Abstract: Summary. A Debye model using two cut-off frequencies corresponding to compressional and shear velocities is used to calculate mineral entropies. This model permits entropy and heat capacity in the Earth to be calculated from seismic profiles, and iteration yields temperature profiles along an isentrope. With an adiabatic temperature profile it is possible to obtain Griineisen’s parameter and thermal expansion as a function of depth. Only in the lower mantle is the calculated Griineisen’s parameter along an isentrope approximately proportional to volume.

Abstract: Summary An acceptably accurate approximation for the sampling distribution of the angle between two sample mean directions, conditional on the observed lengths of the vector resultants, is derived for samples drawn from Fisher populations sharing a common true mean direction. From this a test is given for the null hypothesis that two populations (with a common precision parameter) share a common true mean direction. This test is then compared with the unconditional test derived by Watson. The conditional test is then extended to an approximate test for the case where the two populations do not share a common precision parameter. The conditional test for populations with a common precision parameter is then extended to the case where it is desired to test simultaneously whether several samples could have been drawn from populations sharing a common true mean direction. The pooled, unbiased estimate for the inverse of the precision parameter is determined. From this a test for homogeneity of the precision parameter is derived for the case of several samples having unequal sample sizes.

Abstract: Summary The fold test suggested by McElhinny is shown to be inappropriate. More correctly one should determine whether the mean direction of a group of sites from one limb of a fold may be distinguished statistically from the mean direction of a group of sites from another limb. Details of the application of this test are given with examples. It is shown that the test indicates significance under far less severe folding than does the test suggested by McElhinny.

Abstract: Summary. A palaeomagnetic record of the geomagnetic secular variation during the last 10000 years has been obtained from 10 cores of sediment from Loch Lomond, Scotland, Lake Windermere, North England, and Llyn Geirionydd, North Wales. A time-scale is provided by 30 radiocarbon age determinations and pollen analyses on several of the cores. The main swings and much fine detail of both declination and inclination records repeat well between cores and between lakes, and the overall record is much more detailed than previous European records. The new record shows that neither declination nor inclination swings have been periodic over the past 10000 years, but that the main swings have become progressively shorter in duration during that time. Each swing is characterized by fine detail which enables use of the record as a secondary method of dating other European sediments. The motion of the geomagnetic vector has been predominantly clockwise throughout the time period spanned, but confirms a period of anticlockwise motion from 1100 to 600 bp first discovered by British archaeomagnetic investigations. The record agrees with British and Czechoslovakian archaeomagnetic records, but not with Japanese archaeomagnetic or North American lake sediment records. This suggests that the secular changes are controlled by local growing and decaying, drifting sources, rather than by wobbling of the main geomagnetic dipole.

Abstract: Summary. In view of recent work on seismic scattering by small-scale heterogeneities in the Earth, which has been based on single-scattering perturbation theory (that is, the Born approximation), we attempt to define the region within which this approximation may be regarded as reasonably accurate. Comparison with the exact solution for scattering by an embedded sphere shows that the inequalities obtained, governing the ranges of the parameters of the problem, are appropriate. However, application of these constraints on the parameters imply that, in almost all cases, application of the Born approximation to upper and lower mantle scattering is probably invalid.

Abstract: Summary. Recent results from the analysis of postglacial rebound data suggest that the viscosity of the Earth's mantle increases through the transition region. Models which fit both relative sea-level and free air gravity data have viscosities which increase from a value near 1022 poise in the upper mantle beneath the lithosphere to a value of about 1023 poise in the lower mantle. In this paper we analyse the effect of deglaciation upon the Earth's rotation and thereby show that the observed secular trend (polar wander) evident in the ILS—IPMS pole path, and measurements of the non-tidal acceleration of the length of day, are both consistent with the viscosity profile deduced from postglacial rebound. The two analyses are therefore mutually reinforcing.

Abstract: Summary. The Radon transform or slant stack is becoming a widely used technique for analysing high-quality reflection and refraction data. The transform normally used is applicable to data from a line source in a plane model, that is, one Cartesian coordinate. The theoretical basis for the Radon transform pair for one Cartesian coordinate has appeared in the seismological literature. For a point source in plane or spherical geometry, or a line source in cylindrical geometry only the Radon transform for the direct problem (computation of synthetic seismograms) has been published. To analyse data an approximate inverse transform has been used. In this research note, the exact forms of the generalized Radon transform pairs are completed for a point source in plane or spherical geometry, and for a line source in cylindrical geometry. The differences will be important if the waveforms are being interpreted, and are most significant for near-vertical reflections—the type of data most commonly slant stacked.

