Crustal structure of the Faeroe–Shetiand Channel

Summary. The deep structure of the Faeroe–Shetland Channel has been investigated as part of the North Atlantic Seismic Project. Shot lines were fired along and across the axis of the Channel, with recording stations both at sea and on adjacent land areas. At 61°N, 1.7 km of Tertiary sediments overlies a 3.9–4.5 km s^-1 basement interpreted as the top of early Tertiary volcanics. A main 6.0–6.6 km s^-1 crustal refractor interpreted as old oceanic crust occurs at about 9 km depth. The Moho (8.0 ° 0.2 km s^-1) is at about 15–17 km depth. There is evidence that P _n may be anisotropic beneath the Faeroe–Shetland Channel. Arrivals recorded at land stations show characteristics best explained by scattering at an intervening boundary which may be the continent–ocean crustal contact or the edge of the volcanics.
The Moho delay times at the shot points, determined by time-term analysis, show considerable variation along the axis of the Channel. They correlate with the basement topography, and the greatest delays occur over the buried extension of the Faeroe Ridge at about 60° 15'N, where they are nearly 1 s more than the delays at 61°N after correction for the sediments. The large delays are attributed to thickening of the early Tertiary volcanic layer with isostatic downsagging of the underlying crust and uppermost mantle in response to the load, rather than to thickening of the main crustal ayer.
The new evidence is consistent with deeply buried oceanic crust beneath the Faeroe–Shetland Channel, forming a northern extension of Rockall Trough. The seabed morphology has been grossly modified by the thick and laterally variable pile of early Tertiary volcanic rocks which swamped the region, accounting for the anomalous shallow bathymetry, the transverse ridges and the present narrowness of the Channel.     

Analysis of the Spatial Coherence of Short-Period Acoustic-Gravity Waves in the Atmosphere

Summary The coherence of atmospheric acoustic-gravity waves has been measured in the period range 10–100 s at the Large Aperture Microbarograph Array in south-eastern Montana. The acoustic-gravity waves observed were signals generated by presumed nuclear explosions. The decrease of coherence with increasing distance between pairs of microbarographs is less rapid in the direction of wave propagation than transverse to it. Variation of direction of arrival over a small range of azimuth (±5°) explains the spatial behaviour of coherence in the direction normal to the wave propagation; variation of phase velocity of ±10 ms^-1 explains the behaviour along the direction of wave propagation. Both effects may be due to inhomogeneities in the atmosphere; the velocity variation may be due to the presence in the signal of several normal modes of acoustic- gravity waves, each travelling at a slightly different phase velocity in the range 300–330 ms^-1.     

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

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

Crust and upper mantle shear velocities from controlled sources

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A two ship refraction profile was undertaken on the Australian continental shelf during the Banda Sea geophysical program, carried out by the Woods Hole Oceanographic Institution, the Scripps Institution of Oceanography and the Geological Survey of Indonesia. S waves originating close to the sea bottom were observed to distances of up to 1150 km at an array of stations in northern Australia.
These observations are interpreted as implying S mantle velocities of 4.60 km s^-1 from a depth of 45 km to a depth of 76 km and 4.72 km s^-1 below a depth of 76 km.
Ratios of the P and S travel times (V_p/V_s) have been determined to be 1.74 in the crust rising to a value of greater than 1.79 below a velocity discontinuity at a depth of 200 km. It is inferred that this high value arises because the effect of temperature is greater for S than for P .
Using the data from this and other studies in the shield region of Northern Australia it has been found that the _S travel times are significantly less than predicted by the Jeffreys—Bullen tables.     

Terrestrial Heat Flow and Heat Generation in South-west England

Three heat flow values for south-west England are presented. Two of the sites, Geevor and South Crofty, are operating tin mines on the northern contacts of the Land's End and Carnmenellis Granites, respectively, while the third, Wilsey Down, is a stratigraphical borehole 5 km north of the Bodmin Moor Granite. After applying topographic corrections values of 128·6 mW m^-2 (3·07 μ cal cm^-2 s^-1) for Geevor, 128·9 mW m^-2 (3·08 (A cal cm^-2 s^-1) for South Crofty and 67·3 mW cm^-2 (1·61 cal cm^-2 s^-1) for Wilsey Down, were determined. The value at Wilsey Down is shown to be consistent with that for an environment in which the Hercynian orogeny was the last significant thermal event. An additional heat source term must clearly be involved at Geevor and South Crofty to account for the unusually high values at these sites. Radiogenic heat production has been determined on granites from these sites and in spite of the fact that it is high it does not fully account for the measured heat flow. A compilation of underground temperature measurements made in the nineteenth century suggests that high heat flow is a general feature of the mineralized belt. At least part of this can be explained in terms of hot spring activity recorded widely throughout the area but the ultimate cause remains to be evaluated.     

