical perspective

Abstract: SUMMARY Methods of global spherical harmonic analysis of discrete data on a sphere are placed in a historical context. The paper concentrates on the loss of orthogonality in the direction of latitude, due to the transition from continuous to discretized functions. Special attention is paid to Neumann's (1838) solution to this problem. By recasting the formulae of spherical harmonic analysis into matrix-vector notation, both least-squares solutions and quadrature methods are represented in a general framework of weighted least squares. It is also shown that the two-step formulation of global spherical harmonic computation was applied already by Neumann (1838) and Gauss (1839). Computational modifications to Neumann's method are reviewed as well.

Computation of synthetic seismograms and their partial derivatives for heterogeneous media with arbitrary natural boundary conditions using the Direct Solution Method

Abstract: SUMMARY A new method is presented for calculating synthetic seismograms and their partial derivatives for laterally and vertically heterogeneous media with arbitrary natural boundary conditions. The formulation is derived by adding appropriate surface integrals to the weak form (Galerkin formulation) of the elastic equation of motion to enforce the natural boundary and continuity conditions, and inhomogeneous boundary conditions. Results applicable to media consisting of a combination of fluid and solid regions are presented. The method is called the Direct Solution Method (DSM) (Geller et al. 1990c) because the synthetic seismograms and partial derivatives are computed directly by solving a system of linear equations. In contrast, almost all previous applications of Galerkin methods in seismology have first computed the modes of free oscillation, and only then computed the synthetic seismograms and partial derivatives by summing the modes. As an example of the application of our method, we calculate synthetic seismograms for heterogeneous media which are terminated at the bottom by a thin homogeneous layer with a radiation (energy-absorbing) boundary condition. This method is well suited to computing the quantities necessary to perform linearized inversion for earth structure with respect to a laterally heterogeneous earth model (Geller & Hara 1993). It thus becomes possible to formulate iterative linearized waveform inversion for laterally heterogeneous earth structure on a local and regional scale following the same basic approach used by Hara, Tsuboi & Geller (1993) to invert waveform data for global laterally heterogeneous structure.

The frequency dependence of low field susceptibility in loess sediments

Abstract: SUMMARY The low field susceptibility variations in the Chinese and Tajik loess sequences reflect the palaeoclimatic fluctuations of the Quaternary on the continents in a very direct and complete way. It can be demonstrated that the fine-grained ferromagnetic mineral fraction is largely responsible for the climatically controlled susceptibility enhancement in the loess sediments. In order to gain more information about the origin and quality of the susceptibility enhancement we are scrutinizing the F-factor (F,) which is conventionally used as a measure of the frequency dependence of the susceptibility. For qualitative estimates of the grain sizes present, F, seems to be a rather misleading parameter because it is flawed when variable amounts of minerals with both frequency-independent and frequency-dependent susceptibility are present. We introduce a new concentration-independent F-factor (K) which takes the influence of the mineral fractions with frequency-independent susceptibility on the total susceptibility into consideration. F, is fairly constant in all the investigated sections. This result is explained theoretically following NCel's (1949) fine-particle theory and Stephenson's (1971a) modelling of various single-domain grain distributions. It is found that the grain-size distribution is essentially the same throughout all loess sections investigated and the upper and Iower grain-size limits are essentially invariable. In addition, susceptibility and its frequency dependence were measured at low temperatures on selected loess samples in order to determine the maximum grain size present in the fine-grained magnetic fraction.

A geodynamic framework for interpreting crustal-scale seismic-reflectivity patterns in compressional orogens

