Showing papers in "Geophysical Journal International in 1990"

Current plate motions

Abstract: A global plate motion model, named NUVEL-1, which describes current plate motions between 12 rigid plates is described, with special attention given to the method, data, and assumptions used Tectonic implications of the patterns that emerged from the results are discussed It is shown that wide plate boundary zones can form not only within the continental lithosphere but also within the oceanic lithosphere; eg, between the Indian and Australian plates and between the North American and South American plates Results of the model also suggest small but significant diffuse deformation of the oceanic lithosphere, which may be confined to small awkwardly shaped salients of major plates

Classification of the reversal test in palaeomagnetism

Abstract: SUMMARY It is standard practice that a positive reversal test is claimed on the basis of inability to reject the hypothesis that two distributions share a common mean direction, and thus the claim of a positive reversal test is in fact often based on a lack of information. This is unsatisfactory. Therefore it is suggested that positive reversal tests should be classified according to the amount of information that was available for the test. This amount of information is readily indicated by the critical angle (e.g., at the 95 per cent confidence level) between the two sample mean directions at which the hypothesis of common mean direction for the distributions would be rejected. It is recommended that 5", 10" and 20" be used as the breakpoints in the classification.

Inversion of field data in fault tectonics to obtain the regional stress—III. A new rapid direct inversion method by analytical means

Abstract: SUMMARY A new method for determining the reduced stress tensor with four degrees of freedom (the orientations of the three principal stress axes as well as the ratio of principal stress differences) using fault slip data (or focal mechanisms of earthquakes) is presented. From a computational point of view, the inversion of fault slip data is made in a direct way by purely analytical means; as a result, the determination process is extremely fast and adaptable on small microcomputers. From a physical point of view, the method aims at simultaneously (i) minimizing the angles between theoretical shear stress and actual slip vector and (ii) having relative magnitudes of shear stress large enough to induce slip despite rock cohesion and friction. Examples of application to actual fault slip data sets with good or poor variety of fault slip orientations are shown. The double significance of the basic criterion adopted results in a more realistic solution of the inverse problem than the single minimization of the shear-stria angle.

A new fold test for palaeomagnetic studies

Abstract: SUMMARY Previous techniques for judging the significance of a palaeomagnetic fold test are either invalid or insufficiently flexible. It is shown that under appropriate circumstances an isolated-observation test may be added to the range of statistical tests used to judge a fold test. A powerful new test is developed based on a test statistic that is sensitive to correlation between the distribution of site-mean directions about the overall mean direction and the tectonic corrections. This test is sufficiently flexible that it should cover most circumstances.

Seismicity and associated strain of central Greece between 1890 and 1988

Abstract: SUMMARY We examined the seismicity of central Greece between 1890 and 1988, using macroseismic and instrumental data, to ask two questions: (1) does the seismicity of this period reveal all the major tectonic structures that are known to be active?; and (2) what are the likely strains associated with the seismicity over this period? Many known active structures have been effectively aseismic for the last hundred years, and even the inclusion of all known large events earlier than 1890 reveals no activity associated with the NE coast of Evia, Gulf of Argos, or graben NE of Mt Parnassos. It is clear that even 100 years’ data are inadequate for either a reasonable assessment of seismic risk or for a confident estimation of maximum magnitude. However, we are aware of no earthquakes in central Greece during the last 200 yr that were larger than Ms 7.0. It is probable that the maximum magnitude is restricted by the maximum length of fault segments, which appears to be around 15-20 km. The earthquakes of Ms≥ 5.8 during 1890-1988 can account for a N-S displacement of around 45-70 cm (with maximum and minimum estimates a factor of two greater and smaller than this) across part of a 1890-1900 triangulation network in central Greece that was resurveyed in 1988. The contribution of smaller events may increase this displacement by about 50 per cent. This cumulative seismic displacement is similar to that estimated from the geodetic work (about 100 cm), but a detailed comparison of the two sets of observations will be reported elsewhere. A re-evaluation of all the important earthquakes of 1890–1988 in central Greece is presented in the Appendix, which summarizes information of use to both earth scientists and engineers.

