Showing papers in "Geophysical Journal International in 1979"

Stress and temperature in the bending lithosphere as constrained by experimental rock mechanics

Abstract: Summary. Previous attempts to deduce the stress distribution in the bending lithosphere near a consuming plate margin have relied on the observed bathymetry and an assumed constitutive relation for lithospheric behaviour, eg. perfectly elastic, viscous/perfectly plastic, or elastic perfectly plastic. From the point of view of rock mechanics, each of these approximations fails to describe one or more of several basic phenomena, including brittle failure of rock, temperature dependence of elasticity, and temperature and/or strain rate dependence of ductile behaviour. In order to formulate a more realistic constitutive relation, a limiting yield strength curve, which is primarily a function of temperature, is constructed from data from brittle failure and ductile flow experiments. The moments which can be supported by plates with this constitutive behaviour are compared to the moments calculated from bathymetric profiles. The comparison indicates that moments required by the bathymetric data are consistent with moments supported by plates with experimentally determined constitutive laws as extrapolated to geo- logically reasonable temperatures and strain rates. The stresses developed in such models are required to reach values greater than 100 MPat in the depth range 25-45 km. Geotherms necessary for strength curves consistent with moments calculated from the bathymetric data match those derived from heat flow data for the Aleutian, Bonin, Mariana and Tonga trenches. Of the trenches studied, only the geotherm inferred from the Kuril trench data is significantly different, perhaps implying that the Kuril plate is weaker than the others. The strength curves show that as a first approximation it is better to assume that bending moment is independent of curvature of the plate than to assume that bending moment and curvature are linearly related.

Seismic waves in a stratified half space.

Abstract: Summary. The response of a stratified elastic half space to a general source may be represented in terms of the reflection and transmission properties of the regions above and below the source. For P-SV and SH waves and both buried sources and receivers, convenient forms of the response may be found in which no loss of precision problems arise from growing exponential terms in the evanescent regime. These expressions have a ready physical interpretation and enable useful approximations to the response to be developed. The reflection representation leads to efficient computational procedures for models composed of uniform layers, which may be extended in an asymptotic development to piecewise smooth models.

The use of a priori data to resolve non‐uniqueness in linear inversion

Abstract: Summary. The recent, but by now classical method for dealing with non-uniqueness in geophysical inverse problems is to construct linear averages of the unknown function whose values are uniquely defined by empirical data (Backus & Gilbert). However, the usefulness of such linear averages for making geophysical inferences depends on the good behaviour of the unknown function in the region in which it is averaged. The assumption of good behaviour, which is implicit in the acceptance of a given average property, is equivalent to the use of a priori information about the unknown function. There are many cases in which such a priori information may be expressed quantitatively and incorporated in the analysis from the very beginning. In these cases, the classical least-squares method may be used both to estimate the unknown function and to provide meaningful error estimates. In this paper I develop methods for exploring the resolving power in such cases. For those problems in which a continuous unknown function is represented by a finite number of‘layer averages’, the ultimately achievable resolving width is simply the layer thickness, and perfectly rectangular resolving kernels of greater width are achievable. The method is applied to synthetic data for the inverse‘gravitational edge effect’problem where yi are data, f(z) is an unknown function, and ei are random errors. Results are compared with those of Parker, who studied the same problem using the Backus—Gilbert approach.

Finite deformation during fluid flow

Abstract: Summary. Typical upper mantle circulations obtained by solving Stokes' equation produce finite deformations which differ in important ways from those produced by pure or simple shear. Finite strain, defined by the ratio of the long to the short axis of the deformation ellipse, in most cases shows a steady increase with superimposed oscillations. Similarity solutions for the flow near plate boundaries demonstrate that the observed seismic anisotropy in the oceanic lithosphere can be produced by the finite deformation beneath the ridge axes. The same mechanism should give rise to strong anisotropy in the mantle above sinking slabs. Such anisotropy has not yet been detected, perhaps because the observed high velocities have been attributed to thermal effects. Convection in the mantle remote from plate boundaries produces complicated deformation which varies rapidly with position and will therefore be difficult to map seismically. The fabrics of nodules in lavas and kimberlites suggest that large strains can occur in the mantle under stresses which are too small to produce dislocation movement. The large and complicated finite deformation produced by the convective circulation in the mantle also affects closed geochemical systems, and leads to thorough mixing of any convecting region.