Estimating the crust permeability from fluid-injection-induced seismic emission at the KTB site

During the hydraulic-fracturing experiment in the German Continental Deep Drilling Borehole (KTB) in December 1994, microseismic activity was induced. Here we develop a technique for estimating permeability using the spatio-temporal distribution of the fluid-injection-induced seismic emission. The values we have obtained for the KTB experiment (0.25times10^-16 to 1.0times10^-16 1.0times10^-16 m²) are in a very good agreement with the previous hydraulic-type permeability estimates from KTB deep-observatory studies. In addition, our estimates of the hydraulic diffusivity support the previously calculated value for the upper crust, which is of the order of 1 m² s^-1. However, this estimate now relates to the depth range 7.5-9 km.     

Calibration of the Yellowknife Seismic Array with First Zone Explosions

Summary Recordings from a crustal seismic experiment, which was conducted in the Yellowknife area in 1966, were used for calibration of the Yellow-knife seismic array. In the immediate vicinity of the array the crust is found to be very uniform. A superficial layer with an intercept time of 0–172 ± 0–012s and unknown velocity is underlain by a crust with a P wave velocity of 6.04 ± 0–01 km s^-1 near the top: assuming this velocity constant throughout the second layer, the total thickness of the crust is about 34 ± 2 km. The Mohorovicic discontinuity is horizontal under the array within the resolution of this experiment and the apparent P_n velocity is 8.15 km s^-1. At a distance of a few tens of kilometres the crustal uniformity breaks down. The distances are such that, for most teleseismic signals, the effect of these in homogeneities should be negligible.     

The methodology of quantification of volcanic explosions from broad-band seismic signals and its application to the 2004–2005 explosions at Volcán de Colima, Mexico

A methodology is proposed for the quantification of volcanic explosions based on three parameters derived from broad-band seismic signals: the counter force of the eruption F , the power of the explosion P and the duration of the upward movement of the gas slug in the conduit to the free surface of magma, D . This methodology was applied to the 2004–2005 sequence of explosions at Volcán de Colima, Mexico. The broad-band records of more than 100 explosive events were obtained at a distance of 4 km from the crater. We determined the counter force of the eruption by modelling the low-frequency impulse of the seismic records of 66 volcanic explosions and estimated the power of 116 explosions from the spectra of the high-frequency impulse. The power of Colima explosions spans five orders of magnitude; the counter force spans four orders of magnitude. We show that the power of a volcanic explosion is proportional to the counter force of the eruption. These parameters may be used for the elaboration of a scale of volcanic explosions.     

A boundary layer model for mantle convection with surface plates

Summary. A simple, analytical model for mantle convection with mobile surface plates is presented. Our aim is to determine under what conditions free convection can account for the observed plate motions, and to evaluate the thermal structure of the mantle existing under these conditions. Boundary layer methods are used to represent two-dimensional cellular convection at large Rayleigh and infinite Prandtl numbers. The steady-state structure consists of cells with isentropic interiors enclosed by thermal boundary layers. Lithospheric plates are represented as upper surfaces on each cell free to move at a uniform speed. Buoyancy forces are concentrated in narrow rising and decending thermal plumes; torques imparted by these plumes drive both the deformable mantle and overlying plate. Solutions are found for a comprehensive range of cell sizes. We derive an expression for the plate speed as a function of its length, the mantle viscosity and surface heat flux. Using mean values for these parameters, we find that thermal convection extending to 700 km depth can move plates at 1 cm yr^-1, while convection through the whole mantle can move plates at 4–5 cm yr^-1. Analysis of the steady-state temperature field, for the case of heating from below, shows that the upper thermal boundary layer develops a complex structure, including an 'asthenosphere' defined by a local maximum in the geotherm occurring at depths of 50–150 km.     

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.     

Evidence for a seismic discontinuity near 200km depth under a continental margin

Summary. We present evidence for a seismic discontinuity near 200km depth (the Lehmann Discontinuity) under the passive continental margin of northwest Australia, where continental lithosphere merges into oceanic lithosphere. The velocity contrast across the discontinuity is 0.2–0.3 km s^-1, and is similar to the contrast across discontinuities at similar depths in seismic models for purely continental paths to the east under central Australia. The discontinuity has been shown to be present under continents, oceans and now at continental margins, and is probably a worldwide feature.     

