A long-range seismic profile in Southeastern Australia

Summary. Nine portable seismic stations deployed across the Western Plains of New South Wales recorded signals in the distance range 250–1000 km from large timed explosions at both ends of the line. A velocity—depth model derived from the travel-time data has the following features: a two-layer crust with a thickness of 35 km; a sub-Moho velocity of 7.98 km/s; an abrupt increase to 8.36 km/s at 100 km depth; a further step to 8.72 km/s at 190 km depth, with a low-velocity channel immediately above the discontinuity. The model has several features in common with others derived from long-range profiles in Australia and elsewhere. The data, however, provide the first suggestion of a low P -velocity channel in Eastern Australia.     

The upper mantle structure of the North Sea region from Rayleigh wave dispersion

Summary. The phase velocity dispersion of fundamental mode Rayleigh waves (period range 13–127 s) is determined by the interstation method for three profiles that traverse the North Sea region of northwest Europe. The resulting observations have been combined to produce a regional phase velocity curve with 95 per cent confidence intervals, which belongs to the aseismic continental platform category of Knopoff.
Inversions of the regional phase velocity curve by the'Hedgehog'method indicate that the North Sea region is characterized by an upper mantle low-velocity zone of S -wave velocity 4.35–4.45 km/s between depths of approximately 85–200 km.     

Surface-wave propagation across the Mexican Volcanic Belt and the origin of the long-period seismic-wave amplification in the Valley of Mexico

A network of nine broad-band seismographs was operated from March to May 1994 to study the propagation of seismic waves across the Mexican Volcanic Belt (MVB) in the region of the Valley of Mexico. Analysis of the data from the network reveals an amplification of seismic waves in a wide period band al the stations situated in the southern part of the MVB.
The group velocities of the fundamental mode of the Rayleigh wave in the period range 2–13 s are found to be lower in the southern part of the MVB than in its northern part and in the region south of the MVB. The inversion of dispersion curves shows that the difference in group velocities is due to the presence of a superficial low-velocity layer (with an average S -wave velocity of 1.7 km s^-1 and an average thickness of 2 km) beneath the southern part of the MVB. This low-velocity zone is associated with the region of active volcanism.
Numerical simulations show that this superficial low-velocity layer causes a regional amplification of 8–10 s period signals, which is of the same order as the amplification measured from the data. This layer also increases the signal duration significantly because of the dispersion of the surface waves. These results confirm the hypothesis of Singh et al. (1995), who suggested that the regional amplification observed in the Valley of Mexico is due to the anomalously low shear-wave velocity of the shallow volcanic rocks in the southern MVB     

Pre-drilling reflection survey of the Black Forest, SW Germany

Summary. The unified seismic exploration program, consisting of 345 km of deep reflection profiling, a 200 km refraction profile, an expanding spread profile and near-surface high resolution reflection meaasurements, revealed a strongly differentiated crust beneath the Black Forest. The highly reflective lower crust contains numerous horizontal and dipping reflectors at depths of 13-14 km down to the crust-mantle boundary (Moho). The Moho appears as a flat horizontal first order discontinuity at a relatively shallow level of 25–27 km above a transparent upper mantle. From modelling of synthetic near-vertical and wide-angle seismograms using the reflectivity method the lower crust is supposed to be composed of laminae with an average thickness of about 100 m and velocity differences of greater than 10% increasing from top to bottom. The upper crust is characterised by mostly dipping reflectors, associated with bivergent underthrusting and accretion tectonics of Variscan age and with extensional faults of Mesozoic age. A bright spot at 9.5 km depth is characterised by low velocity material suggesting a fluid trap. It appears on all of the three profiles in the centre of the intersection region. The upper crust seems to be decoupled from the lowest crust by a relatively transparent zone which is' also identified as a low-velocity zone. This low velocity channel is situated directly above the laminated lower crust. The laminae in the Rhinegraben area are displaced vertically to greater depths indicating an origin before Tertiary rift formation and a subsidence of the whole graben wedge.     