The use of FFT techniques in physical geodesy

Abstract: SUMMARY The fast Fourier transform (FFT) technique is a very powerful tool for the efficient evaluation of gravity field convolution integrals It can handle heterogeneous and noisy data, and thus presents a very attractive alternative to the classical, time consuming approaches, provided gridded data are available This paper reviews the mathematics of the FFT methods as well as their practical problems, and presents examples from physical geodesy where the application of these methods is especially advantageous The spectral evaluation of Stokes’, Vening Meinesz’ and Molodensky’s integrals, least-squares collocation in the frequency domain, integrals for terrain reductions and for airborne gravity gradiometry , and the computation of covariance and power spectral density functions are treated in detail Numerical examples illustrate the efficiency and accuracy of the FFT methods Key words: FFT, physical geodesy, spectral methods 1 INTRODUCTION Physical geodesy is the branch of geodesy which uses measured gradients of the anomalou6gravity potential T to determine a unique and coherent representation of the terrestrial gravity field at the Earth’s surface and in outer space The anomalous potential T is the difference between the actual gravity potential of the Earth and the reference potential of an ellipsoid with the same mass, flattening, and angular rotation rate as the Earth An approximation of T is needed to model geodetic measurements, to predict perturbations of satellite orbits, to determine global ocean circulation patterns, to assist global geophysics, and to support oil and mineral exploration In recent years, the amount of data available for the solution of this problem has increased dramatically, both in quantity and in type This has made the data processing problems more severe and has created a demand for efficient numerical solutions Since much of the data is available in gridded form, the use of fast spectral techniques was clearly appropriate Progress in the application of these methods to geodetic problems has been rapid during the last three years and it is almost certain that, because of their efficiency and accuracy, they will become standard procedures for a number of applications However, it has also become clear that geodetic and, more generally, geophysical data often present specific problems not usually encountered in typical electrical engineering applications The problems are with the heterogeneity of the data, the complicated surface on which they are given, the uneven spatial distribution, and the non-uniformity of the data noise This paper will discuss the use of

The 1960 Chile earthquake: inversion for slip distribution from surface deformation

Abstract: SUMMARY A total of 166 observations of sea-level change, 130 measurements of elevation difference, and 16 determinations of horizontal strain provide an excellent view of the (quasi-)static source process of the great 1960 Chilean earthquake. These surface deformation data were employed in classical uniform slip fault models as well as more recently developed models that allow spatial variability of slip. The best uniform slip planar (USP) model is 850km long, 130km wide, and dips 20°. Seventeen metres of fault displacement contributed to a USP moment of 9.4 times 1022 N m. The variable slip planar (VSP) model concentrates slip on a 900 km long, 150 km wide band parallel to the coast. Several peaks of slip with dimensions of 50–100 km appear in this band and are thought to represent major subduction zone asperities. Important fractures of the oceanic lithosphere bound the 1960 rupture and are offered as a potential source of fault segmentation within the Chilean subduction zone. The VSP moment for 1960 earthquake totals 9.5 times 1022 N m, about one fifth of the value estimated for the foreshock-mainshock sequence from seismic methods. Except for areas out to sea, geodetic resolution on the fault is fairly uniform. Thus, it is unlikely that slip missed by the network could increase the VSP moment much beyond 1.8 times 1023 N m. Several patches of moment, isolated from the main body at 80–110 km depth, are found down dip in the VSP model and are presumably indicative of aseismic slip. One patch at the northern end of the rupture is probably associated with the initiation phase of the mainshock, although the time sequence of the relationship is unknown. Tide gauge records suggest that another patch between 40° and 43° S, responsible for the observed strain and uplifts inland at those latitudes, is not of coseismic origin, but derives from in-place, post-seismic creep over several years. Apparently, great 1960-type events are not typical members of the ∼ 128 yr earthquake cycle in south-central Chile. The Nazca-South America boundary here is characterized by a variable rupture mode in which major asperities are completely broken by great earthquakes only once in four or five earthquake cycles. The more frequent large earthquakes, that geographically overlap the great events, fill in between the locked zones.