Abstract: Summary. Remanent acquisition curves, remanent hysteresis curves and alternating field demagnetization curves were determined for a number of artificial rock specimens containing well-defined grain-size fractions between 5 and 250 μm of natural magnetite, titanomagnetite and hematite. From these curves, the remanent acquisition coercive force H'cr, the remanent coercive force Hcr and the median destructive field of IRM H½I were determined. Theoretically these parameters should be the same for an assembly of non-interacting, homogeneously distributed, randomly oriented single-domain grains. For a given hematite specimen H'cr, Hcr and H½I have about the same value in spite of the strong grain-size dependence of these parameters. For each specimen of magnetite and titanomagnetite the value of H'cr is larger than Hcr which again is larger than H½I. However, the ratios H'cr/Hcr and H½I/Hcr appear to have a (different) constant value. An interesting relationship which appears to hold for dispersed magnetite, titanomagnetite or hematite grains between 5 and 250 μm, independently of grain-size, quantity and packing density of the magnetic material, is:

Abstract: Data from Japanese local seismograph networks suggest that the stresses in double seismic zones are in-plate compression for the upper zone and in-plate tension for the lower zone; the stresses do not necessarily appear to be down-dip. It may therefore be possible to identify other double seismic zones on the basis of data which indicate that events with differing orientations of in-plate stresses occur in a given segment of slab. A global survey of published focal mechanisms for intermediate depth earthquakes suggests that the stress in the slab is controlled, at least in part, by the age of the slab and the rate of convergence. Old and slow slabs are under in-plate tensile stresses and the amount of in-plate compression in the slab increases with increasing convergence rate or decreasing slab age. Young and fast slabs are an exception to this trend; all such slabs are down-dip tensile. Since these slabs all subduct under continents, they may be bent by continental loading. Double seismic zones are not a feature common to all subduction zones and are only observed in slabs which are not dominated by tensile or compressive stresses. Unbending of the lithosphere and upper mantle phase changes are unlikely to be the causes of the major features of double zones, although they may contribute to producing some of their characteristics. Sagging or thermal effects, possibly aided by asthenospheric relative motion, may produce the local deviatoric stresses that cause double zones.

Abstract: The effects of the oceans, which are subject to a resonance due to a free rotational eigenmode of an elliptical, rotating earth with a fluid outer core having an eigenfrequency of (1 + 1/460) cycle/day, on the body tide and nutational response of the earth to the diurnal luni-tidal force are computed. The response of an elastic, rotating, elliptical, oceanless earth with a fluid outer core to a given load distribution on its surface is first considered, and the tidal sea level height for equilibrium and nonequilibrium oceans is examined. Computations of the effects of equilibrium and nonequilibrium oceans on the nutational and deformational responses of the earth are then presented which show small but significant perturbations to the retrograde 18.6-year and prograde six-month nutations, and more important effects on the earth body tide, which is also resonant at the free core notation eigenfrequency.

Abstract: Summary. A new earthquake catalogue for Greece has been formed to cover the instrumental period 1901–78, in particular 605 earthquakes for the period 1917–63 inclusive are relocated using first arrival data from the International Seismological Summary. These relocations incorporate macro-seismically and other well-controlled master events into an ensuing joint epicentre determination technique. The largest annual average shift is 165 km for 78 earthquakes during the decade after 1917, decreasing to 17 km for the later years 1957–63. Magnitudes are redetermined mainly using readings from the Swedish network and Uppsala since as early as 1908. Catalogue completeness exists for magnitudes around 5.5 for at least the last 60 years, but magnitudes of about 4.7 are completely reported only during the most recent 15 years. The new epicentral positions lead to a better delineation of the seismic zones than has previously been achieved. The majority of shallow earthquakes are contained in a belt parallel to the Hellenic Arc which extends north into Albania, in the south-east the epicentral locations give a more diffuse extension of this zone into the west coast of Turkey. Depths of intermediate earthquakes along the Hellenic Arc tend to relocate to shallower depths, in the south-west part of Crete no earthquake focus deeper than 100 km is found, although in the south-eastern section of the arc a tendency to increased depths is observed. A second zone starts at Leukas Island in the west and extends through central Greece to near Volos on the east coast, where it divides into two branches, which are less well defined, but which eventually join the seismicity of western Turkey. A third zone follows the Saronikos and Corinth gulfs.