Thermal evolution of the Earth's core

Abstract: Summary. The persistence of the magnetic field of the Earth demands a constant energy source for the last three thousand million years, and this provides a constraint on the thermal evolution of the core. The equations of global energy and entropy balance are used to estimate the power source required for a specific magnetic field. The amount of power required depends on the exact nature of the source. Three possibilities are considered here: radioactive heating in the core itself, loss of internal energy due to cooling and freezing of the outer core to form the inner core, and cooling of the whole core with consequent differentiation to form the inner core with release of gravitational energy. The last of these includes all the sources except for radioactive heating, but the introduction of some radioactivity into this calculation would be a simple matter. For radioactive heating alone, 1013W is required for the dynamo. This is just within the limits set by the observed surface heat flux (4 x lOI3 W) and what some geochemists believe to be the heating due to K40. Cooling itself cannot release enough heat to power the dynamo because the required cooling rate is so high that the inner core would be a very recent feature of the Earth. The release of gravitational energy can produce a magnetic field of 100-200 gauss, with the inner core growing slowly to its present size over 4Ga, and a heat release of 2.5 x lO'*W. A lower heat flux is required because of the greater efficiency of conversion of gravitational energy into magnetic fields than heat. When pursuing the calculations backwards in time, the rate of energy release is found to be proportional to the mass of the inner core. A surprising feature of this model, which assumes a constant rate of cooling at the top of the core, is that the useful power available for the dynamo increases with time, so that the field should be stronger now than it was in the past, although only by about 30 per cent.

The frequency dependence of Q in the Earth and implications for mantle rheology and Chandler wobble

Abstract: For most solids the ‘high temperature background’ attenuation dominates at low frequencies and temperatures greater than about one-half the melting temperature. It is likely to be important in the mantle at seismic frequencies. The same mechanism also contributes to transient creep at low stresses and low total strains. A relaxation spectrum is found which satisfies the frequency dependence of laboratory Q and the time dependence of transient creep data. This makes it possible to provide a physical interpretation of the parameters in Jeffrey's modified Lomnitz creep function. Q is predicted to increase as ω^α in the lower Q regions of the mantle. At high and low frequencies Q should increase as ω and ω^(−1), respectively. The location of the ω^α band depends on temperature and therefore shifts with depth. At high temperatures, seismic waves are on the low-frequency side of the absorption band and Q decreases with frequency. Far from the melting point and at sufficiently high frequencies Q should increase linearly with frequency. We use Chandler wobble, tidal and free oscillation data to estimate that α is ∼ 1/5 to 1/3, consistent with laboratory measurements of transient creep and internal friction at high temperature. A preliminary attempt is made to estimate the transient creep response of the mantle from Q measurements. The inferred viscosity agrees well with direct measurements. The effect of anelasticity is to lengthen the calculated period of the Chandler wobble by 5-20 days, depending on the Chandler wobble Q. A Q of 300 for the wobble, which is within the experimental uncertainty of recent determinations, gives the observed period after correcting for the effect of the oceans.

Subduction of the Nazca plate beneath Peru: evidence from spatial distribution of earthquakes