Earthquake mechanisms in the Hellenic Trench near Crete

Abstract: SUMMARY In the Hellenic Trench south of Crete convergence between the southern Aegean Sea and Africa occurs at a rate of at least 60 mm yr-1. Previously published first motion fault plane solutions show a variety of different fault orientations and types, but are not well constrained. Furthermore, the lack of reliable focal depths for these earthquakes has obscured any simple pattern of deformation that might exist. Nonetheless, the mechanisms of these earthquakes have strongly influenced views of the tectonics in the Hellenic Trench. We have improved estimates of the fault parameters and focal depths for 14 of these earthquakes, using long-period P- and SH-waveforms. The earthquake mechanisms fall into four groups: (a) normal faults with a N-S strike in the over-riding material above the subduction zone; (b) low-angle thrusts with an E-W strike at a depth of about 40 km; (c) high-angle reverse faults with the same strike but shallower focal depths than (b); (d) events within the suducting lithosphere with approximately E-W P axes. The thrusting in groups (b) and (c) is probably the mechanism by which the sediments of the Mediterranean Sea underplate and uplift Crete. These events have slip vectors in the direction 025 ± 12° which represents the convergence direction between Crete and Africa along the SW-facing boundary of the Hellenic Trench. One of the events in group (d) occurred beneath the Mediterranean Ridge and involved high-angle reverse faulting with a WNW-ESE P axis: almost perpendicular to the direction of shortening deduced from folds at the surface. The Mediterranean Sea floor in this region appears to be in a state of E-W compression, for reasons that are not clear.

Long-wavelength S-wave velocity structure throughout the mantle

Abstract: SUMMARY Long-wavelength S-wave velocity anomalies are obtained for the entire mantle by inverting 6OOO long-period (40-100s) SH body waves and lo00 long-period (100-500 s) Love waves. In the shallow mantle, the patterns of fast and slow velocity anomalies correlate well with age and tectonics. In the depth range of 400-1000 km, we find two antipodal fast velocity anomalies, one in the western Pacific and the other in South America. They form a fairly strong I = 2 anomaly, Moreover the 1 = 2 anomaly is the largest spherical harmonic component throughout the entire mantle except for in the layer of 1000-1300 km depth. A fast-velocity anomaly near the Caribbean, previously reported by several other investigators, is found between the depths of 1000 and 1300km. With diminishing amplitudes, it can be traced to Canada and the Aleutians and may be related to old subducted slabs. In the same depth interval, similar size anomalies can be found in the middle of the Pacific and in the Indian Ocean. These could be checked by more detailed studies to judge the reliability of this study. Heterogeneity power has a peak at the surface and a secondary peak at the bottom of the mantle (D”), consistent with the existence of chemical and/or thermal boundary layers. At the base of the mantle we also find a fast-velocity anomaly which surrounds the Pacific Ocean. This feature has been found by other researchers previously, but its interpretation is difficult due to the trade-off between velocity anomaly and (core-mantle) boundary deformation.

Holocene glacial rebound and sea-level change in NW Europe

Abstract: SUMMARY Observations of Late Pleistocene and Holocene sea-level change relative to the crust exhibit very considerable variations across NW Europe in consequence of the response of the Earth’s crust to the deglaciation of Fennoscandia and of the water added to the oceans from the melting of all Late Pleistocene ice sheets. Inversion of sea-level observations from a site near the centre of the Fennoscandian ice sheet and from three sites located beyond the margin of the ice sheet at the time of maximum glaciation yield a range of plausible models for the Earth’s response and for the ice models. Further constraints on this range of models is placed by a comparison of observed sea-levels with predicted values at other sites near the former ice sheet margins. The resulting mantle parameters are: upper mantle viscosity (3-5) X 10’’ Pa s; lower mantle viscosity (2-7) X loz1 Pa s; lithospheric thickness 100150 km. These values represent effective parameters that describe the response of the Earth to surface loading of short to intermediate wavelengths on a time-scale of 104yr. The lower mantle viscosity is poorly constrained but the marked increase from upper to lower mantle is a characteristic of all plausible solutions. The inversion places a constraint on the total volume of ice in the Fennoscandian ice sheet such that the equivalent sea-level rise from this contribution is about 13-14 m. A less well-determined constraint of about 10 m equivalent sea-level rise is suggested for the Barents-Kara ice sheet. The inversion also indicates that a small amount of melt-water, from ice sheets far away from Europe, continued to be added into the oceans during Late Holocene time so as to raise the equivalent sea-level by about 3 m during the past 6000 yr, consistent with similar inversions of data from sites in the Australian and Pacific regions.

