Showing papers in "Geophysical Journal International in 1987"

Active tectonics of the Adriatic Region

Abstract: Seismicity and fault-plane solutions show that the active deformation in the Adriatic region is very varied. West of Messina, N–S shortening occurs with slip vectors representative of the overall Africa–Eurasia motion. Along the length of peninsular Italy, NE–SW extension on normal faults is the dominant style of deformation, but changes to N–S shortening in N. Italy. Inland central and northern Yugoslavia is deforming on strike-slip and thrust faults, and an intense belt of NE–SW shortening continues south along the coast from central Yugoslavia into Albania. South of Albania the shortening in coastal regions is in a more easterly direction. The most remarkable feature of the region is the low level of seismicity in the Adriatic Sea itself, compared with the intense activity in the high topographic belts that border it on the SW, NW and NE. The relatively rigid behaviour of the Adriatic allows its motion relative to Eurasia to be described by rotation about a pole in N. Italy. Anticlockwise rotation about this pole accounts, in a general way, for the change in style and orientation of the deformation in the circum-Adriatic belts. Historical and recent seismicity account for approximately equal rates of extension in central Italy and shortening in southern Yugoslavia of about 2 mm yr−1; however, these are uncertain by at least a factor of two, and are anyway likely to be underestimates of the true motion, because of the unknown contribution of aseismic creep. The Adriatic region resembles, in some ways, other relatively stable continental blocks, such as Central Iran and the Tarim Basin, that are caught up within the distributed deformation of the Alpine–Himalayan Belt. The Adriatic, however, is bounded on three sides by the relatively stable Eurasia plate. Its boundary with the African plate is short and ill-defined by seismicity, but is likely to be located in the Southern Adriatic, near the Strait of Otranto. The present day seismicity shows that the Adriatic, although once perhaps ‘a promontory of Africa', is no longer behaving in this way, and the motions on its boundaries do not directly reflect the Africa–Eurasia convergence.

Shear-wave splitting in the upper-mantle wedge above the Tonga subduction zone

Abstract: Summary. Intermediate-period seismograms of shear waves from deep earthquakes are examined for shear-wave splitting diagnostic of seismic velocity anisotropy in the upper-mantle wedge between the subducting Tonga slab and stations in the Fiji Islands. Earthquakes with near-vertical incidence angles are selected to minimize complications of converted phases and the free surface. Horizontal-component seismograms for 25 deep earthquakes recorded at station LAK on the Lau Ridge show splitting of up to 0.9 s with an average of 0.45 * 0.25 s between fast and slow S-wave components. Anisotropy in the upper mantle is the most likely cause of the splitting. The ESE orientation of the fast velocity direction at station LAK corresponds to an axis of symmetry of the anisotropic mantle and is approximately perpendicular to magnetic lineations in the Lau Basin and the northeastern portion of the South iiji Basin. The observations are consistent with a model of flow alignment of olivine (100) and orthopyroxene (001) axes parallel to the direction of basin extension. Seismograms for eight deep earthquakes at station SVA on Viti Levu show up to 1.0 s splitting with an average of 0.65 * 0.29 s and a weak NNE preferred orientation of fast velocity. Polarizations of shear-wave particle motions corrected for anisotropy are within 36" 5 26" of polarixations predicted for nine moment tensor solutions. The magnitude of shearwave splitting and estimates of the thickness of the lithosphere in the Fiji region imply that the anisotropy cannot be contained in the lithosphere alone. Either the anisotropy is distributed in both the lithosphere and asthenosphere, or the asthenosphere alone is anisotropic.