Abstract: Summary. This paper reports a detailed re-examination of the spatial distribution of shallow- and intermediate-depth earthquakes located beneath Peru between latitudes 3° and 18°S. The events are classified into two groups according to the quality of the locations. The results of this paper confirm and considerably strengthen the findings of Barazangi & Isacks, which have taken on significance beyond Peru because of their implications regarding the tectonics of western North America during the Laramide orogeny. The more accurately located events that occurred beneath central and northern Peru clearly define a shallow-dipping (about 10°) inclined seismic zone that extends inland for a distance of about 700 km from the Peru trench. Although the intermediate-depth events are not uniformly and continuously distributed along the strike of the arc, the available data are best explained by a single Benioff zone that defines the descending Nazca plate beneath central and northern Peru. In southern Peru the Benioff zone is well determined and has a dip of about 30°. The transition from the relatively flat Benioff zone in central Peru and the steeper Benioff zone in southern Peru is abrupt and is interpreted as a tear in the descending Nazca plate. Considerable seismic activity occurs within the upper 50 km of the overriding continental South America plate. This activity is well separated from the shallow-dipping Benioff zone in central and northern Peru, and probably occurs in the crustal part of the South American lithosphere. All events that occur outside the spatial pattern of events as described above are based on few and generally insufficient data. Azimuthal coverage at teleseismic distances is poor, and no reliable pP readings are available to control hypocentral depths. A search for ScS-to-P converted phases at the upper boundary of the descending Nacza plate beneath central Peru has failed to provide any clear and consistent data. This is in contrast to the well-documented observations from Japan. The flat geometry of the descending Nazca plate beneath central and northern Peru requires that the thickness of the continental South American lithosphere does not exceed about 130 km in this region. A remarkable correlation exists between the flat geometry of the descending Nazca plate and the absence of Quaternary volcanism on the overriding South America plate. This is in marked contrast to the presence of abundant volcanism above the steeper segment of the descending Nazca plate in southern Peru, and suggests that generation of arc-type volcanism requires a mantle wedge of asthenospheric material between the subducted and overriding plates. The flat geometry of the descending plate may also be the cause for the broad deformation of the Andean Cordilleras. Many factors may be responsible for the observed flat geometry, such as the fact that the Nazca plate is relatively young (< 50 Myr old), and hence is thinner and more buoyant than the older western Pacific plate. Also, it may be significant that the upper South American lithosphere is thick, in comparison to the upper western Pacific plates, and is possibly actively overriding the descending Nazca plate. Finally, the width of the flat Peru segment along the strike of the arc is probably controlled by the existence of the aseismic submarine structural features near the southern limit (the Nazca ridge) and the northern limit (the Carnegie ridge) of the flat segment. These features are probably zones of weakness along which the Nazca plate is being transversely torn.

Lengths of intermediate and deep seismic zones and temperatures in downgoing slabs of lithosphere

Abstract: Summary. If intermediate and deep earthquakes occur in the coldest portions of the downgoing slabs of lithosphere, then different lengths of seismic zones represent different temperatures in the slabs. As the slab descends through the aesthenosphere, it warms primarily by conduction of heat through its upper and lower surfaces. Isotherms are advected downwards to distances approximately proportional both to the rate of subduction and to the square of the thickness of the lithosphere. Consequently, the lengths of seismic zones should be approximately proportional to the product of the rates and the squares of these thicknesses. As these thicknesses are approximately proportional to the square root of the age of the lithosphere, the lengths ought therefore to be approximately proportional to the product of the convergence rates times the ages. Although there is considerable scatter, observed lengths are approximately proportional to such products, and are not simply related to the rate, the age or the thickness alone. The data crudely fit the relationship: length = rate × age/10. Using this relationship, we infer that the Philippine sea and Pacific plates move slowly, if at all, with respect to one another and that the Farallon plate may have been too young to be subducted to a great distance beneath western North America in the Palaeogene. Calculations of temperatures at the depths of the deepest events suggest that these cut-off temperatures increase from about 600 ± 100°C at 200 km to 830 ± 50°C at 650 km depth, but the cut-off potential temperature is approximately constant. Assuming that the strength is a thermally activated parameter, and using the activation energy for olivine, a crude estimate of activation volume is obtained from the dependence of the cut-off temperature on depth.

Observational constraints on the chemical and thermal structure of the Earth's deep interior

Abstract: Summary. The observations of the periods of free oscillation of the Earth provide direct constraints on the density distribution in the Earth. These in turn allow constraints to be placed on the size of departures from a state of adiabaticity and chemical homogeneity. These departures are quantified in terms of a stratification parameter ‘8’ first introduced as an index of chemical homogeneity. The resolving power theory of Backus & Gilbert is used to determine the ability of the observed free oscillations to constrain η in the lower mantle and outer core. The results suggest that the outer core is not strongly chemically stratified although a significantly thermally stable core cannot be excluded. The free oscillations also apparently require a compositional difference between the inner and outer cores.