Spectroscopic Analysis of Global Tide Gauge Sea Level Data

Abstract: Yearly and monthly global tide-gage sea-level data are fitted to numerically generated tidal data in order to search for the 18.6-yr lunar nodal tide and 14-month pole tide. Both of these tides are clearly evident, with amplitudes and phases that are consistent with a global equilibrium response. The ocean's response to atmospheric pressure is studied with the least-squares fit technique. Consideration is given to the global rise in sea level, the effects of postglacial rebound, and the possible causes of the enhanced pole tides in the North Sea, the Baltic Sea, and the Gulf of Bothnia. The results support O'Connor's (1986) suggestion that the enhanced pole tide in these regions is due to meteorological forcing rather than a basin-scale resonance. Also, the global average of the tide-gage data show an increase in sea level over tha last 80 yr of between 1.1 and 1.9 mm/yr.

Single‐Link Cluster Analysis As A Method to Evaluate Spatial and Temporal Properties of Earthquake Catalogues

Abstract: SUMMARY Single-link cluster analysis is a straightforward method to quantitatively measure the degree of clustering or isolation of groups of elements in a set, such as a catalogue of earthquakes. to apply single-link cluster analysis to a set of N earthquakes, individual earthquakes are first linked to their nearest neighbours to form event sub-groups. the process is then repeated and each sub-group linked to its nearest neighbour, recursively, until N-1 links are found to join all the earthquakes. This paper shows how knowledge of these links can be used to divide a group of earthquakes into any number of spatial clusters. With various modifications this analysis can identify earthquake nests, isolated events, aftershock sequences, and zones of seismic quiescence. to illustrate this method we apply it to; (i) a global data set of 2178 earthquakes having mb of 5.8 or greater reported by the International Seismological Centre (ISC) between 1964 and February 1986, and (ii) sets of earthquakes having mb of 4.9 and greater as reported by the ISC, occurring in Central America and in the Aleutians. to facilitate comparison with real data, this study also investigates the distribution of link lengths for synthetic events placed randomly on 1-D (circular line), 2-D (circular area) and 3-D (spherical volume) geometries. If we remove links between nuclear explosions from the ISC data, the resulting link distribution is quite similar to that for synthetic events placed along a 1-D fault. For the shortest links, the similarity improves markedly if the synthetic fault has a finite width, or if relative location errors cause events to be located away from the fault trace, with a standard deviation of about 15 km.

Smoothness criteria in surface wave tomography

Abstract: SUMMARY We discuss and develop further the methods of surface wave tomography in the frame of the geometric ray approximation. The general approach for determining the lateral phase or group velocity distribution, which is a standard 2-D tomography problem, involves linearization, representation of the unknown function as a series in some basis functions, and evaluation of the coefficients by the methods of linear algebra. If the wave paths cover the area under investigation non-uniformly, the basis functions should not be chosen a priori, but constructed proceeding from the pattern of paths. Different criteria for constructing the basis functions are compared, and a relation between them is considered. A more preferable approach is joint interpretation of phase and group velocity data for different periods, because it allows the information about phase velocity variations to be enlarged due to the use of the group velocity data. Both the phase and group traveltimes are represented as linear functionals of the unknown phase slowness corrections. A specific form of the data kernels allows the basis functions to be represented as a product of two functions, one depending on the horizontal coordinates, and the other on frequency.