A stable finite element solution for two-dimensional magnetotelluric modelling

Abstract: Summary. We report herein on a finite element algorithm for 2-D magnetotelluric modelling which solves directly for secondary variations in the field parallel to strike, plus the subsequent vertical and transverse auxiliary fields, for both transverse electric and transverse magnetic modes. The governing Helmholtz equations for the secondary fields along strike are the same as those for total field algorithms with the addition of source terms involving the primary fields and the conductivity difference between the body and the host. Our approach has overcome a difficulty with numerical accuracy at low frequencies observed in total field solutions with 32-bit arithmetic far the transverse magnetic mode especially, but also for the transverse electric mode. Matrix ill-conditioning, which affects total field solutions, increases with the number of element rows with the square of the maximum element aspect ratio and with the inverse of frequency. In the secondary formulation, the field along strike and the auxiliary fields do not need to be extracted in the face of an approximately computed primary field which increasingly dominates the total field solution towards low frequencies. In addition to low-frequency stability, the absolute accuracy of our algorithm is verified by comparison with the TM and the TE mode analytic responses of a segmented overburden model.

Fractal structure of spatial distribution of microfracturing in rock

Abstract: Summary. A constant stress fracture experiment of Oshima granite was carried out at the confining pressure of 40MPa. Hypocentres of 2064 acoustic emissions were located during the experiment. Using the ‘correlation integral’, we found that the spatial distribution of hypocentres of acoustic emission is a fractal, and that the fractal dimension decreases with the evolution of rock fracturing. The spatial distribution of earthquake’s hypocentres reveals fractals ranging from regional to worldwide distribution. If we extrapolate from laboratory measurements, it is possible to predict the occurrence of large earthquakes by the decrease in the fractal dimension.

Glacial rebound and relative sea-level variations: a new appraisal

Abstract: Summary. Late Pleistocene and Holocene changes in sea-level at technically stable shorelines contain information on the melting history of the ice sheets and on the response of the Earth to changes in surface loads on time scales of 104yr. Some separation of parameters defining the ice loads and the Earth's response can be achieved by examining sea-level change at different sites around the world and at different epochs. This paper examines the mathematical requirements for obtaining satisfactory solutions for the sea-level equation rather than emphasizing results for the Earth's response to the surface loads. an examination of the equation expressing the sea-level variation on a viscoelastic earth indicates that careful consideration needs to be given to both the definition of the ice load and the geometry of the redistributed water load if geophysically significant Earth-response parameters are to be deduced from observations of relative sea-level. the spectrum of the 5°x 5° area-mean description of the ice load, widely used in glacial rebound studies, contains significant power at high degrees (n > 30) and can result in substantial spatial variations in sea-level unless a thick lithosphere is introduced in the Earth's response function. A smoothed 1°× 1° description of the ice model avoids these high-degree oscillations in relative sea-level and produces satisfactory sea-level results without requiring a thick lithosphere. At far field sites, the Holocene sea-levels are sensitive to the Earth's response to the meltwater loading in the vicinity of the site and a high-degree spatial resolution of coastline geometry is required to model the mantle flow driven by the differential loading about the site. At some sites convergence of the solution to the sea-level equation is not obtained, even when the solution is expanded to degree 180. Convergent solutions at lower degrees can be achieved by introducing earth models with a moderately thick lithosphere. If high-resolution ice and meltwater load models are not used then the tendency will be to overestimate the effective elastic thickness of the lithosphere. Estimates of mantle viscosity are also of questionable value if these high resolution models are not used. A preliminary examination of sea-levels at sites in both the near- and far-field places an upper limit of about 50–80 km on the effective lithospheric thickness. These sea-level observations are also consistent with an upper mantle viscosity of about 1021 Pas with a lower mantle viscosity in the range of 1021-1023Pa s.

Explosive volcanic eruptions - VI. Ejecta dispersal in plinian eruptions: the control of eruption conditions and atmospheric properties

Abstract: Summary. A simple model is developed to relate the maximum down-wind and cross-wind ranges of pyroclasts forming a plinian airfall deposit to the dynamic processes in the eruption cloud from which they fall and the atmospheric wind conditions in the area. The eruption cloud dynamics are in turn related to the eruptive conditions in the vent (vent radius, exsolved magmatic volatile weight fraction, velocity with which material passes through the vent, and mass eruption rate), some or all of which can be deduced if the appropriate field measurements can be made. Some aspects of the stability of convecting volcanic eruption clouds are investigated, and the effects on eruption cloud height of the local atmospheric temperature profile and the value adopted for the entrainment constant (which relates the horizontal flow speed of atmospheric air entering the column to the vertical rise speed of the column material) are explored. It is confirmed that eruption-cloud rise height and pyroclast dispersal are mainly controlled by the mass eruption rate (per unit length of active fissure in the case of linear vents) and, hence, the heat input rate to the cloud; but a significant subsidiary dependence on the amount of exsolved magma volatiles is also found. The eruption cloud model is validated by application to observed historic eruptions, and its use in the analysis of palaeo-eruptions is discussed.