The turbulent structure of the bottom boundary layer in a tidal current

Abstract: Summary. Measurements of turbulence in the neutrally stratified bottom boundary layer of a tidal current are described. It is shown that whereas the bottom boundary layer is similar to its atmospheric counterpart, in terms of appropriately scaled spectra of the turbulent velocity fluctuations and the Reynolds stress, there is less evidence to suggest that it is structurally similar when measured in terms of the Reynolds stress tensor components. These differences appear to be more pronounced in deep flows where there is some evidence to suggest that the indices of structural similarity may vary over the tidal cycle; this behaviour may be due in part to the presence of inactive motions in the boundary layer. The statistical properties of the turbulence are described and in particular the high sampling variability of the Reynolds stress is shown to be associated with intermittently large momentum fluxes occurring in the boundary layer. The use of an intermittency factor in determining the burst period is also discussed. Suspended sediment is shown to have no effect on the turbulent structure of this example of a boundary layer. Estimates of the drag coefficient for a range of sediment types and bed forms have been obtained and finally it is shown that levelling errors are a major source of inaccuracy in boundary layer stress measurements.

The fundamental solution in dynamic poroelasticity

Abstract: Summary. In this paper we study Biot’s full, time-dependent equations of dynamic poroelasticity with a view to understanding the effect of pore fluid on seismic wave propagation. Typical values of the constants appearing in the equations which are relevant to the rock surrounding earthquake sources are estimated from values appearing in the recent literature. We investigate the disturbance due to an instantaneous point body force acting in a uniform whole space. In fact we calculate the tensor fundamental solution since this has spherical symmetry, which is strongly exploited in our method of solution. The introduction of four scalar potentials enables us to reduce the problem to two decoupled second-order systems, each consisting of two coupled wave equations with friction in one space and one time dimension. By a further transformation these systems are expressed as symmetric hyperbolic systems of the first order, which are then solved by Laplace transforms. Because the dispersion equations are of higher than second degree only the large time saddle-point contributions are calculated. From these several phenomena emerge. (a) A P wave propagating with the P-wave speed appropriate to the ‘solid’ obtained by constraining the fluid to move with the solid matrix. However, instead of a 6 pulse shape familiar in elastodynamics this P wave has the shape of a Gaussian which appears to diffuse in a frame of reference moving with the P-wave speed. (b) An S wave with similar shape to P. (c) A long-term diffusion which is what one obtains from the equations reduced by setting the inertial terms to zero as in consolidation theory. We also investigate in an appendix a special case of dynamical compatibility in which the P wave remains sharp (i.e. a 6 pulse) and one of our two systems can be solved explicitly. The pulse diffusion amounts to a dissipation of the high frequency content qf seismic waves at a rate proportional to the square of the frequency.

Dispersion function computations for unlimited frequency values

Abstract: Summary Progress in the matrix method for the calculation of the seismic surface wave dispersion function for a layered elastic media started with the beginning of the electronic digital computer age. The use of Thomson–Haskell formulation, Knopoff's method or any other published method has had the persistent problem of loss of precision at high frequencies. The severity of the high-frequency limitation problem varies among the approaches but exists in all of them. In this paper we present a novel method to determine the dispersion function for Rayleigh waves in layered elastic media. In this method there is no limitation on the value of the frequency.

Earthquake sequences on a frictional fault model with non-uniform strengths and relaxation times

Abstract: Summary. The space and time characteristics of earthquake sequences, including a main shock, aftershocks and the recurrence of major shocks in a long time range, are investigated on a frictional fault model with non-uniform strengths and relaxation times, which is subjected to a time-dependent shear stress. Aftershocks with low stress drop take place successively in spaced regions so as to fill the gaps which have not yet been ruptured since the main shock, while those with high stress drop occur in and around the regions left unruptured during the main faulting. The frequency decay of aftershocks with time follows a hyperbolic law with the rates p consistent with observations. There are good linear relations in logarithmic scales for source area versus frequency and seismic moment versus frequency of the generated aftershocks. The b-value obtained in the present experiments appears slightly larger than that for observations. It was found that more heterogeneous distribution of the fault strength give smaller p and larger b-values. The recurrence of major shocks, particularly of very large shocks with high stress drop, is often preceded by a completely silent period of activity or very low activity with a small number of foreshocks. The major shocks take place successively in adjacent unruptured regions and sometimes show slow-speed migrations. These results provide explanations to various observations of earthquake sequences.