Formation and growth of deep mantle plumes

Summary. If whole mantle convection occurs in the Earth's mantle, then the core–mantle boundary constitutes the lower boundary layer for mantle convection. This boundary layer appears to be unstable on a small scale, and thus may be a source of plumes of hot matter which penetrate the mantle and occasionally even the lithosphere (producing hot spots). A finite-amplitude numerical code is used to study the formation of such plumes and their growth through the mantle. The plumes are restricted to being two-dimensional sheets rather than cylinders. The initial conditions consist of a steadily convecting mantle, and plumes are produced by introducing a perturbation in the form of either a pulse or a steady stream of heat into the bottom of the mantle. Two main results are obtained: (1) A critical perturbation size has been found for a mantle with a Rayleigh number of 10⁷. Small perturbations produce plumes which fail to penetrate the mantle, and instead are swept up by the pre-existing convective pattern, while large perturbations succeed in penetrating the mantle and reaching the lithosphere. The critical perturbation size is shown empirically to be proportional to the effective bouyancy and to a factor related to the shape of the perturbation. A perturbed region 150km wide and 60 km deep should produce a successful plume when the temperature perturbation is 200K or more. (2) Deep mantle plumes appear to require on the order of 50–100Myr to penetrate the mantle; episodic plumes on shorter time-scales appear unlikely. A similar time is required for plumes forming in an initially static, uniform temperature mantle.     

A near-regional explosion source model for tuff

Summary. A variety of near-regional (300 km) data, including spectral amplitudes of Pg , surface-wave forms, and close-in (5–10 km) accelerograms have been used to build an elastic seismic source model for a 1-Mton explosion in tuff at near-regional distances. The model consists of: (1) a pressure pulse which injects 3 × 10¹² cm³ of volume into the medium, (2) a vertical, upward force impulse that imparts 10¹⁸ dyn-s of momentum to the medium, each source component having a time duration of 0.6 s and a depth of 1.3 km. The force impulse appears to be required by two considerations: (a) the striking similarity, apart from sign, of explosion surface waves with those of their cavity collapses, (b) the observation of considerable SV energy leaving the source of the 1-Mton explosions JORUM and HANDLEY . Scaling curves have been constructed which fit the proposed source model. These scaling curves employ: very slow decrease, as (yield)^−0.10 of the primary corner frequency; decay as (frequency)⁴ or (frequency)³ to high frequency. While these scaling curves are unconventional, they appear to be the only ones which can satisfy the near-regional data. The slow scaling with yield of the spectral carner frequency suggests that it is caused by something other than the equivalent elastic radius, e.g. the time duration of motion at the source. The results, at odds with similar studies at teleseismic distances, suggest that significantly different equivalent elastic sources are required at near-regional (as compared with teleseismic) distances; therefore, the effect of the upward impulse might not be seen at teleseismic distances. Consequently, these results probably do not pertain to the seismic discrimination problem at teleseismic distances.     

Crustal model of the Bransfield Rift, West Antarctica, from detailed OBS refraction experiments

The first detailed deep seismic refraction study in the Bransfield Strait, West Antarctica, using sensitive OBSs (ocean bottom seismographs) was carried out successfully during the Antarctic summer of 1990/1991. The experiment focused on the deep crustal structure beneath the axis of the Bransfield Rift. Seismic profile DSS-20 was located exactly in the Bransfield Trough, which is suspected to be a young rift system. Along the profile, five OBSs were deployed at spacings of 50-70 km. 51 shots were fired along the 310 km profile. This paper gives the first presentation of the results. A detailed model of the crustal structure was obtained by modelling the observed traveltimes and amplitudes using a 2-D ray-tracing technique. The uppermost (sedimentary?) cover, with velocities of 2.0-5.5 km s⁻¹, reaches a depth of up to 8 km. Below this, a complex with velocities of 6.4-6.8 km s⁻¹ is observed. The presence of a high-velocity body, with V _p= 7.3-7.7 km s⁻¹, was detected in the 14-32 km depth range in the central part of the profile. These inhomogeneities can be interpreted as a stage of back-arc spreading and stretching of the continental crust, coinciding with the Deception-Bridgeman volcanic line. Velocities of 8.1 km s⁻¹, characteristic of the Moho, are observed along the profile at a depth of 30-32 km.     