Abstract: SUMMARY 2-D, plane-strain finite-element models are used to calculate crustal deformation patterns within model compressional orogens and the results are compared with natural examples. The fundamental driving mechanism for model deformation is provided by asymmetric detachment and underthrusting of mantle lithosphere, with deformation being rooted at the stress singularity where the mantle detaches. Deformation steps up into the overlying crust and spreads laterally according to the rhcological properties assumed for the crust. The crust is modelled using plastic (frictional) and viscous rheologies and incorporates the effects of compositional layering and variable geothermal gradients. Overall deformation is determined by balancing the internal strength of the crust against the sum of applied boundary stresses plus the gravitational stress induced by mass redistribution within the deforming orogen. The amount of convergence in the models is limited to 150 km or less and so the results are not strictly comparable with larger orogens involving greater convergence. Nevertheless, larger orogens seem to evolve through stages that may be understood in terms of the models provided that consideration is confined to smaller scale components that are limited in time and space. The continuum deformation calculated by the finite-element models is qualitatively converted to equivalent pattcrns of discrete faults and shear zones by associating these discrete features with the areas of highest cumulative strain using the results of sandbox experiments as guides. Significant styles of deformation include: (1) coupling between step-up shears and subhorizontal detachments at one or more levels; (2) localization of upper crustal deformation by subduction of the lower crust; and (3) pinning of deformation by surface denudation. A partial catalogue of such model styles is presented in which the strength of basal coupling between crust and mantle, the number of rhcological layers within the crust, the thickness of subducted lower crust, and the extent of surface denudation are varied systematically. Compressional orogens with a wide range of ages are characterized by crustal-scale seismic reflectivity similar to patterns of fault/shear zones represented in this catalogue of styles. This similarity suggests that crustalscale seismic-reflection data can be used to constrain the dynamic processes involved in compressional orogcnesis.

Magnetostratigraphic Data From Late Quaternary Sediments From the Yermak Plateau, Arctic Ocean: Evidence For Four Geomagnetic Polarity Events Within the Last 170 Ka of the Brunhes Chron

Abstract: SUMMARY Palaeomagnetic investigations of two sediment cores recovered from RV Polarstern near the eastern slope of the Yermak Plateau (sites PS 1533 and PS 2212) reveal convincing evidence for four polarity events of the Earth's magnetic field during the last 170 Ka. A comprehensive rock magnetic study of the sediments proved that fine-grained magnetite is the principal carrier of the remanent magnetization. No changes in magneto-mineralogy across the polarity transitions in the sediments investigated were found. Calcareous nannofossil biostratigraphy, AMS-14C (accelerated mass spectrometry) and oxygen isotope data, and 10Be and 230Th stratigraphies yielded age ranges of 24–29 Ka for the Mono Lake event, 34–43 Ka for the Laschamp event, 72–86 Ka for the Norwegian-Greenland Sea event and 118–128 Ka for the Blake event. Two reverse polarity samples at the base of core PS 2212–3 KAL are interpreted as the termination of the Biwa I event (171-181 Ka). the events exhibit full inversion of inclination in both cores. the data suggest that the transition process of the Earth's magnetic field during such polarity events requires some 1 Ka.

The Use of Velocity Spectrum For Stacking Receiver Functions and Imaging Upper Mantle Discontinuities

Abstract: SUMMARY In order to improve the signal-to-noise ratio of receiver function data, it is typical to stack receiver functions calculated from events at similar distances and back azimuths. We have adapted the velocity spectrum stacking (VSS) technique, used extensively in reflection seismology, to the receiver function method in order to stack data with different ray parameters, thereby improving further the signal-to-noise ratio. Perhaps more importantly, by producing the velocity spectrum stacks we take advantage of the differences in the shapes of the moveout curves of converted phases and reverberations to identify and separate the various phases and to infer velocity structure. By comparison of velocity spectrum stacks produced from the observed data at the IRIS/IDA broad-band station at Obninsk, Russia (OBN) with those produced from PREM synthetics we have identified Ps phases from the 400 and 670 km discontinuities. the P-to-S (Ps) converted phase from the 400 km discontinuity observed at OBN is much larger, compared with that of the 670 km discontinuity, than is predicted by PREM. This suggests a higher velocity contrast at 400 km than in PREM. By adapting a bootstrap method to assess the depth and velocity estimates determined by VSS, we find that the Ps phase from the 670 km discontinuity is best imaged with higher than PREM upper mantle P and S velocities and at a depth of 663 ± 3 km. We find no evidence of a 220 km discontinuity beneath OBN in these data.