The deep seismicity of the Tyrrhenian Sea

Abstract: Summary. 'l'he study reappraises the deep seismicity of the Tyrrhenian Sea. Careful examination of the quality of reported hypocentres shows that the earthquakes define a zone dipping NW, about 200 km along strike, 50 km thick, and reaching a depth of about 500 km. The zone is slightly concave to the NW at a depth of 300 km, but, contrary to many previous reports, is not tightly concave, nor are there significant spatial gaps in the seismicity, which is effectively continuous with depth. Seismicity is, however, concentrated in the depth interval 250-300 km, where the dip of the seismic zone changes from 70" (above 250 km) to a more gentle dip of 45" at greater depths. Seven fault-plane solutions are available for the largest earthquakes in this depth interval, all of them consistent with a P-axis down the dip of the seismic zone, and all of them requiring movement on faults out of the plane of the subducting slab. Two deep earthquakes near Naples lie well outside the main zone of activity; for one of which a fault-plane solution is available that has aP-axis not aligned with the dip of the seismic zone. The tightly concave slabgeometry favoured by other reports is supported mainly by the location of these events near Naples, which we think may represent deformation in a separate, probably shallower dipping, piece of subducted lithosphere. The lack of shallow seismicity, and particularly of thrust faulting earthquakes, at the surface projection of the Benioff zone suggests that active subduction has ceased. Estimates of the convergence rate responsible for subduction in the last 10 Myr far exceed the present convergence rate of Africa and Eurasia, suggesting that the subduction was related instead to the stretching and thinning of the crust in the Tyrrhenian Sea.

Changes in the Earth's rotation and low-degree gravitational field induced by earthquakes

Abstract: Analytical formulas based on the normal-mode theory are used together with a spherically symmetric earth model and the centroid-moment tensor solutions for earthquake sources to compute the earthquake-induced changes in the earth's rotation and low-degree harmonics of the gravitational field for the period 1977-1985. Spectral and statistical analyses are conducted on these changes. It is found that the earthquake-induced changes are two orders of magnitude smaller than those observed; most of these changes show strong evidence of nonrandomness either in their polarity or in their directions.

Compaction-induced inclination shallowing of the post-depositional remanent magnetization in a synthetic sediment

Abstract: Summary. A synthetic sediment comprised of kaolinite, distilled water and either equidimensional or acicular magnetite was given a post-depositional remanent magnetization (PDRM) by being stirred in a magnetic field. This sediment was compacted under pressures which varied continuously from 0 to 0.14 MPa in a water-tank consolidometer and to higher pressure steps (~2.53 MPa) in a standard soil consolidometer. Compaction took place in the same magnetic field in which the sample was given its PDRM. The compaction caused shallowing of the sample's magnetic inclination. This shallowing was found to be a function of the sample's initial magnetic inclination and the degree of sample compaction; tan (I,) = (1 - aAv) tan (lo), where IR is the remanent inclination after compaction, AV is the volume change, I. is the initial magnetic inclination, and a is an empirically derived constant. The data show a maximum inclination shallowing of 12" for an initial inclination of 54", in good agreement with the maximum inclination shallowing predicted by the above equation. We propose an electrostatic mechanism to be the cause of the inclination shallowing. In this model positively charged magnetite grains adhere to the surface of negatively charged clay grains with their long dimension parallel to the clay grain's surface. As the clay grains become reorientated due to compaction the easy axes of magnetization are rotated away from the axis of compression. Alternating field demagnetization data reveal that our samples have shallower characteristic magnetizations than their post-compaction NRMs, implying that the smaller, higher coercivity grains are most affected by the compaction process. These data support our model since a surface charge mechanism for inclination shallowing would predict that the smallest magnetic grains would be preferentially affected.