Seismotectonic implications of relocated aftershock sequences in Iran and Turkey

Abstract: Summary. Six aftershock sequences in Iran and Turkey are relocated using existing teleseismic data. Two of these are in the Zagros mountains where local fieldwork has failed to detect subcrustal seismicity but published teleseismic locations show depths greater than 100 km. All apparently deep events are shown to be small and badly recorded with poor depth resolution. There is thus no evidence for active lithospheric subduction in the Zagros. Relocations of other sequences in Iran and Turkey are used with fault plane solutions, satellite photographs and surface faulting to provide new insight on the geometry of faulting and crustal deformation of those regions. Linear seismic trends from these sequences are shown to cut older geological structures and do not always bear a simple relation to surface faulting. In such cases aftershock activity may be on primary buried faults whose behaviour is not simply revealed in surface structure and deformation. A linearized inversion scheme is used to investigate the trade-off between resolution and uncertainty in the hypocentral parameters. The ultimate resolution of teleseismic locations is shown to be limited by the quality of arrival time data.

A thermal history model for the Earth with parameterized convection

Abstract: Summary. A thermal history model for the Earth is described in which the energetically important effects of convection are parameterized through the Nusselt number. The validity of the resulting quasi-steady-state thermal model is shown to depend upon the separation of two time-scales—a dynamic time-scale associated with the overturn time for an assumed mantle-wide convective circulation, and a thermal time-scale associated with the cooling of the planet. Provided the initial thermal state of the Earth was ‘hot’, the assumption of a time-scale separation can be shown under certain conditions, to be valid throughout the Earth's history. In this connection, the temperature-dependent mantle rheology plays a key role in regulating the thermal history. It is shown that the present-day, gross thermal structure of the Earth can be understood within the context of a quasi-steady-state model which is driven mainly by primordial heat. The notion of whole-mantle convection is shown to be consistent with several additional observational constraints, including the observed mean lithospheric thickness and the mean plate velocities. We briefly consider the extension of the parameterized thermal model to Venus.

Post-shock temperatures in minerals

Abstract: An experimental technique was developed for measuring post-shock temperatures in a wide variety of materials, including those of geophysical interest such as silicates. The technique uses an infrared radiation detector to determine the brightness temperature of samples shocked to pressures in the range 5 to approximately 30 GPa; in these experiments measurements were made in two wavelength ranges (4.5 to 5.75 microns and 7 to 14 microns). Reproducible results, with the temperatures in the two wavelength bands generally in excellent agreement, were obtained for aluminum-2024 (10.5 to 33 GPa, 125 to 260 C), stainless steel-304 (11.5 to 50 GPa, 80 to 350 C), crystalline quartz (5.0 to 21.5 GPa, 80 to 250 C), forsterite (7.5 to 28.0 GPa, approximately 30 to 160 C) and Bamble bronzite (6.0 to 26.0 GPa, approximately 30 to 225 C). It is concluded that release adiabat data should be used, wherever available, for calculations of residual temperature, and that adequate descriptions of the shock and release processes in minerals are more complex than generally assumed.

The pulse-probe method of conductivity measurement

Abstract: Summary. An alternative to the steady heating of a cylindrical probe, in the ‘needle-probe’ method of conductivity measurement, is the observation of the thermal decay from a short, calibrated, heat pulse. The theoretical solution is the time-differential of that for the former method, and requires only the measurement of point temperatures rather than the determination of a gradient. A careful analysis of the theoretical decay function shows that it should be possible to make accurate conductivity measurements in as little as three probe ‘time constants’ if external information is available on the heat capacity of the medium. A self-contained method, using two temperature determinations from a record about six time-constants long, can be used where such information is not available. The theory was tested by measurements on the ocean-floor, and the data correspond to the theory when a correction was applied for some internal probe conduction problems.