Wave propagation in anisotropic linear viscoelastic media: theory and simulated wavefields

Abstract: SUMMARY The anisotropic linear viscoelastic rheological relation constitutes a suitable model for describing the variety of phenomena which occur in seismic wavefields. This rheology, known also as Boltzmann’s superposition principle, expresses the stress as a time convolution of a fourth rank tensorial relaxation function with the strain tensor. The first problem is to establish the time dependence of the relaxation tensor in a general and consistent way. Two kernels based on the general standard linear solid are identified with the mean stress and with the deviatoric components of the stress tensor in a. given coordinate system, respectively. Additional conditions are that in the elastic limit the relaxation matrix must give the elasticity matrix, and in the isotropic limit the relaxation matrix must approach the isotropic-viscoelastic matrix. The resulting rheological relation provides the framework for incorporating anelasticity in time-marching methods for computing synthetic seismograms. Through a plane wave analysis of the anisotropic-viscoelastic medium, the phase, group and energy velocities are calculated in function of the complex velocity, showing that those velocities are in general different from each other. For instance, the energy velocity which represents the wave surface, is different from the group velocity unlike in the anisotropic-elastic case. The group velocity loses its physical meaning at the cusps where singularities appear. Each frequency component of the wavefield has a different non-spherical wavefront. Moreover, the quality factors for the different propagating modes are not isotropic. Examples of these physical quantities are shown for transversely isotropic-viscoelastic clayshale and sandstone. As in the isotropic-viscoelastic case, Boltzmann’s superposition principle is implemented in the equation of motion by defining memory variables which circumvent the convolutional relation betweeh stress and strain. The numerical problem is solved by using a new time integration technique specially designed to deal with wave propagation in linear viscoelastic media. As a first application snapshots and synthetic seismograms are computed for 2-D transversely isotropicviscoelastic clayshale and sandstone which show substantial differences in amplitude, waveform and arrival time with the results given by the isotropic and elastic rheologies.

A method for correcting geographically separated remanence directions for the purpose of archaeomagnetic dating

Abstract: SUMMARY Spatial variations in the geomagnetic field must be taken into account if secular variation master curves and directional magnetic dates are to be optimized. Two methods for relocating remanence vectors have been proposed and in this paper their relative accuracies are compared using a numerical model based on the present-day field. A method which converts archaeomagnetic directions via a virtual geomagnetic pole is shown to be the more efficient transformation. For an‘archaeomagnetic region’the size of the British Isles, (900 km radius), the maximum error in relocating vectors to a central location is predicted to be of the order of 1.2°.

Analysis of conductive and convective heat transfer in a sedimentary basin, demonstrated for the Rheingraben

Abstract: SUMMARY The identification and quantification of conductive and convective components in the heat transfer of a sedimentary basin is demonstrated for the Rheingraben. Three different methods of varying complexity as well as three independent data sets are employed: (1) energy budget considerations based on hydraulically perturbed thermal data from shallow boreholes ( lo00 m), and (3) 2-D finite difference modelling of the fully coupled fluid flow and heat transport equations on a vertical cross-section of the entire Rheingraben. Energy budget considerations yield a conductive basal heat flow density of 84 + 40/-10 mW mP2, and in good agreement with this Peclet number analysis, gives median values in the range 90 f 35 mW m-’. In the first case, the basement is formed by low permeable, tertiary sediments at about 500 m depth, and in the second by the transition from the sedimentary graben fill to the crystalline basement at depths of between 2000 and 4000m. It is shown how results from numerical modelling support the flow field assumptions made by methods (1) and (2), as well as the value of 80 f 10 mW m-’ for average basal heat flow density entering the graben from below. Conversely, the Peclet number range Pe I 1.2 inferred from method (2) can be applied for a (at least partial) calibration of the fully coupled hydrothermal model calculatioris. This technique is suggested as a potentially interesting thermal method for constraining regional-scale permeability. An interpretation of heat transport is presented that satisfies the experimentally established patterns of both temperature and heat flow density in the Rheingraben. Moreover, it is demonstrated that the thermal anomalies along the western rim of the graben (such as Pechelbronn, France or Landau, Germany) can be convincingly explained by a basin-wide, deep rooted E-W groundwater circulation that locally enhances a background basal heat flow density of about 80 mW m-’ on average by 50 per cent and at individual sites by as much as 120 per cent.