Limits on lateral density and velocity variations in the Earth's outer core

Abstract: An approximate analysis is given for the likely fractional lateral density variations (δρ/ρ) in the outer core, caused by large scale-length fluid dynamical processes. It is first shown that fractional density and fractional seismic velocity variations are probably comparable, so that fluid dynamic arguments have relevance to seismic data. In regions of nearly neutral stability in the outer core, an analysis of convective vigour indicates an upper bound of δρ/ρ≲10^(−8). If the outer core possesses one or more layers of strong static stability then stationary contributions to δρ can be larger, if they are associated with axisymmetric (m = 0) harmonics, because of stabilizing zonal winds. Baroclinic instabilities nevertheless limit δρ/ρ≲ 10^(−6) but may not exist if the static stability is sufficiently large. Shear instabilities always limit δρ/ρ≲ 10^(−4). Magnetic field effects suggest comparable or more stringent upper bounds. It is concluded that scientists undertaking analysis of the Earth's geoid or seismic travel times or normal modes can safely assume that there are negligible lateral variations in the outer core.

The earthquake of 1980 November 23 in Campania—Basilicata (southern Italy)

Abstract: Summary Teleseismic waveforms, local ground acceleration, elevation changes, surface faulting and aftershocks are used to investigate the three-dimensional geometry of fault movement in the destructive earthquake (Ms= 6.9) of 1980 November 23 in Campania–Basilicata (southern Italy). Twelve kilometres of surface faulting has been identified following this earthquake. The re-determined epicentre and focal mechanism, and the focal depth of 10 km, determined by modelling long-period teleseismic body waves, show that the hypocentre was on a downward projection of the surface faulting, and that the seismogenic normal fault was approximately planar, with a dip of 60°, from the hypocentre to the surface. Further analysis of the long-period body-waves indicates that, within 10s of the origin time of the earthquake, motion occurred on three discrete fault-segments extending for 30 km along strike. Fault rupture in the earthquake propagated predominantly towards the NW. An overall moment tensor for the earthquake is obtained from the inversion of long-period GDSN and WWSSN data, and shows that the total scalar moment of 26×1018 Nm is approximately double that accounted for by the fault motion in these first three subevents. We use teleseismic body-waves, locally recorded ground acceleration and aftershocks to investigate the position, timing and orientation of the additional seismic sources responsible for the remaining seismic moment. These data suggest that a fourth subevent occurred about 13 s after the first motion, approximately 20 km SE of the hypocentre of the first subevent. Two later fault ruptures also occurred, beneath the hanging wall of the earlier ruptures, about 20 and 40 s after the first motion. Long-period body waves and elevation changes are consistent with these occurring on normal faults, dipping at about 20°NE, at the base of the upper-crustal seismogenic zone. The total of six subevents that we identify for this earthquake account for almost all of the scalar moment in the overall moment tensor.

MT and reflection: an essential combination

Abstract: Summary. At many localities in the world there have been coincident comprehensive electromagnetic (EM) studies and seismic reflection profiles conducted. Unfortunately, over many more regions the seismic reflection images are interpreted without the constraints afforded by electrical conductivity information. This paper is an attempt to convince the reader that a collocated magnetotelluric (MT) study should, in almost every case, be made wherever a seismic reflection survey is undertaken. Examples are shown from six studies in which the EM results aided the geological/tectonic interpretations of the seismic sections. Also, difficulties with the MT technique are discussed, and the interpretations of conducting zones within the lower crust are examined. Finally, a generalised model is proposed for the continental crust that may account for both the reflectivity and conductivity of the zone at the top of the lower crust.

Computation of high-frequency seismic wavefields in 3-D laterally inhomogeneous anisotropic media

Abstract: Summary. An algorithm for the computation of travel times, ray amplitudes and ray synthetic seismograms in 3-D laterally inhomogeneous media composed of isotropic and anisotropic layers is described. All 21 independent elastic parameters may vary within the anisotropic layers. Rays and travel times are evaluated by numerical solution of the ray tracing equations. Ray amplitudes are determined by evaluating reflection/ transmission coefficients and the geometrical spreading along individual rays. The geometrical spreading is computed approximately by numerical measurement of the cross-sectional area of the ray tube formed by three neighbouring rays. A similar approximate procedure is used for the determination of the coefficients of the paraxial ray approximation. The ray paraxial approximation makes computation of synthetic seismograms on the surface of the model very efficient. Examples of ray synthetic seismograms computed with a program package based on the described algorithm are presented.