Abstract: Summary. A normal mode superposition approach is used to synthesize complete seismic codas for flat layered earth models and the P-SV phases. Only modes which have real eigenwavenumbers are used so that the search for eigenvalues in the complex wavenumber plane is confined to the real axis. In order to synthesize early P-wave arrivals by summing a number of‘trapped’modes, an anomalously high velocity cap layer is added to the bottom of the structure so that most of the seismic energy is contained in the upper layers as high-order surface waves. Causality arguments are used to define time windows for which the resulting synthetic seismograms are close approximations to the exact solutions without the cap layer. The traditional Thomson—Haskell matrix approach to computing the normal modes is reformulated so that numerical problems encountered at high frequencies are avoided and numerical results of the locked mode approximation are given.

Abstract: The earth is believed to possess a free nutational mode due to its rotating, elliptical, fluid core, with an eigenfrequency of approximately (1 + 1/460) cycle per sidereal day as seen from the sidereally rotating earth. This free 'core nutation' has not yet been undisputably observed. Furthermore, there has been considerable doubt that any known mechanism could excite this mode to an observable level. It is shown here that diurnal atmospheric and oceanic loading of the earth's surface provides an efficient excitation mechanism which depends critically on the physical damping of the mode. Possible effects of the mode on geodetic measurements are discussed. The effects of 'wobble' and 'nutation' on astrometric observations are also considered.

Abstract: Summary The Thessaloniki (northern Greece) earthquake sequence appears to have occurred along faults forming a graben structure. This graben, situated in the border region between the Serbomacedonian massif and the Vardar zone, is bounded to the south-west by clearly exposed north-west striking north-east dipping normal faults. Relative hypocentre determinations, fault-plane solutions, surface faulting and the aftershock distribution suggest that some of these faults have been reactivated during the 1978 earthquakes. The source parameters of the mainshock (mb = 6.1, Ms = 6.4) were determined by computing body-wave synthetic seismograms in the time domain and comparing them with the observed. This modelling constrained the orientation of faulting determined by the P-wave fault-plane solution. It also constrained the source depth to 6 |Mp 2 km. Similar depths were calculated by a relative relocation method for the other three large events of the sequence. The dislocation time-function required for the far-field had a total duration of 9 |Mp 1.5 s, the body-wave moment was 5.2 |Mp 1.8 times 1025 dyne cm. For a fault length of 35 km and a fault width of 17 km (both estimated from the aftershock distribution) the static stress drop was found to be 4 bar. An accelerograph record of the mainshock shows two distinct events, 3—4 s apart. These two events were unresolved by the long-period data in the far-field but the unusually long duration of the mainshock time-function suggests an overall slow energy release which probably occurred as a sequence of events close in space and time. Seismic energy released in this fashion can account for waveform complexities observed at some stations.

Abstract: Summary An analytic one-dimensional magnetotelluric inversion scheme is described. The main characteristics of this new scheme are its simplicity and its minimal requirements in computer time and storage space. The basic idea of the scheme is that for a given period T only the structure above a certain depth matters. The scheme therefore starts with the shortest periods of the available data set and tries to explain the observed response, specifically the apparent resistivity pa(T) and the phase o(T), in terms of a two-layer structure. Shifting successively to longer periods, discrete new layers are introduced at progressively greater depth. Some stabilizing features simultaneously keep the inversion process from diverging and hold the necessary number of layers to a minimum. The properties of the scheme are analysed by studying its performances when working on synthetic and real field data. The scheme can handle fairly scattered data and is very stable. It has, in fact, never been observed to diverge. Examples are given of how the scheme might be handled in practice, especially regarding the use of the causal dispersion relations with which it is possible to ensure that the model proposed by the inversion scheme will return a response function in agreement with the initial data.