Teleseismic Sn: a guided wave in the mantle

The short-period seismic phase Sn has been interpreted by Stephens & Isacks as a lid wave' in which the seismic energy is constrained to the uppermost few tens of kilometres of the mantle. We have extended their normal-mode interpretation for structures both with and without low-velocity zones (LVZ) in the upper mantle. We have used spherical, anelastic models of the Earth. For a model of an oceanic mantle with a LVZ, we agree that Sn is a lid wave for sources above 200–250 km, if only the onset of Sn is considered. The later portions of the Sn wave train sample the structure as deeply as the 420-km discontinuity. For deeper foci, the pseudo-lid wave does not appear to be generated; even the onset of Sn samples the deeper mantle structure. For a model of a continental mantle without a LVZ, in general, sources at all depths above the 420-km discontinuity appear to generate teleseismic Sn which samples the entire mantle as deeply as the discontinuity and which travels with a velocity significantly greater than the lid velocity. Thus the velocity of Sn may be an important diagnostic to determine whether or not a LVZ exists in the upper mantle.     

Transitional continental–oceanic structure beneath the Norwegian Sea from inversion of surface wave group velocity data

We have analysed the fundamental mode of Love and Rayleigh waves generated by 12 earthquakes located in the mid-Atlantic ridge and Jan Mayen fracture zone. Using the multiple filter analysis technique, we isolated the Rayleigh and Love wave group velocities for periods between 10 and 50  s. The surface wave propagation paths were divided into five groups, and average group velocities calculated for each group. The average group velocities were inverted and produced shear wave velocity models that correspond to a quasi-continental oceanic structure in the Greenland–Norwegian Sea region. Although resolution is poor at shallow depth, we obtained crustal thickness values of about 18  km in the Norwegian Sea area and 9  km in the region between Svalbard and Iceland. The abnormally thick crust in the Norwegian Sea area is ascribed to magmatic underplating and the thermal blanketing effect of sedimentary layers. Maximum crustal shear velocities vary between 3.5 and 3.9  km  s⁻¹ for most paths. An average lithospheric thickness of 60  km was observed, which is lower than expected for oceanic-type structure of similar age. We also observed low shear wave velocities in the lower crust and upper mantle. We suggest that high heat flow extending to depths of about 30  km beneath the surface can account for the thin lithosphere and observed low velocities. Anisotropy coefficients of 1–5 per cent in the shallow layers and >7 per cent in the upper mantle point to the existence of polarization anisotropy in the region.     

Simultaneous inversion of seismic data

Summary. The resolving power of different data sets, consisting of surface-wave dispersion measurements and S travel times, are compared for a continental structure. The shear velocity in the low-velocity zone can be resolved in some detail with higher-mode phase-velocity data. Sufficient resolution for small density contrasts (0.03 g cm⁻³) until depths of ∼ 300 km can be reached if higher-mode group velocities are available as well, even at a precision as low as 0.10 km/s. At greater depths the density is not resolved, and here travel-time data are superior to higher modes in resolving the shear velocity.     

An anisotropic model of the Earth's crust in the Baikal rift zone

Summary. An attempt is made to explain the existence of intracrustal low-velocity layers in rift zones by using an anisotropic model. It is supposed that the anisotropy is due to the preferred orientation of micas and amphiboles in metamorphic rocks forming the upper crust. Analysing the velocity distribution along different directions in anisotropic media the authors conclude that the low-velocity layer must be isotropic (randomly oriented) with an anisotropic lid.     

Receiver functions at the stations of the German Regional Seismic Network (GRSN)

Several years of broad-band teleseismic data from the GRSN stations have been analysed for crustal structure using P -to- S converted waves at the crustal discontinuities. An inversion technique was developed which applies the Thomson-Haskell formalism for plane waves without slowness integration. The main phases observed are Moho conversions, their multiples in the crust, and conversions at the base of the sediments. The crustal thickness derived from these data is in good agreement with results from other studies. For the Gräfenberg stations, we have made a more detailed comparison of our model with a previously published model obtained from refraction seismic experiments. The refraction seismic model contains boundaries with strong velocity contrasts and a significant low-velocity zone, resulting in teleseismic waveforms that are too complicated as compared to the observed simple waveforms. The comparison suggests that a significant low-velocity zone is not required and that internal crustal boundaries are rather smooth.     