Efficient calculation of complete differential seismograms for laterally homogeneous earth models

Abstract: SUMMARY Differential seismograms, the sensitivity of the seismogram to perturbations of parameters of the earth model, are essential for waveform modelling with iterative non-linear optimization techniques. The growing number of broad-band highdynamic-range fixed and portable stations provide a rich data set of regional distance events with accurate locations and source mechanisms that are ideal for modelling lithospheric structure. An efficient technique for calculating differential seismograms for complete synthetic seismograms in laterally homogeneous earth models will significantly decrease the computational burden of seismic-waveform inverse modelling. The technique we present here is equivalent to computing three synthetic seismograms in multilayered models, contrasted with N + 1 seismograms for a typical brute-force approach, which reduces the work to 3/(N + 1) or about a 90 per cent saving for 30 layers. The lateral-homogeneity requirement will yield path-averaged structures which may form the basis for later tomographic studies to resolve lateral variations.

Crustal structure, gravity anomalies and flexure of the lithosphere in the vicinity of the Canary Islands

Abstract: SUMMARY Simple elastic plate models have been used to calculate the flexure of the lithosphere caused by volcanic loading at the Canary Islands and sediment loading at the Moroccan continental margin. By comparing the calculated flexure to observations based on seismic refraction and free-air gravity anomaly data, constraints have been placed on the long-term (>106 years) elastic thickness of the lithosphere, Te. The best fit between the calculated and observed flexure in the vicinity of the Canary Islands is for Te= 20 km. This value of Te also explains seismic reflection data in regions that flank the island provided that the lithosphere underlying the Moroccan margin is sufficiently weak (Te < 5 km) for sediment loading to contribute little to the island flexure. a backstripping study, in which gravity data are used to constrain the value of Te, supports the suggestion that the lithosphere underlying the Moroccan margin, like its conjugate at the Baltimore Canyon Trough, is weak. Although sediment loading at the Moroccan margin appears therefore to have exerted little influence on the structure of the Canary Islands, there is evidence that island flexure may have influenced the stratigraphic development of the Moroccan margin, especially in the region of the ‘slope anticline’. A Te, of 20 km is about 15 km lower than would be expected on the basis of the thermal age of the oceanic lithosphere that underlies the Canary Islands. Similar low values have been reported from oceanic islands in the Pacific ‘superswell’ region of French Polynesia where they have been attributed to re-heating of the lithosphere by one or more hotspots. A general E-W age progression of the volcanic rocks suggests that the Canary Islands were also generated by a hotspot. They lack, however, the topographic swell and gravity/geoid high which usually accompanies these features. One possibility is that the low Te values are the result of a pre-existing weakness in the oceanic crust. A Te of 20 km is large enough, however, for a significant part of the mantle to still be involved in the support of the island loads. A more likely explanation is thermal weakening by a hotspot which has been localized enough to reduce Te but not to produce a swell or gravity/geoid high. Irrespective of its cause, the low Te suggests that oceanic lithosphere does not necessarily progressively increase its strength with age, so that even 140 Ma old lithosphere is vulnerable and can, in some regions, be significantly weakened by later thermal and mechanical processes.

The magnetostrophic rise of a buoyant parcel in the Earth's core

Abstract: Accepted 1993 September 21. Received 1993 September 21; in original form 1993 April 30 SUMMARY The dynamics of a buoyant parcel (or blob) of fluid released from the mushy zone on the inner core boundary (ICB) is considered. Estimates of the density defect and of the rise velocity are obtained from consideration of mass conservation and magnetostrophic force balance. When Lorentz and Coriolis forces are of comparable orders of magnitude, the disturbance remains localized in a neighbourhood of the blob even in the absence of viscous effects, and an inviscid analysis is possible. The instantaneous velocity and magnetic field associated with a given localized buoyancy distribution are then determined, and on the assumption that the velocity is approximately uniform throughout the blob, its trajectory from ICB to core-mantle boundary (CMB) is deduced. Both westward drift and poleward migration of erupting field loops are indicated. The effects of turbulent diffusion on the blob are considered. The prospects for constructing a dynamo theory based on this ‘blob scenario’ are discussed.

The pole-pole 3-D Dc-resistivity inverse problem: a conjugategradient approach

Abstract: SUMMARY The pole-pole 3-D DC-resistivity inverse problem is solved by converting the inverse problem into an objective-function optimization problem, using the adjoint equation to compute the gradient of the objective function, and using a conjugategradient minimization. Two examples of the application of the resulting inversion algorithm are given. First, a large synthetic data set is inverted, and second, the inversion algorithm is used to invert E-SCAN field data of relevance to mineral exploration.