Abstract: Summary Ocean bottom magnetotelluric responses from three ages on the Pacific plate have been inverted to reveal the presence of a conductive zone whose depth is a function of lithospheric age. The conductivity achieves its maximum value (˜ 0.1 Sm-1) at depths near 70, 120 and 180 km for lithospheric ages of 1, 30 and 72 Myr respectively. The existence of this conductive structure has firmly been established by constructing a number of different acceptable models, and also by obtaining unique averages over depth of the conductivity. An explanation in terms of partial melting in the upper mantle has produced lower bounds for the volume fraction of melt which must exist in order to produce the high conductivity. Beneath JDF (t= 1 Myr) the lower bound lies in the range 0.04–0.12; beneath CAL (t= 30 Myr) it lies in the range 0.02–0.10; and beneath NCP (t= 72 Myr) it lies in the range 0.008–0.02. The distributions of partial melt show that the major portion of melt beneath JDF lies between 40 and 90 km and that melting has ceased by depths of about 170 km at all three sites. Geotherms have been computed and they are qualitatively similar to those predicted from a convective system where the temperature increases rapidly through a boundary layer to achieve a maximum value, and then (possibly) decreases with depth. The depth at which the maximum temperature is reached is about 125 km for CAL and about 175 km for NCP, and the maximum temperatures are estimated to be 1400–1500° C. Lastly, as an independent check upon the partial melt explanation, the location and magnitude of the seismic low-velocity zone has been correlated positively with the computed distribution of partial melt at all three ocean bottom sites.

Abstract: Summary Linear inverse theory is used to deconvolve a data set when the blurring function or source wavelet is (approximately) known. Rather than attempting to find one of infinitely many models which fits the data this paper uses the methods of Backus and Gilbert to generate localized averages of the model which are unique except for a statistical uncertainty caused by errors in the data. The averages, their statistical error and the associated averaging window completely codify our knowledge about the model. Averages with lower standard deviations can be had by sacrificing resolution and the investigator is free to choose those results which are most meaningful. Moreover, this method is optimum in the sense that no other averaging window can be constructed which has greater resolving power and yet produces averages with the same statistical accuracy. Our deconvolution method in the time domain is shown to be very similar to finding the inverse filter of the source wavelet, and indeed the averaging window is simply the convolution of these two functions. However, by investigating the trade-off between resolution and accuracy we have shown that the data errors can be much more important than the parameters of the Wiener optimum inverse filter such as the length of that filter and the desired location of the output spike. In the frequency domain the equations for trading off accuracy and resolution have been developed and the computations are seen particularly to be simple because no matrix inversion is required. Sufficient examples will be presented to show the importance of incorporating the observational errors into the deconvolution procedure. Additionally, we have shown how to reduce the sidelobes of the averaging windows by shaping them into Gaussian functions of a predetermined width, have looked at the effects of using a zero area source function characteristic of seismic problems, and have attempted a deconvolution when the wavelet was only approximately known. The frequency domain deconvolution filter derived here is also compared quantitatively with those derived intuitively by Helmberger and Wiggins and Deregoiwski. Lastly, we show how information in the averages and averaging windows can be used to construct a parametric model, composed of a series of delta functions, which fits the data. Such a model is of importance in seismological and spectroscopic studies.

Abstract: Summary. The South-west Indian Ridge, the contact between the African and Antarctic plates, lies between the Bouvet Triple Junction in the South Atlantic and the Indian Ocean Triple Junction about 2100 km east of Madagascar. From the vicinity of Prince Edward Island at 40° E it trends north-easterly and it is segmented by a suite of deep north—south gashes terminating on the north-east with two spectacular meridional fracture zones, the ‘Atlantis II’ and the ‘Melville’, at 57° 30′E and 60° 30′E respectively. From there northeast to the Indian Ocean Triple Junction at 25° 30′S, 70° 00′E the ridge trends N75° E; it is characterized by a triangle of rough topography with the triple junction at the eastern apex. From all available data an instantaneous pole of relative motion for Africa/Antarctica was computed; it lies at 8.4° N, 42.4° W, with a rate of 0.15° Myr−1. Since the marked change in the direction and rate of spreading in the Madagascar, Crozet and Central Indian Basins that occurred in the Eocene (44 Ma, Anomaly 19), the poles of relative motion for the African, Indian and Antarctic plates have changed very little. We fixed the Africa/Antarctica and Africa/India poles and computed that for India/Antarctica. We justified this pole by comparisons of predicted isochrons with observed magnetic lineations and determined the tectonic history of the triple junction. Since the Eocene (44 Ma, Anomaly 19), this junction has moved as rapidly east-wards with respect to Africa as Antarctica has moved south. The resultant geometry and slow spreading account for the triangle of rough topography produced by the South-west Indian Ridge east of the Melville Fracture Zone. The triple junction evolved as a stable ridge-ridge-ridge type with the South-east Indian Ridge remaining approximately constant in length. It was not resolved whether this constancy in length is maintained by frequent ridge jumps or by oblique spreading on the South-west and Central Indian Ridges near the triple junction.