Upper mantle shear structure of North America

Summary. The waveforms and travel times of S and SS phases in the range 10°–60° have been used to derive upper mantle shear velocity structures for two distinct tectonic provinces in North America. Data from earthquakes on the East Pacific Rise recorded at stations in western North America were used to derive a tectonic upper mantle model. Events on the north-west coast of North America and earthquakes off the coast of Greenland provided the data to investigate the upper mantle under the Canadian shield. All branches from the triplications due to velocity jumps near 400 and 660 km were observed in both areas. Using synthetic seismograms to model these observations placed tight constraints on heterogeneity in the upper mantle and on the details of its structure. SS–S travel-time differences of 30 s along with consistent differences in waveforms between the two data sets require substantial heterogeneity to at least 350 km depth. Velocities in the upper 170 km of the shield are about 10 per cent higher than in the tectonic area. At 250 km depth the shield velocities are still greater by about 4.5 per cent and they gradually merge near 400 km. Below 400 km no evidence for heterogeneity was found. The two models both have first-order discontinuities of 4.5 per cent at 405 km and 7.5 per cent at 695 km. Both models also have lids with lower velocities beneath. In the western model the lid is very thin and of relatively low velocity. In the shield the lid is 170 km thick with very high elocity (4.78 km s^-1); below it the velocity decreases to about 4.65 km s^-1. Aside from these features the models are relatively smooth, the major difference between them being a larger gradient in the tectonic region from 200 to 400 km.     

Upper mantle structure beneath the Mid-Atlantic Ridge north of the Azores based on observations of compressional wave

Summary. Four seismic refraction profiles have been interpreted which serve to indicate the structure of the lithosphere near the Mid-Atlantic Ridge close to the Azores. An east–west profile which crosses the ridge axis yields a crustal structure. Although energy is propagated across the ridge axis within the crust the axial region marks a clear barrier to propagation within the mantle. A profile parallel to the axis (4 my isochron) shows, below a 7.6 km/s layer, a low-velocity zone underlain by an 8.3 km/s refractor 9 km below the sea bed. On profies normal to the ridge axis higher velocities, which are observed on lines shot towards the ridge, can be attributed to this refractor if it has a dip of several degrees away from the ridge. On another profile parallel to the axis (9 my isochron) a velocity of about 8.3 km/s is only found to exist much deeper at about 30 km depth. These observations are interpreted in the light of seismic refraction results recently obtained by Lewis & Snydsman and of quantitative petrological models, such as that of Bottinga & Allègre. A velocity model based on Bottinga & Allègre's model allows us to understand our results qualitatively. In particular the two 8.3 km/s refractors at 9 and 30 km depth correspond to two different residual peridotite layers. The upper layer contains 1.5–2 wt per cent water and as the lithosphere moves away from the ridge axis the temperature in this layer becomes low enough to start hydration reactions. These cause the low-velocity zone observed at 4 my and the total disappearance of the shallow level refractor before 9 my.     

Seismic and electromagnetic evidence of dehydration as a free water source in the reactivated crust

According to recent estimates, the continental mid-crust contains 35–40 per cent amphibolites. Heating of the crust by an underlying mantle plume, for example beneath continental rifts, high plateaus, and areas of intraplate volcanic activity, releases water. Dehydration of amphibole-bearing rocks at depths of 20–40  km occurs mainly in the temperature range 650–700 °C, and this releases about 0.4  wt per cent of water.
  Seismic tomography studies of the crust in the Kirgyz Tien Shan Range, where the age of the tectonic activity is less than 30  Ma, revealed a low-velocity zone in the mid-crust. The velocity of P waves was 0.4  km  s⁻¹ lower than in normal crust. MT sounding data in the region show the existence of a low-resistivity layer with an average resistivity of about 25  Ω  m at the depth of the low-velocity layer. The spatial correlation of the observed anomalous layers and calculated effect of fluid phase on seismic and electric parameters of rocks suggests the presence of aqueous fluids released by the heating of the mid-crust.     