Models of aftershock occurrence

Abstract: Summary. The Omori formula for aftershock activity can be shown to be a consequence of each of two models of a complex earthquake. In the first case. aftershocks are assumed to be caused by subsequent slip on asperities of a fault which are locked during the fracture in the main shock. In the second, aftershocks are caused by catastrophic coalescence of nearby small fractures with the fracture surface of the main shock. Hence, the first source model describes aftershock activity in the interior of a main-shock rupture zone, while the second describes aftershock activity near the outer boundary of a main-shock rupture zone. Stress corrosion cracking is assumed to be the physical cause of the quasistatic growth of the main fracture zone at the expense of the area of the locked patches in the first model and is also the cause of the growth of the small satellite fractures in the second.

Seismic waves in stratified anisotropic media. II: Elastodynamic eigensolutions for some anisotropic systems

Abstract: Summary. The differential system for elastic waves in stratified anisotropic media is, as in the isotropic case, most simply solved by constructing a propagator from the eigenvalues and eigenvectors. In general, the eigensolutions must be found numerically, which may lead to an enormous amount of computation if the ultimate goal is to construct synthetic seismograms. However, if the stiffness tensor for each layer displays a horizontal plane of symmetry, simple analytical solutions exist and the expense of numerical solution can be avoided. We consider monoclinic, orthorhombic, hexagonal (including transversely isotropic), and istropic symmetries and give analytical expressions for the eigensolutions for each of these forms when the alignment is such that a horizontal symmetry plane exists. LThe analytical solutions are concise enough to make the construction of complete synthetic seismograms (a task which involves the construction and manipulation of 3-D functions) a realistic endeavour. While not applicable to arbitrary anisotropy, these results have widespread application as most of the anisotropies hypothesized in seismology require a horizontal plane of elastic symmetry.

Multiplet-multiplet coupling due to lateral heterogeneity: asymptotic effects on the amplitude and frequency of the Earth's normal modes

Abstract: Summary. Starting with the first-order formulation of quasi-degenerate splitting theory for the normal modes of a laterally heterogeneous earth, we have obtained an asymptotic expression for the coupling terms corresponding to neighbouring multiplets along the same dispersion branch as the mode considered, valid to order 1/l, where l is the angular order of this mode (l≥ 1). We show that, to order zero, these coupling terms introduce a small shift in epicentral distance into the expression for the long period seismogram obtained by normal mode summation. This shift depends on the difference between the great circle and the minor arc averages of the local frequency. the coupling terms thus permit us to reconcile results obtained by normal-mode summation and by a propagating wave approach, as far as the dependence on structure of the phase of surface waves is concerned. To order 1/l, the coupling terms result in a perturbation in the amplitude of the mode considered, which depends on spatial derivatives of the local frequency and thus on the structure in the vicinity of the source station great circle path. We show that this term is equivalent to that which is found using ray perturbation methods for propagating surface waves. We compare and discuss the assumptions underlying both approaches and illustrate, by an example, the potential of the asymptotic normal-mode formulation for improved modelling of lateral heterogeneity in the earth.

Applications of seismic travel-time tomography

Abstract: Summary. This paper describes the application of tomography to seismic travel-time inversion. There are various implementations of travel-time tomography. In reflection tomography, sources and receivers are on the surface of the Earth and the principal seismic events are reflections from subsurface velocity discontinuities. In transmission tomography, sources and/or receivers may be buried beneath the surface and the events correspond to direct, or unreflected, arrivals; this is the analogue of medical tomography. There are also cases in which both direct as well as reflected arrivals are important, such as in Vertical Seismic Profiling. The latter is a direct application of the first two, but is not discussed in any detail here. It is also shown how the iterative use of travel-time tomography and depth migration can produce much enhanced subsurface images. Examples of both transmission tomography and reflection tomography combined with depth migration illustrate the methods.