Abstract: Summary. A seismic study of the Lesser Antilles arc has been carried out, first for the period 1950–1978, for which we can use local seismic networks to draw maps of instrumental seismicity, then for the period 1530–1950, for which we have catalogues of felt earthquakes. The striking feature of the spatial distribution of foci is the cluster of epicentres in the northern half of the arc; all large earthquakes (M > 7.5) are located north of 14° latitude. Seismicity cross-sections through the arc show a variable dipping subduction zone along the arc; the deep seismic zone is steeper in the centre of the arc than on the extremity. The time-space diagram for historical seismicity, and the evidence of a seismic gap at the east of Guadeloupe lead us to consider the northern half arc as a likely site for a large earthquake in the near future. The seismic slip rate calculated from all major earthquakes since 1530 is of much greater value than that obtained from recent plate tectonic models, suggesting that the recurrence rate of earthquakes is more than many hundreds of years with a possible aseismic creep.

Abstract: Summary We consider spontaneous cracks spreading out areally over a fault plane in an infinite medium as a realistic earthquake source model. The boundary integral equation technique described in Das is used to determine the displacements and stresses everywhere on the crack plane. For faults spreading out symmetrically in all four quadrants over infinite planes of constant yield strength, we find that the terminal rupture velocity VIIICR in the direction of purely anti-plane rupture is given by VIIICR < β. In the purely in-plane direction of rupture propagation, we find the terminal rupture velocity VIICR to be given by VIICR < α or VIICR < 0.5α, depending on the yield strength. We also find that crack propagation in the purely antiplane direction does not influence crack propagation in the purely in-plane direction and vice versa. For infinitely long shear cracks of finite width, the slip at a point is found to grow more slowly after the arrival of a shear wave diffracted from the nearer crack edge. The slip virtually ceases after the shear arrival from the further crack edge. This implies that for such faults the slip is controlled by the fault width. For a rectangular fault in an infinite medium, our final dynamic solution is in closer agreement with the static elliptical crack solution of Eshelby than with the static rectangular ‘dislocation model’ solution of Chinnery. We find the relationship between the average slip ū and average dynamic stress drop τe on such a rectangular fault of half-width W to be τe= Cμ(ū/W), where c ˜ 0.7. The slip at an interior point is again controlled by the fault width for faults that are much longer than wide so that the slip may stop in the region where the fault initiated before the completion of the rupture process. This implies that two rectangular faults of varying lengths but of the same width have the same slip for the same average stress drop, a fact clearly contradicted by observations that fault slip increases with earthquake size for great earthquakes occurring along the same fault. This contradiction is resolved if stress drops are also bigger for bigger earthquakes.

Abstract: Summary. The geomagnetic field at the Earth's surface may be represented in terms of spherical harmonic coefficients. The determination of the coefficients from observed values of the field, and the evaluation of a field value from a set of coefficients, may be regarded as linear inverse problems of discrete and continuous types respectively. Well-known procedures of inverse theory are found to provide new insight into some of the hazards of conventional methods of analysis, as well as suggesting some novel methods of approach. The methods are applied to a dataset of measurements of all three components of the field from 80 magnetic observations. Determining coefficients from observations is usually effected by least-squares analysis after truncating the harmonic series at some degree and order. Inverse methods all seem to have their counterpart in least-squares theory, but they do not help decide the best truncation level. An alternative approach, Parker's method of linear inference, has been developed to apply to spherical harmonic series. The results are similar to least squares but the error bounds are more pessimistic. In evaluating the field components from a finite set of coefficients it is only possible to find a weighted average of the field value over a certain area of a spherical surface. The averaging function appropriate to the usual spherical harmonic sum formula has oscillations and is not a well-localized average. Alternative formulae are suggested which give a better, localized average. The methods are applied to fields downward continued to the core-mantle boundary. Most striking results are obtained when the covariance matrix for the spherical harmonic coefficients is used to damp the field determination. This procedure retains the necessary information about the original distribution of the data and it is found to eliminate some of the features of magnetic charts in the oceans where data coverage is poor, showing that these anomalies are not real.