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

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

THE RANGE OF EXISTENCE OF STONELEY WAVES IN AN INTERNAL STRATUM. II. ANTISYMMETRIC VIBRATIONS

Summary. We derive the frequency equation and the condition of existence of Stoneley type waves with antisymmetric vibrations which can be propagated along the interfaces between an internal stratum and two adjacent halfspaces of identical elastic properties, all perfectly elastic, homogeneous and isotropic. The ranges of existence of such waves are next obtained by numerical computation and the results are presented both in tabular form and graphically.
For large values of the frequency, the frequency equation for these waves, like the one for waves with symmetric vibrations discussed in an earlier paper*, reduces to the velocity equation of Stoneley waves propagated along the interface between two halfspaces.
For a low-velocity stratum, the results are similar to those for waves with symmetric vibrations. It is found that, for a pair of media for which waves of some frequency can exist, as the frequency of the waves or the thickness of the stratum is decreased, there is a cut-off value of either below which such waves cannot be propagated.
For a high-velocity stratum, the results are in sharp contrast with those for waves with symmetric vibrations. Stoneley waves of all frequencies can exist for some pairs of materials. As the frequency of the waves or the thickness of the stratum is decreased, that is, as the ratio of the wave-length to the thickness of the stratum is increased, the regions of existence expand until a certain limiting region of existence is obtained when this ratio tends to infinity.
Another difference between waves with symmetric and antisymmetric vibrations is that whereas for the former the phase velocity could not be less than the smaller of the two Rayleigh wave velocities, for the latter there is no theoretical lower limit.     

Forward modelling receiver functions for crustal structure beneath station TBZ (Trabzon, Turkey)

We use teleseismic three-component digital data from the Trabzon, Turkey broadband seismic station TBZ to model the crustal structure by the receiver function method. The station is located at a structural transition from continental northeastern Anatolia to the oceanic Black Sea basin. Rocks in the region are of volcanic origin covered by young sediments. By forward modelling the radial receiver functions, we construct 1-D crustal shear velocity models that include a lower crustal low-velocity zone, indicating a partial melt mechanism which may be the source of surfacing magmatic rocks and regional volcanism. Within the top 5 km, velocities increase sharply from about 1.5 to 3.5 km s⁻¹. Such near-surface low velocities are caused by sedimentation, extending from the Black Sea basin. Velocities at around 20 km depth have mantle-like values (about 4.25 km s⁻¹ ), which easily correlate to magmatic rocks cropping out on the surface. At 25 km depth there is a thin low-velocity layer of about 4.0 km s⁻¹. The average Moho velocity is about 4.6 km s⁻¹, and its depth changes from 32 to 40 km. Arrivals on the tangential components indicate that the Moho discontinuity dips approximately southwards, in agreement with the crustal thickening to the south. We searched for the solution of receiver functions around the regional surface wave group velocity inversion results, which helped alleviate the multiple solution problem frequently encountered in receiver function modelling.
Station TBZ is a recently deployed broadband seismic station, and the aim of this study is to report on the analysis of new receiver function data. The analysis of new data in such a structurally complex region provides constraining starting models for future structural studies in the region.     