Velocity structure of upper-mantle transition zones beneath central eurasia from seismic inversion using genetic algorithms

We present velocity constraints for the upper-mantle transition zones beneath Central Siberia based on observations of the 1982 RIFT Deep Seismic Sounding (DSS) profile. The data consist of seismic recordings of a nuclear explosion in north-western Siberia along a 2600 km long seismic profile extending from the Yamal Peninsula to Lake Baikal. We invert seismic data from the mantle transition zones using a non-linear inversion scheme using a genetic algorithm for optimization and the WKBJ method to compute the synthetic seismograms. A statistical error analysis using a graph-binning technique was performed to provide uncertainty values in the velocity models.
Our best model for the upper-mantle velocity discontinuity near 410 km depth has a two-stage velocity-gradient structure, with velocities increasing from 8.70–9.25 km s⁻¹ over a depth range of 400–415 km, a gradient of 0.0433 s⁻¹, and from 9.25–9.60 km s⁻¹ over a depth range of 415–435 km, a gradient of 0.0175 s⁻¹. This derived model is consistent with other seismological observations and mineral-physics models. The model for the velocity discontinuity near 660 km depth is simple, sharp and includes velocities increasing from 10.15 km s⁻¹ at 655 km depth to 10.70 km s⁻¹ at 660 km depth, a gradient of 0.055 s⁻¹.     

Crustal structure of the Mid-Atlantic ridge crest at 37° N

Summary. A structural model of the Mid-Atlantic Ridge at 37° N is proposed on the basis of travel-time data and synthetic seismograms. At the ridge axis the crust is only 3 km thick and overlies material with an anomalously low'upper mantle'velocity of 7.2 km s⁻¹. Crustal thickening and the formation of layer 3 and a layer with velocity 7.2–7.3 km s⁻¹ takes place within a few kilometres of the axis, producing a 6–7 km thick crust by less than 10 km from the axis. A normal upper mantle velocity of 8.1 km s⁻¹ exists within 10 km of the axis. Shear waves propagate across the axis, thus precluding the existence of any sizeable magma chamber at shallow depth.     

Seismic refraction profiles between Cyprus and Israel and their interpretation

Summary. A long seismic refraction profile was carried out between southern Israel and Cyprus. The seismic energy was generated by 33 sea shots each of 0.8 t explosives and was recorded by land stations in Israel and Cyprus and by ocean bottom seismographs deployed along the profile.
The results showed that the continental crust of southern Israel thins towards the Mediterranean underneath a northward thickening sedimentary cover. Cyprus is underlain by a 35 km thick continental crust thinning south-wards and extending to Mt Eratosthenes. Between Mt Eratosthenes and the Israel continental shelf the crystalline crust is composed of high velocity (6.5 km s^-1)material and is about 8 km thick. It is covered by 12–14 km of sediments and may represent a fossil oceanic crust.     

Crustal and upper mantle structure of the northwestern North Island, New Zealand, from seismic refraction data

The crustal and upper mantle structure of the northwestern North Island of New Zealand is derived from the results of a seismic refraction experiment; shots were fired at the ends and middle of a 575 km-long line extending from Lake Taupo to Cape Reinga. The principal finding from the experiment is that the crust is 25 ± 2 km thick, and is underlain by what is interpreted to be an upper mantle of seismic velocity 7.6 ± 0.1 km s⁻¹, that increases to 7.9 km s⁻¹ at a depth of about 45 km. Crustal seismic velocities vary between 5.3 and 6.36 km s⁻¹ with an average value of 6.04 km s⁻¹. There are close geophysical and geological similarities between the north-western North Island of New Zealand and the Basin and Range province of the western United States. In particular, the conditions of low upper-mantle seismic velocities, thin crust with respect to surface elevation, and high heat-flow (70–100 mW m⁻²) observed in these two areas can be ascribed to their respective positions behind an active convergent margin for about the past 20 Myr.     

On the penetration of a hot diapir through a strongly temperature-dependent viscosity medium

Summary. The ascent of a hot spherical body through a fluid with a strongly temperature-dependent viscosity has been studied using an axisymmetric finite element method. Numerical solutions range over Peclet numbers of 10⁻¹– 10³ from constant viscosity up to viscosity variations of 10⁵. Both rigid and stress-free boundary conditions were applied at the surface of the sphere. The dependence of drag on viscosity variation was shown to have no dependence on the stress boundary condition except for a Stokes flow scaling factor. A Nusselt number parameterization based on the stress-free constant viscosity functional dependence on the Peclet number scaled by a parameter depending on the viscosity structure fits both stress-free and rigid boundary condition data above viscosity variations of 100. The temperature scale height was determined as a function of sphere radius. For the simple physical model studied in this paper pre-heating is required to reduce the ambient viscosity of the country rock to less than 10²² cm² s⁻¹ in order for a 10 km diapir to penetrate a distance of several radii.     

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.