Abstract: Summary. A method of simultaneous reduction is presented for determining strain rates from multiple triangulation surveys where common triangulation stations have been used, but the angles of the old survey have not necessarily been reobserved. This method is applied to triangulation in the northern South Island, at the southern end of the Tonga-Kermadec-Hikurangi subduction zone. From a profile of shear strain across the Indian-Pacific plate boundary, the displacement of the Indian plate relative to the Pacific is calculated to be 54 * 9 mm yr-' at an azimuth of 84" f lo", in remarkable agreement with the motion predicted by global plate tectonic models. Most of this motion occurs within a 15Okm wide zone bounded on the east by the Hikurangi Trough. Within this zone the motion is partitioned: near the Hikurangi Trough no slip is occurring at the upper surface of the subducting Pacific plate (the subduction thrust) and motion is predominantly thrusting normal to the trough axis: to the west is a region of predominantly dextral strike slip faulting. This pattern is consistent with Fitch's model of oblique subduction. To the south of the profile, a change is observed in the azimuth of the faulting along a line which marks the southern extent of the subduction slab, indicating the end of the partitioned motion.

Abstract: Summary Quarry blasts recorded along three lines on the central Iberian Meseta are used in an attempt to interpret the crustal structure The results of the interpretation of the data, together with published surface wave and earthquake data, suggest a layered structure of the crust having the following features: the basement, in some areas covered by up to 4 km of sediments, has a P-velocity of 61 km s−1; a low-velocity layer, between 7 and 11 km depth, seems to exist on the basis of both P and S interpretation of seismic data; a thick middle crust of 12 km has a P-velocity of 64 km s−1 and overlies a lower crust with a mean P-velocity of 69 km s−1 and a possible slight negative gradient; the mean vp/vs ratio for the crust is about 175; the Moho is reached at about 31 km depth and consists of a transition zone at least 15 km thick The P-velocity of the upper mantle is close to 81 km s−1 and the S-velocity about 45 km s−1, which gives a vp/vs ratio of 18 for the uppermost mantle A tentative petrological interpretation of the velocities and composition of the layers is given

Abstract: Summary Seismic refraction measurements were carried out using ocean bottom seismometers over foundered continental crust in Orphan Basin and Flemish Pass, on the continental margin north-east of Newfoundland. Tau—p travel time inversion, synthetic seismogram analysis and conventional layered model calculations were applied to the data. The results show that these outer regions of the margin are underlain by thinned continental crust, with a total depth to the M discontinuity of about 22 km. There are two main crustal layers with P-velocities of about 6.1 and 7.0 km s-1 which appear to be homogenous, and separated by sharp interfaces. These are overlain by a layer in which P-velocities are about 5.5 km s−1; this is interpreted to be Precambrian or Palaeozoic basement. Mesozoic and Cenozoic sediments cover the basement rocks, and are over 4 km thick. The results imply that crustal thinning to about 50 per cent of the original crustal thickness occurred. The gravity anomaly data show that the thin crust has a maximum horizontal extent of about 450 km, from the ocean—continent boundary near Orphan Knoll landward to the outer continental shelf. The subsidence history and thermal evolution of the region was computed, assuming that the observed thinning is produced by horizontal extension of the lithosphere. It is suggested that extension can only satisfy the observed crustal structure and elevation of the margin during the rift phase if more extension took place in the lower lithosphere than in the upper lithosphere. The computed subsidence is compared to the observed subsidence and the total amounts of subsidence are similar. However, the shape of the observed subsidence curves measured in deep exploratory wells differs significantly from the predicted subsidence, assuming cooling began when final continental breakup occurred in the Late Cretaceous. The temperature distribution within the lithosphere due to extension may be related to the flexural rigidity of the plate as a function of both time and position across the margin. Therefore, it is suggested that the response of the lithosphere to sediment loading, and the large amplitude of the gravity ‘shelf-edge’ anomalies are directly related to the thermal history of the region.