The crustal magma chamber of the Katla volcano in south Iceland revealed by 2-D seismic undershooting

Abstract: SUMMARY Results of a 2-D, seismic undershooting experiment on the Katla central volcano in south Iceland are reported. Large localized traveltime anomalies (0.4 s) are observed on an array within the Katla caldera. The traveltimes are forward modelled using a wavefront tracker developed in Appendix A. Thus, non-linear effects encountered in traveltime tomography are avoided as well as common problems with ray tracing in the presence of strong lateral heterogeneity. An extreme variation in compressional velocity is required to extend over a significant volume in order to model the data. The resulting model is not unique, but constraints on the allowable range of velocities (2.5-6.0 km s-') render the basic features well constrained. A clear S-wave shadow is closely associated with delays in traveltime due to a shallow slow anomaly. Low-amplitude P waves go hand in hand with early arrivals due to thin structural features flanking the slow anomaly. The model is interpreted in terms of a magma chamber containing extensively molten rock. The magma chamber is shallow, with a bottom at a depth of about 1.5 km below sea-level (3.0 km below surface), and measures about 5 km across. The depth of the chamber is roughly at the level of buoyant equilibrium for basaltic melt in the crust. Owing to poor vertical resolution at shallow depths in the undershooting geometry the top of this shallow magma chamber is not well resolved. On the other hand, the bottom of the chamber is well resolved. The chamber is underlain by rocks of average or high velocity for that depth. The magma chamber is a persistent feature, big enough (10 km3) to supply magma for large eruptions and to supply heat to permit remelting of hydrated basaltic crust to produce silicic magmas at shallow levels. The chamber is fed by magma fracturing from below. The model agrees with phenomenological models of magma chambers in Iceland based on geological observations and provides a quantification of those models in terms of depth and size. On the other hand, it is fundamentally different from recent models of magma chambers at mid-ocean ridges which may be more akin to the pervasive region of partial melt at depth beneath Iceland. This underlines the important effect of the Icelandic hotspot on tectonics and volcanism in Iceland and implies a substantially different crustal and thermal structure in Iceland from that of 'normal' mid-ocean ridges.

3-D network ray tracing

Abstract: SUMMARY An efficient algorithm of the network shortest path calculation of rays and traveltimes of the first arrivals is described. The traveltime error of such computations is estimated. The rough estimate of the relative traveltime error is evaluated locally at all network nodes prior to network ray tracing, and is minimized by means of a proper choice of the sizes of forward stars. In this way, the structure of the network is adjusted for a particular model and for a particular computer memory. After network ray tracing, the error estimate is refined and the absolute error bounds of the calculated traveltimes are evaluated. The method and its accuracy are demonstrated on numerical examples.

The 1953 earthquake in Cephalonia (Western Hellenic Arc): coastal uplift and halotectonic faulting

Abstract: SUMMARY Geomorphological, marine biological and radiometric data in combination with earlier reports reveal that the M, =7.2, 1953 Cephalonia earthquake, the most destructive but least studied earthquake in Greece this century, was associated with a 0.3-0.7m quasi-rigid-body uplift and westward tilting of the central part of the island. Another palaeoseismic event, around 1500 yr BP, associated with coastal uplifts was also identified. Structural data indicate that the 1953 uplift is bounded by two subparallel, east-dipping major reverse faults and corresponds to a piston-like motion. This structurally unusual pattern of seismic deformation is detached from the deformation of the basement (conspicuously a thrust) and different from the long-term deformation pattern of the area; it is probably due to the particularities of salt tectonics: a -1500m thick salt layer acts as a regional dCcollement, while thinner layers are sandwiched between the carbonate thrust sheets that compose the overburden and reduce their friction during fault movement. This crustal anisotropy is responsible for the observed anomalous attenuation of seismic waves during the 1953 and historical earthquakes. The 1953 seismic surface deformation mimics long-term halotectonic patterns, but is not directly indicative of the regional stress-field, for it reflects uplift-induced stresses only.