Stochastic analysis of global traveltime data: mantle heterogeneity and random errors in the ISC data

Abstract: Analysis of global traveltime data has been formulated in terms of the stochastic properties of the Earth's heterogeneity pattern and random errors in the data. The formalism relates the coherency of traveltime residuals within bundles of rays (summary rays) of varying size to the spherical harmonic power spectrum of the slowness field of the medium. It has been applied to mantle P-wave data from the ISC catalogue. The measure of coherency is the variance within summary rays. It is estimated within bins in source depth, epicentral distance and the scale size of the area defining a summary ray. The variance at infinitesimal scale length represents the incoherent component of the data (random errors). The variation of the variance with scale length contains information about the autocorrelation function or power spectrum of slowness perturbations within the Earth. The variation with epicentral distance reflects the depth variation of the spectrum. The formalism accounts for the uneven distribution (clustering) of stations and events. We find that estimates of random errors correlate well with complexities on the traveltime curve of P-waves. The variance peaks at 1.0–2.0 s^2 at Δ ≈ 20°, where triplications occur on the traveltime curve, drops to 0.15–0.8s^2 at teleseismic distances, and rises to 0.4–1.3 s^2 approaching the core shadow, where the traveltime curves of P-waves and PcP-waves merge. These estimates should be considered upper bounds for the random error variance of the data. The signal to random noise ratio in the teleseismic ISC P-wave data is about S/N ≈ 2. Inversion of the scale-dependent structural signal in the data yields models that concentrate heterogeneity strongly in the upper mantle. The product of correlation length and power drops by about two orders of magnitude from the surface of the Earth to the lower mantle. About half of this quantity in the upper mantle is due to small-scale features (<300km). The lower mantle is devoid of small-scale structure. It contains 0.1 per cent velocity variations at a characteristic scale of about 1000km. This corresponds to a spectral band-width of l ≈ 7. The D″ layer at the bottom 100–200 km of the mantle shows up as a distinct layer in our results. It has 0.3 per cent velocity variations at a characteristic scale of 350km. The top of the lower mantle contains 0.3 per cent velocity variations on a scale of 500km and also contains some small-scale power. These results are compatible with previous deterministic lower mantle studies, although some details differ. The strength of heterogeneity in the upper mantle may obscure attempts to model the Earth's deep interior.

A Back-Projection Method For Imaging Large-Scale Lateral Variations of Lg Coda Q With Application to Continental Africa

Abstract: SUMMARY A new method is developed which uses back-projection tomography to regionalize large-scale lateral variations of coda Q for Lg waves which have traversed long continental paths. Successful use of this method requires precise and stable single-trace measurements of Lg coda Q (Xie & Nuttli 1988). the method converges rapidly and requires minimal computer storage. It also allows quantitative estimation of resolution and error in imaging lateral variations of coda Q. the spatial resolution of this method is limited by the uneven spatial coverage of the data base, by our limited knowledge of Lg coda generation, and by the trade-off between the stability and the spatial resolution of the coda Q inversion. the method is applied to a large set of digital Lg coda data from Africa, where large-scale lateral variations of Lg coda Q are found to correlate well with major tectonic features. Most of Africa is stable, and like other stable regions, has relatively high coda Q values. the lowest values of coda Q are associated with the African rift system. Other regions of low-Q values include the Atlas mountains and Cape Fold Belt, regions of Mesozoic and younger deformation. the lateral variation of frequency dependence of Lg coda Q correlates, in most regions, with that of Q at 1 Hz. Our analysis of the spatial resolution of this method indicates that the resolving power of Lg coda Q imaging is comparable to that of velocity tomography using long-period surface waves. Using empirical approaches, we estimate that uncertainties in Lg coda Q and its power-law frequency dependence parameter are less than about 60 and about 0.2, respectively, for most of Africa.