Regionalized shear velocity models of the Pacific upper mantle from observed Love and Rayleigh wave dispersion

Abstract: Summary. Group and phase velocities for periods between 16 and 110 s have been determined using the single-station method along 33 Rayleigh-and 30 Love-wave paths across the Pacific. These dispersion data were used to detect variations in upper mantle structure beneath the Pacific. Taking both regional changes and azimuthal anisotropy into consideration, regionalized group and phase velocities were derived for four regions of differing age. The boundaries of the regions were chosen to correspond to isochrons of 20, 50 and 100 Myr, as inferred from geomagnetic lineations. A small, but significant degree of azimuthal anisotropy (less than 0.8 per cent) was found to occur throughout the Pacific if the degree and direction of maximum velocity were assumed to be uniform everywhere. The average direction of maximum velocity was found to be 85°± 6° as measured clockwise from north. Regionalized group and phase velocities for Rayleigh waves at the larger periods increase systematically with increasing age of the ocean floor. For Love waves, departures from a systematic increase occur in the youngest (0–20 Myr) and oldest (> 100 Myr) portions of the Pacific. The dispersion data were corrected for the effects of anelasticity and were inverted to obtain shear velocity models for each of the four regions using the stochastic inversion method. A comparison of the shear wave models derived from Rayleigh waves in the four regions indicates that average shear velocities at depths between 30 and 110 km increase rapidly with age. This increase occurs mainly because the depth to the base of the lithosphere increases; however, the shear velocities of the lithosphere also increase for the regions out to 100 Myr in age. Slightly lower velocities in the upper part of the lithosphere for greater ages may be due to a large influx of mid-plate volcanism during the Cretaceous. Separate inversions of Love-and Rayleigh-wave velocities indicate that polarization anisotropy can be resolved for the lithosphere. Polarization anisotropy cannot, be resolved for the low-velocity zone beneath the Pacific using these data, except possibly in the youngest regions, where lateral complexities of the East Pacific Rise are apt to affect the data adversely. Assuming that we can adequately invert our Love-and Rayleigh-wave data separately to obtain information on anisotropy, we find that average SV velocities in the lithosphere increase from about 4.3 to 4.6 km/s, and average SH velocities increase from about 4.5 to 4.7 km/s, from the youngest to oldest regions. Average velocities in the upper portion of the low-velocity zone increase from about 4.1 to about 4.25 km/s for the same regions. The existence of anisotropy in the lithosphere is constant with the results of seismic refraction experiments in the Pacific and supports peroxide as the predominant upper mantle material. The occurrence of anisotropy through-out the entire lithosphere, suggests a situation in which periodite crystallizes at the base of the lithosphere from a partially molten low-velocity zone and olivine crystals become aligned in response to the stress field within the Pacific plate.

Rotational and elliptical splitting of the free oscillations of the earth.

Abstract: *Summary. We present a table of rotational and elliptical splitting parameters for earth model 1066A, including all terms through second order in rotation and first order in ellipticity. An algorithm for calculating the second-order Coriolis splitting by summing over all modes which are coupled to first order is given in detail. Coupling to secular (or zero frequency) modes, as well as the usual seismic modes, can provide significant contributions to these splitting parameters.

Lateral variations of S velocity in the upper mantle from higher Rayleigh modes

Abstract: Summary. The generalized inverse theory has been applied to interpret several sets of higher mode data, previously obtained for the United States and the Pacific Ocean. The depth-resolving power of these data allows us to find the distribution of S velocity down to about 600 km. The main lateral variations of S velocity are found in the uppermost 250 km, the south-western United States showing the lowest velocities and the central-north-eastern United States the highest velocities. Between 250 and 500 km an opposite situation seems to occur, western velocities being the greatest ones, but these lateral variations are 3 to 5 times less than above and they cannot be surely established under the variance estimated for the data. Finally no lateral variations are resolved between 500 and 700 km. Some remarks may be made about the corresponding absolute models: (1) the agreement is good with published models, built with the fundamental mode alone; (2) the slight lowvelocity zone which is not required when inverting the fundamental mode alone in the central and north-eastern United States, is required when highermode data are added; (3) a rather strong increase of the S-velocity gradient is found near 360 km depth, both for the average data across the United States and the Pacific Ocean.

The analysis of 6‐component measurements of a random electromagnetic wave field in a magnetoplasma – I. The direct problem

Abstract: Summary This is the first of a series of papers, the general subject of which is how to interpret a set of simultaneous measurements of the three electric and three magnetic components of a random electromagnetic wave field in a magnetoplasma. The point at which the measurements are made is assumed to be stationary with respect to the plasma. In this first paper, the following problems are treated: how to define, within the framework of classical electrodynamics, a distribution function that characterizes the statistics of a linear random electromagnetic wave field in a lossless magnetoplasma; the direct problem of predicting the statistical properties of measurements of the six components of a field of this type, when the distribution function is known.