Random stress and earthquake statistics: spatial dependence

Abstract: SUMMARY We calculate the 3-D stochastic rotations of focal mechanisms (disorientations) caused by stresses arising from the presence of many small, random, point defects that may surround the tip of an earthquake fault. These random stresses can be shown to be distributed according to a Cauchy distribution; as a consequence the next episode of fracture of a fault cannot be planar. The disorientation of focal mechanisms of these new rupture episodes is closely approximated by a rotational Cauchy distribution. As observed previously, the geometry of fault systems for natural earthquakes is consistent with these observations, since it too can be modelled by the Cauchy distribution. These observations indicate how the 3-D rupture process in rocks and other materials can be modelled. We also calculate distributions of disorientations caused by a uniformly random 3-D rotation of sources. These distributions are governed by symmetry properties of earthquake focal mechanisms. To take into account the source symmetry, we compare the Cauchy distribution which is due to influence of random stresses to the distributions caused by random rotations.

Stacking gravity tide observations in central Europe for the retrieval of the complex eigenfrequency of the nearly diurnal free-wobble

Abstract: Summary. We have used tidal gravity measurements from six stations in central Europe to investigate the resonance in the diurnal tidal band, caused by inertial coupling between the mantle and outer core of the Earth. By the use of stacking it was possible to determine the eigenfrequency and quality factor of this eigenmode, commonly called the ‘nearly diurnal free-wobble’. We assessed the effect of systematic errors from the ocean correction to the tidal measurements employing a Monte-Carlo method. The observed eigenfrequency is 1 + 1/(434 f 7) cycles per sidereal day, and is significantly higher than predicted by theories. The observed quality factor is (2.8 ? 0.5) x 10’.

Seismic reflection coefficients from mantle fault zones

Abstract: Summary. Several bright reflections from structures within the mantle can be seen on BIRPS' deep seismic reflection profiles. We have calculated apparent reflection coefficients for the brightest of these events and obtain values around 0.1. It is not possible to produce such large reflections by either compositional layering or seismic anisotropy if olivine and pyroxene are the only significant minerals in the mantle. These large reflections can be produced by a mafic layer or a partially hydrated layer within normal peridotite. The brightest reflections seem to be best explained as major faults or shear zones within the mantle.

Multiple‐taper spectral analysis of terrestrial free oscillations: part I

Abstract: We present a new method for estimating the frequencies of the Earth's free oscillations. This method is an extension of the techniques of Thomson (1982) for finding the harmonic components of a time series. Optimal tapers for reducing the spectral leakage of decaying sinusoids immersed in white noise are derived. Multiplying the data by the best K tapers creates K time series. A decaying sinusoid model is fit to the K time series by a least squares procedure. A statistical F-test is performed to test the fit of the decaying sinusoid model, and thus determine the probability that there are coherent oscillations in the data. The F-test is performed at a number of chosen frequencies, producing a measure of the certainty that there is a decaying sinusoid at each frequency. We compare this method with the conventional technique employing a discrete Fourier transform of a cosine-tapered time-series. The multiple-taper method is found to be a more sensitive detector of decaying sinusoids in a time series contaminated by white noise.

On the electrical conductivity of the mid‐mantle‐I. Calculation of equivalent scalar magnetotelluric response functions

Abstract: Summary The problem of global conductivity sounding has been re-posed in order to investigate the possible existence of large-scale lateral heterogeneities in mid-mantle conductivity. A response function (Z/H) is robustly estimated and scaled into an equivalent scalar magnetotelluric impedance by assuming the geomagnetic field spatial variations are adequately described by a P10 spherical harmonic. Data found to be inconsistent with this representation in either the time, frequency or spatial domain are excluded from the analysis. Making use of this relationship, robust statistical techniques and careful data handling procedures have been applied to a new global magnetic database containing well in excess of 106 daily mean values (1358 observatory-years spanning the period 1883-1980). A set of spatially distributed scalar frequency domain impedance functions for a variety of tectonic provinces result from this analysis. Response functions consistent with both the P10 assumption and local 1-D earth structure were found for 22 of the 79 observatories examined. The set of acceptable response functions is presented, along with a summary of data quality for the observatories represented in the database.