Asymptotic theory for diffusive electromagnetic imaging

Abstract: SUMMARY We propose an asymptotic theory for diffusive electromagnetic imaging. Three steps are required to perform this imaging. (1) A high-frequency solution is first constructed which mimics the one usually found in wave-propagation phenomena. (2) This solution, valid for a smooth continuous description of the resistivity in the medium, is used in a first-order Born approximation leading to a linear relation between the resistivity perturbation of the subsurface and the perturbation of the electric signal obtained at the free surface. (3) This linear relation is asymptotically inverted by using an iterative quasi-Newtonian inversion based on a least-squares criterion developed by Jin et al. (1992). Although the extension to smooth heterogeneous reference medium is possible, we have only tested the inversion scheme for homogeneous reference media as Zhdanov & Frenkel (1983) previously did with another method.

Magnetic Enhancement of Chinese Loess‐The Role of γFe2O3?

Abstract: SUMMARY Various mechanisms have been proposed for the observed link between palaeoclimate and magnetic susceptibility in the Chinese loess. Many authors have recently pointed to the controlling influence that ultrafine (stable single domain/ superparamagnetic) magnetite/maghemite has on the magnetic susceptibility signal. However, there is still no clear evidence as to the origin of this susceptibility-enhancing magnetic fraction. We have collected samples from two sites representing the range of magnetic susceptibilities to be found in the Chinese loess record. Magnetic measurements indicate that susceptibility enhancement, throughout the loess plateau, is associated with the concentration of a ferrimagnetic fine fraction. This magnetic fraction has a uniform, non-variable grain-size distribution which spans the stable single-domain/superparamagnetic boundary. High-field thermomagnetic analysis reveals a trend in behaviour with increasing susceptibility, towards linear, more reversible curves. We propose that both the particular grain size of the enhanced ferrimagnetic fraction and thermomagnetic reversibility, can be explained by size-induced phase transitions in Fe2O3, which results in maghemite being the energetically favoured phase for certain grain sizes. Thus, the production of Fe2O3, which spans this grain-size range, is the only requirement for magnetic enhancement in the loess.

Inversion of 3-D DC resistivity data using an approximate inverse mapping

Abstract: SUMMARY We present an iterative algorithm for inverting 3-D pole-pole DC resistivity data. The algorithm utilizes an AIM (approximate inverse mapping) formalism and iterative inversions are carried out by performing updates in both model space (AIM—MS) and data space (AIM—DS) by using an approximate inverse mapping with an exact forward mapping. In the approximate inverse mapping, the potential anomaly is expressed as a depth integral of the logarithmic conductivity perturbation convolved horizontally with a known kernel. Fourier transforming the data equation decouples wavenumber components and the Fourier transform of the conductivity anomaly is recovered by performing 1-D linear inversions at each wavenumber. Inverse Fourier transforming the 1-D inversion results produces the sought conductivity. The AIM methodology avoids the generation and inversion of a full 3-D sensitivity matrix and is consequently fast and efficient. Only one forward modelling is performed at each iteration. The algorithm is tested with synthetic data and a field data set from an epithermal region.

Location of the Africa-Australia-India Triple Junction and Motion Between the Australian and Indian Plates: Results From an Aeromagnetic Investigation of the Central Indian and Carlsberg Ridges

Abstract: Accepted 1994 June 7. Received 1994 May 23; in original form 1993 May 24 SUMMARY Prior studies have proposed and examined the hypothesis that India and Australia are separate rigid plates separated by a wide, near-equatorial, E-W striking, plate boundary. Attempts to place narrow limits on the location of the Africa-AustraliaIndia triple junction have been hindered, however, by the lack of useful magnetic profiles crossing the eastern Carlsberg Ridge and northern Central Indian Ridge. Herein we present near-ridge portions of new profiles from an aeromagnetic survey of the Carlsberg Ridge east of 66"E and of the Central Indian Ridge north of 19"s. These new data are used to estimate 35 new spreading rates averaged from the middle of chron 2A (3.03 Ma) to the present. All other plate motion data along the Central Indian and Carlsberg ridges are also analysed to investigate the present kinematics of the Indian Ocean, especially the motion and boundary between the Indian and Australian plates. Unlike prior efforts, we objectively estimate uncertainties in the strikes of transform faults along the Carlsberg and Central Indian ridges.