A seismic refraction investigation of crustal structure beneath the Lesser Antilles island arc

Abstract: Summary. A seismic refraction experiment along am across the Lesser Antilles island arc in the eastern Caribbean has shown that the crust beneath the arc contains two refractors with average seismic velocities of 6.2 and 6.9 km/s. The upper, 6.2 km/s refractor varies in depth and shows large fluctuations in its seismic velocity. The layer betweeen the refractors has an average thickness of about 10 km, and varies between 2 and 20 km. The total thickness of the crust of the arc is about 30 km beneath Martinique and about 35 km beneath St Vincent. The approximate depths of the Moho beneath the centres of Grenada and Tobago Troughs either side of the arc are 21 and 25 km respectively. Both these troughs, although of different origin, are underlain by crust of modified oceanic type.

The tidal dynamics of the Irish and Celtic Seas

Abstract: Summary. Current meter data collected over periods of more than 14 day from the Irish and Celtic Seas are harmonically analysed and presented in maps of tidal stream information. Making use of the analysed current data, and by constructing time series of frictional and inertial stresses which are also harmonically analysed, harmonic constituents of the surface tidal slopes at current meter stations are obtained. Using these with data collected from offshore tide gauges, and in conjunction with coastal tide data, cotidal maps are drawn with some confidence for M2, S2, O1 and K1, the M2 chart resolving the discrepancy which exists between the different charts of the Celtic Sea already produced. Cotidal maps for M3 and M4 are also presented. The mean over a tidal cycle of the energy flux for M2, S2 and O1 is also presented in the form of the total energy flux in these constituents which crosses different sectional lines. A flux of 44 × 106 kW is observed to enter the Celtic Sea from the Atlantic and this is compared with previous estimates. An energy budget is also performed for M2, including all the effects of astronomical forcing and Earth tides to enable comparison to be made between the true energy inflow and the estimated frictional dissipation. Finally, comparison is made between the mean of the instantaneous energy flux and the sum of the energy fluxes associated with the major harmonics.

Magnetic remanence acquisition in Finnish lake sediments

Abstract: Summary. Magnetic studies have been carried out on organic sediments from five Finnish lakes to determine the carrier(s) of the stable NRM and to find how the remanence is acquired. Single or pseudo-single domain ‘magnetite’ is thought to carry the NRM. It was found that drying and cooling sediment samples resulted in a loss of NRM which was attributed to the misalignment of small magnetic particles. Low-field experiments were carried out on sediment samples in different physical states and from the results of these investigations it was concluded that post-depositional processes are important in the acquisition of an NRM. Stabilization of magnetic grains is thought to be due to the growth of gels in the organic sediment rather than to dewatering.

The Nootka Fault Zone — a new plate boundary off western Canada

Abstract: Summary. Relative motion across a boundary between the main Juan de Fuca plate and its northern extension, the Explorer plate, had earlier been suggested from sea-floor magnetic anomaly analysis and from earthquakes recorded on the western Canada land seismic network. The location of the boundary, called the Nootka fault zone, and the motion across it have been examined through seismic reflection profiles, accurate location of earthquakes with an array of ocean bottom seismometers and through analysis of magnetic, gravity and bathymetric data. The fault zone extends from a ridge-fault—fault triple point at the northern end of the Juan de Fuca ridge to a fault—trench—trench triple junction at the margin off north-central Vancouver Island. The active portion of the fault zone is about 20 km wide, and has produced extensive disturbance in the 0.5 to 1 km of overlying sediments. Magnetic anomaly analysis suggests present left-lateral strike slip motion of about 3 cm/yr, with convergence at the margin being more rapid to the south than to the north of the fault zone. Because of rapidly changing spreading parameters on the Explorer and Juan de Fuca ridges over the past 5 Myr the Nootka fault zone has had a very complex history.