Mechanisms of isostatic compensation in the vicinity of the East African Rift, Kenya

Abstract: Summary The topography and gravity anomalies in the area surrounding the East African Rift in Kenya can be modelled as the sum of the effects of surface and subsurface loading of an elastic plate. Assuming surface and subsurface loading are independent processes, the observed coherence between the 2-D Fourier transforms of Bouguer gravity and topography provides a constraint on the effective elastic thickness or flexural rigidity of the plate. Distinct linear segments of the log gravity power spectrum suggest that components of the gravity field with wavelengths of 250–1000 km are generated predominantly by a density contrast at a depth of about 32 km. Most shorter wavelength gravity anomalies are probably associated with source depths of less than 1 km and indicate variations in thickness of low density sedimentary or volcanic layers or lateral variations in density of the surface rocks. A simple density model based on these estimates and the geology of Kenya consists of a cover layer averaging 0.5 km thick with density 2300 kg m−3, a layer 32 km thick with density 2800 kg m−3, and an underlying half-space with density 3200 kg m−3. Using this density model and assuming loading due to relief on all three density interfaces, the elastic thickness that best predicts the observed coherence in the least squares sense is 25 km (Flexural rigidity of 1.4 × 1023Nm). Amplitudes of loads on the density interfaces can be calculated based on the model response. Topography with wavelength greater than 650 km is locally compensated making surface and subsurface loading indistinguishable. At shorter wavelengths, rift volcanics and volcanic cones including Mts Kenya, Elgon and Kilimanjaro can be identified as surface loads. The largest amplitude subsurface load is an upward directed, regionally supported load beneath the Kenya Dome that may correspond to a region of hot, low density mantle recognized by other geophysical studies. In creating the topography for which surface and sub-surface loading are distinguishable, surface loading is much more important than doming due to subsurface loading.

Observation and modelling of fault‐zone fracture seismic anisotropy ‐ I. P, SV and SH travel times

Abstract: Summary. Three-component VSP borehole seismograms taken in the vicinity of an active normal fault in California show strong systematic shear-wave splitting that increases with proximity to the fault. Using Cervený's method of characteristics for ray tracing in anisotropic heterogeneous media and Hudson's formulation of elastic constants for media-bearing aligned fractures, we have fitted a suite of P, SV and SH hanging-wall and foot-wall travel times with a simple model of aligned fractures flanking the fault zone. The dominant fracture set is best modelled as parallel to the fault plane and increasing in density with approach to the fault. The increase in fracture density is non-uniform (power law or Gaussian) with respect to distance to the fault. Although the hanging-wall and the foot-wall rock are petrologically the same unit, the fracture halo is more intense and extensive in the hanging wall than in the foot wall. Upon approach to the fault plane, the fracture density or fracture-density gradient becomes too great for the seismic response to be computed by Hudson–Cervený procedures (the maximum fracture density that can be modelled is about 0.08). Within this 25 m fracture domain it appears more useful to model the fault and near field fractures as a low-velocity waveguide. We observe production of trapped waves within the confines of the intense fracture interval.

Hatton Bank (northwest U.K.) continental margin structure

Abstract: Summary. The continent-ocean transition near Hatton Bank was studied using a dense grid of single-ship and two-ship multichannel seismic (mcs) profiles. Extensive oceanward dipping reflectors in a sequence of igneous rocks are developed in the upper crust across the entire margin. At the landward (shallowest) end the dipping reflectors overlie continental crust, while at the seaward end they are formed above oceanic crust. Beneath the central and lower part of the margin is a mid-crustal layer approximately 5 km thick that could be either stretched and thinned continental crust or maybe newly formed igneous crust generated at the same time as the dipping reflector sequence. Beneath this mid-crustal layer and above a well defined seismic Moho which rises from 27 km (continental end) to 15 km (oceanic end) across the margin, the present lower crust comprises a 10-15 km thick lens of material with a seismic velocity of 7.3 to 7.4 kds. We interpret the present lower crustal lens as underplated igneous rocks left after extraction of the extruded basaltic lavas. A considerable quantity of new material has been added to the crust under the rifted margin. The present Moho is a new boundary formed during creation of the margin and cannot, therefore, be used to determine the amount of thinning.