Abstract: Summary In one-dimensional magnetotelluric modelling the standard deviation e is often used as an indicator of the degree of fit between the field measurements and the calculated model response The topography of e in the space of the model parameters has been studied and found to be rather simpler than expected The absolute minimum seems to be quite isolated from other minima In general no such other local minima were found To find the minimum it was not necessary, therefore, to look for a computing routine capable of jumping out of localized minima But the search routine had to be capable of moving along a valley with an exceedingly level floor, as the minimum is often at large distances from the initial model In this respect it was important to work with logarithmic coordinates Since the absolute minimum emin can be found, it often becomes possible to split emin into three separate components: (1) the scatter of the original field data, (2) the departure of these data from one-dimensionality and (3) a component that will occur if one attempts to model the data with a structure comprising too few layers Knowing this last contribution it becomes easy to decide which is the smallest number of layers necessary to model a given data Sci

Abstract: Summary. Polarization anomalies in seismic shear wavetrains, diagnostic of propagation through anisotropic media, have now been observed in dilatancy zones in seismic regions. Stress-induced dilatancy will open cracks with preferred orientations, which will be effectively anisotropic to short-period seismic waves. The polarization anomalies are due to the shear waves splitting, in propagation through anisotropic media, into components with different polarizations and different velocities. This writes characteristic signatures into the shear wavetrains. The paper examines ways in which the differential shear-wave anisotropy (the delay between the split shear-waves) varies with direction by plotting stereograms of the relative delays, and their polarizations, for possible dilatancy symmetry-systems. It seems likely, that if sufficient observations of these anomalies can be obtained at each stage of the dilatancy episode, it will be possible to estimate the symmetry directions of the dilatancy and the geometry of the stress-induced crack-system, as well as monitoring the progress of the dilatancy episode.

Abstract: Summary. Earthquakes under coastal northern Chile, from 18° S to 33° S occur at two very different depths. The shallower interplate events have the typical thrust mechanisms associated with subduction of the Nazca Plate under South America. The deeper (close to 70 km) events have normal fault mechanisms with a tension axis approximately parallel to the dip of the downgoing slab. We study in detail a. couple of Ms∼ 7.5 events that took place about 60 km north of Valparaiso, Chile. The 1971 July 9 event was a shallow thrust interplate event at 40 km depth, while the 1965 March 28 earthquake had a normal fault mechanism and a depth of 72 km. We carry out a detailed analysis of long period P-and surface waves observed in the WWSSN network of stations in order to improve fault plane solutions and depth determinations. For the 1971 event we find a seismic moment of 5.6 ∼ 1027 dyne cm from both P-and surface waves. The source area for this event is relatively small compared to that of other underthrust events with similar moments. Assuming a circular fault shape and a rigidity of 7 ∼ 1011 dyne cm−2 we find a stress drop of 38 bar. The 1965 event is more complex; seismic moments of 1.8 and 1.0 ∼ 1027 dyne cm are found from surface and body waves respectively. The stress drop estimated from P-waves and a circular fault model is close to 91 bar. The existence of earthquakes at two different depths near the Chilean coast may be interpreted in terms of a double layered Benioff zone, as has been found in Honshu, Kuriles or central Peru. Unfortunately the hypo-central determinations of events in this region are not precise enough to separate the two layers. An alternative method to demonstrate the existence of a double Benioff zone is to look for compressional events associated with the upper layer of the down-going slab. A search among available fault plane solutions is ambiguous; three shallow coastal events are found in Stauder's catalogue with horizontal pressure axes normal to the trench, but they might be intra-continental plate events.

Abstract: The long period luni-solar tidal potential is known to cause periodic changes in the earth's rotation rate. It is found that the effect of a dissipationless fluid outer core is to reduce the amplitudes of these tidal perturbations by about 11 percent. When the fluid core effect is added to Agnew and Farrell's (1978) estimate of the effect of an equilibrium ocean, the result is in accord with observation. The effects of dissipative processes within the fluid core are also examined. Out-of-phase perturbations are found which could be as large as about 10 ms at 18.6 yr. It is concluded, however, that the poorly understood decade fluctuations in the earth's rotation rate will prohibit observation of this effect.