Seismic risk in southern Europe through to India examined using Gumbel's third distribution of extreme values

Abstract: Summary. A technique is described for the analysis of seismicity using Gumbel's third asymptotic distribution of extreme values. Seismicity of southern Europe through to India, nominally for the period 1900–74, is subdivided in a cellular manner, without recourse to tectonic discrimination between regions, and a covariance analysis on the three parameters of Gumbel's distribution is performed for each cell of seismicity. The results indicate that the upper bound to the magnitude of earthquake occurrence is often uncertain although it is discernible, while curvature of the earthquake occurrence distributions is usually established. Uncertainties in the forecasts of largest earthquakes, with a return period of 75 yr, are distinctly improved by taking into account the large and negative covariance which is measured between the curvature and upper bound to earthquake magnitude for the observed seismicity. These results are then used to map seismic risk for southern Europe through to India.

Asthenospheric counterflow: a kinematic model

Abstract: Summary. Present-day plate motions imply that about 240 km3 of oceanic lithosphere is created by sea-floor spreading and destroyed by subduction per year. A greater volume of asthenosphere will be dragged along by plate motions. Given the fluxes generated at plate boundaries, the horizontal direction and net rate of counterflow required to maintain mass balance is determined globally by a simple analytical model. Time-dependent calculations indicate that the motions are approximately valid in the hotspot reference frame over the past 5 Myr. Under most plates, the model return flow is opposite to the lithospheric motion in the hotspot frame. The counterflow dominates the resisting stresses to plate motion, so driving force models based on plate drag alone are not valid where the directions of plate motion and counterflow differ. The most marked departure of the two directions is under the North American plate. The model counterflow directions indicate that the sources of mantle hotspots are not located within the asthenosphere. Model flux balances demonstrate exchange of material between asthenospheric reservoirs located beneath different plates. Suggestions of southward asthenospheric motion under the North Atlantic, based on physical features around Iceland and strontium isotope geochemistry, are consistent with the direction of flow predicted by the model.

Magnitude corrections for attenuation in the upper mantle

Abstract: Summary. The mb:Ms relation for explosions at the Nevada Test Site (NTS) differs from those for explosions in other parts of the world. There is considerable evidence that this results mostly from high body-wave attenuation in the upper mantle beneath the western US. The authors have developed an empirical magnitude correction for body-wave attenuation and applied it to both source and receiver ends of the teleseismic body-wave path. The results imply that mb values are lower for NTS explosions than for Soviet explosions of comparable yield and seismic coupling. The authors have also developed and applied a source-depth correction to account for pP-P interference in the P-wave arrival. The body-wave magnitude resulting from these corrections is designated mo to distinguish it from other definitions of mb. Values of mQ determined for a world-wide set of large explosions show that a single mQ: yield relation is a fair fit to the data for the explosions with high seismic coupling. However, grouping the explosions under two mQ:yield relations gives a better fit to the data. All the studied explosions in salt or granite or below the water table fit a common Ms:yield relation. Explosions from North America, Eurasia and Africa have a common mQ: Ms relation.

Re-determination of the decade fluctuations in the rotation of the Earth in the period 1861–1978

Abstract: Summary The ‘decade’ fluctuations in the Earth's rate of rotation are derived from a comparison of the universal time-scale, based on the Earth's diurnal rotation, with: (a) the dynamical time-scale of the lunar ephemeris from 1861 to 1955 (using the collected times of occultations of stars), and (b) the international atomic time-scale from 1955 to 1978. It is concluded from these fluctuations that the torques which operated on the Earth's mantle varied in magnitude in a characteristic period of about 30 yr and reached a maximum of 1018Nm around 1900.

A comprehensive model current system for high-latitude magnetic activity - I. The steady state system

Abstract: Summary. Magnetic variations at high latitudes represent the combination of magnetic perturbations from several different current systems. The relative effects of each of the contributory current systems vary from event to event, resulting in changes in magnetic perturbation patterns in a given local time sector which are a function of several variables, e.g. level of magnetospheric activity, spectrum of precipitating particles responsible for ionospheric conductivity, configuration of interplanetary magnetic field, etc. In this paper we extend the work of Hughes & Rostoker in which the steady state perturbation pattern in each local time sector was established from ground-based meridian line magnetometer data. We shall define the various current systems which contribute to the high-latitude magnetic perturbation pattern and we shall construct the model composite current system which best explains the high-latitude observations in the winter months. We shall discuss changes in this pattern as a function of season and as a function of the configuration of the interplanetary magnetic field. Finally, we shall relate the geometrical configuration of the ionosphere—magnetosphere current flow to the convection of plasma in the magnetosphere.