Green functions and associated sources in infinite and stratified poroelastic media

Abstract: Summary The purpose of this paper is to study Green functions for porous saturated media and to present an example of numerical simulation using these Green functions. In the first part the equations of the porous media are recalled and the principal results concerning the features of the waves are given. The second part is devoted to the determination of sources and Green functions. We give the analytical solution, then we study some of its properties. In the third part we focus on the construction of synthetic seismograms. We show examples of bi-dimensional computation in a semi-infinite stratified porous medium, for a double dipole or a fluid injection.

Some remarks on the Gaussian beam summation method

Abstract: Summary. Recently, a method using superposition of Gaussian beams has been proposed for the solution of high-frequency wave problems. The method is a potentially useful approach when the more usual techniques of ray theory fail: it gives answers which are finite at caustics, computes a nonzero field in shadow zones, and exhibits critical angle phenomena, including head waves. Subsequent tests by several authors have been encouraging, although some reported solutions show an unexplained dependence on the ‘free’ complex parameter e which specifies the initial widths and phases of the Gaussian beams. We use methods of uniform asymptotic expansions to explain the behaviour of the Gaussian beam method. We show how it computes correctly the entire caustic boundary layer of a caustic of arbitrary complexity, and computes correctly in a region of critical reflection. However, the beam solution for head waves and in edge-diffracted shadow zones are shown to have the correct asymptotic form, but with governing parameters that are explicitly e-dependent. We also explain the mechanism by which the beam solution degrades when there are strong lateral inhomogeneities. We compare numerically our predictions for some representative, model problems, with exact solutions obtained by other means.

Magnetotellurics along the Fennoscandian Long Range profile

Abstract: Summary. Estimates of magnetotelluric transfer functions along the Fennoscandian Long Range (FENNOLORA) profile are presented and discussed in relation to the major lithotectonic subdivisions of the Swedish part of the Baltic shield. None of the transfer functions are found to be consistent with a one-dimensional earth structure, thereby making a quantitative interpretation difficult. However, the geological units are found to display characteristic differences in the obtained transfer functions, and in some areas a quantitative interpretation can be justified. The most interesting structure indicated is a low resistive (4 am) crustal structure of lateral extension more than 150 km centred around the Skellefte ore district. A minimum thickness for this structure of 15 km is inferred.

Migration - why doesn't it work for deep continental data?

Abstract: Summary. Conventional migration of deep seismic reflection data often produces disappointingly poor results even when the original unmigrated data are of high quality and are relatively noise free. Surprisingly, deep data often subjectively appear to be best migrated at velocities which are up to 50% less than appropriate interval velocities derived from crustal refraction experiments or directly from stacking velocities. The explanation for this behaviour is that near surface features distort and attenuate the seismic wave field and produce apparent discontinuities in deep reflections. Since these discontinuities are spurious they are not associated with the appropriate large diffractions which real discontinuities at depth would produce. During the process of migration reflections are invented in order to cancel out the missing diffractions thereby producing a “smiley” section which appears overmigrated. Since the lateral extent of individual smiles increases with increasing migration velocity, increasing two-way-time, and increasing seismic wavelength, the effect” is almost unnoticeable on conventional shallow seismic data but is overwhelming for deep crustal data.

Coincident seismic reflection/refraction studies of the continental lithosphere: a global review

Abstract: Summary. Vertical-incidence reflection profiling has identified several characteristic features of the continental Iithosphere including a generally transparent upper crust, a reflective lower crust, reflections from the crust-mantle boundary, and a commonly transparent upper mantle. The underlying physical causes of these characteristic features remain poorly understood. This review summarizes additional information brought to bear on the physical properties of these characteristic crustal structures through the use of coincident wide-angle refraction profiling.