A wide-angle seismic traverse through the Variscan of southwest Ireland

A wide-angle seismic profile across the western peninsulas of SW Ireland was performed. This region corresponds to the northernmost Variscan thrust and fold deformation. The dense set of 13 shots and 109 stations along the 120  km long profile provides a detailed velocity model of the crust.
  The seismic velocity model, obtained by forward and inverse modelling, defines a five-layer crust. A sedimentary layer, 5–8  km thick, is underlain by an upper-crustal layer of variable thickness, with a base generally at a depth of 10–12  km. Two mid-crustal layers are defined, and a lower-crustal layer below 22  km. The Moho lies at a depth of 30–32  km. A low-velocity zone, which coincides with a well-defined gravity low, is observed in the central part of the region and is modelled as a Caledonian granite which intruded upper-crustal basement. The granite may have acted as a buffer to northward-directed Variscan thrusting. The Dingle–Dungarvan Line (DDL) marks a major change in sedimentary and crustal velocity and structure. It lies immediately to the north of the velocity and gravity low, and shows thickness and velocity differences in many of the underlying crustal layers and even in the Moho. This suggests a deep, pre-Variscan control of the structural development of this area. The model is compatible with thin-skinned tectonics, which terminated at the DDL and which incorporated thrusts involving the sedimentary and upper-crustal layers.     

Three-dimensional image of the anisotropic bodies beneath central Honshu, Japan

Shear-wave splitting from local deep earthquakes is investigated to clarify the volume and the location of two anisotropic bodies in the mantle wedge beneath central Honshu, Japan. We observe a spatial variation in splitting parameters depending on the combination of sources and receivers, nearly N–S fast in the northern region, nearly E–W fast in the southern region and small time delays in the eastern region. Using forward modelling, two models with 30 and 10 per cent anisotropy are tested by means of a global search for the locations of anisotropic bodies with various volumes. The optimum model is obtained for 30 per cent anisotropy, which means a 5 per cent velocity difference between fast and slow polarized waves. The northern anisotropic body has a volume of 1.00° (longitude) × 0.5° (latitude) × 75 km (depth), with the orientation of the symmetry axis being N20°E. The southern anisotropic body has a volume of 1.25° × 1.25° × 100 km with the symmetry axis along N95°E. Our results show that the anisotropic bodies are located in low-velocity and low- Q regions of the mantle. This, together with petrological data and the location of volcanoes in the arc, suggests that the possible cause of the anisotropy is the preferred alignment of cracks filled with melt.     

The Hatton Bank continental margin—I. Shallow structure from two-ship expanding spread seismic profiles

Summary. The Hatton Bank passive continental margin exhibits thick seaward dipping reflector sequences which consist of basalts extruded during rifting between Greenland and Rockall Plateau. Multichannel seismic reflection profiling across the margin reveals three reflector wedges with a maximum thickness near 7 km, extending from beneath the upper continental slope to the deep ocean basin. We present results of the velocity structure within the dipping reflector sequences at eight locations across the margin, interpreted by synthetic seismogram modelling a set of multichannel expanding spread profiles parallel to the margin. At the top of some reflector sequences, we observe a series of 100 m thick high- and low-velocity zones, which are interpreted as basalt flows alternating with sediments or weathered and rubble layers. At the profile locations, the base of the dipping reflectors correlates with P -wave velocities near 6.5 km s⁻¹. However, elsewhere the reflectors appear to extend significantly deeper than the inferred 6.5 km s⁻¹ velocity contour, indicating that the velocity structure may not be controlled solely by lithological boundaries but also by metamorphic effects. Shear-waves were observed on two lines, permitting the calculation of Poisson's ratio. The decrease in Poisson's ratio from 0.28 to near 0.25 in the upper 5 km of crust may also indicate the effect of metamorphism on seismic properties, or alternatively may be explained by crack closure under load.     

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

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

A relative P-wave delay study between Eskdalemuir and Charnwood Forest

Summary. The relative P-wave delay between CWF, a permanent seismic station on the Precambrian rocks of Charnwood Forest in the English Midlands and EKA, the Eskdalemuir Seismological Array, shows a large azimuthal variation of 1.3 s. This is examined and is consistent with a thinning of the crust from EKA to CWF, together with a considerable thickness of high velocity (most probably greater than 7.0 km s⁻¹) lower crust beneath CWF. The Southern Uplands Fault, approximately 42 km to the north-west at its closest approach to EKA, seems to be associated with a large anomaly in the relative P -wave delay. Raypaths from events originating between azimuths 260 to 350° from EKA apparently pass through anomalously high velocity material entering the crust just to the south of the fault.