Evidence of a laterally variable lower mantle structure from P‐ and S‐waves

Abstract: SUMMARY Examples of Grafenberg-array data showing anomalous P-waves which typically arrive 3-5s after the direct P-wave and which have a slowness 0.7-0.8 sdeg-' smaller than direct P are presented. This additional phase is most frequently observed for events located in the NE portion of the southern Kurile Island subduction zone 73"-80" from Grafenberg, but systematically disappears for events in the SW portion of this zone. Because of the magnitude of the slowness difference, these observations cannot be attributed to a complex source rupture process nor to multipathing through the descending slab. Likewise, they may not be accounted for by near-receiver structure because these phases are not seen for all Kurile events. If present they appear at all stations of the array but they follow direct P too closely to be a multiple from the Moho. Therefore, we conclude they are very likely caused by lower mantle velocity structure. The most likely explanation is the presence of a P velocity jump of about 3 per cent approximately 290 km above the core-mantle boundary, since such a reflector in the lowermost mantle not only gives a good fit of traveltimes and slowness but is also able to model the waveform and the amplitudes of this additional P phase. The distribution of bounce points on this reflector for the Kurile events indicates a lateral extension of this velocity anomaly under northern Siberia of about 150 km by at least 200 km. The best fitting S-wave model has a reflector in the same depth, but the velocity contrast seems to be only about 2 per cent suggesting a different behaviour of the P and S velocity in D". Few events from other regions in this distance range are suitable for a definitive analysis of this kind. From among this group some observations indicate a lower mantle anomaly under the Lomonosow Ridge and under northern Greenland; but since the lower mantle under western Siberia, northern Novaya Zemlya, the Azores Islands region and the USSR-Afghanistan border region does not produce an additional phase in the Grafenberg recordings it is very unlikely that such a velocity anomaly in the lowermost mantle is a global feature.

The ocean-continent boundary off the western continental margin of Iberia—I. Crustal structure at 40°30′N

Abstract: SUMMARY The western continental margin of the Iberian peninsula has the characteristics of a rifted non-volcanic margin with half-graben and tilted fault blocks seen in several places. The ocean-continent boundary (OCB) is therefore expected to be where thinned continental crust and oceanic crust are juxtaposed, as at many similar margins worldwide. It is particularly useful to locate the OCB off western Iberia in order to constrain the pre-rift fit of North America to Iberia and, by implication, the shape of the proto-Bay of Biscay. The fit is only marginally constrained by sea-floor spreading magnetic anomalies because anomaly 34 is believed to be far to the west of the OCB and it is even possible that all older oceanic crust was created during the Cretaceous constant polarity interval. The best way to distinguish oceanic crust from thinned continental crust appears to be the crustal seismic velocity structure. Therefore in 1986 a series of seismic refraction lines was shot parallel to, and normal to, the continental margin. These lines enabled us to bracket the location of the OCB. A further constraint on the location was obtained by modelling an east-west magnetic profile which included the enigmatic J-anomaly. This anomaly can be explained as either just pre-anomaly MO or as part of the Cretaceous constant polarity interval, depending on whether spreading began about 127 or after 118 Myr ago, respectively. The evidence favours the former explanation. Lastly the depth to acoustic basement was contoured from a compilation of seismic reflection profiles. This indicated a new fracture zone at 41"15'N which offsets the OCB. A few key reflection profiles also suggest that the OCB can be identified by an abrupt landward step-down in acoustic basement. We conclude that the OCB in the eastern Iberia Abyssal Plain lies between 12'10' and 12'30'W and has a trend just east of north. This westerly location is consistent with recent estimates of the location of the OCB off the Grand Banks but brings into question the proposed location at about 11"W of the OCB in the Tagus Abyssal Plain.

The density and shear velocity contrast at the inner core boundary

Abstract: SUMMARY A systematic search of short-period GDSN seismograms from 1980 to 1984 at ranges from 20" to 90" identifies two probable PKiKP arrivals. PKiKPIPcP amplitude ratios for these phases are consistent with previous studies. However, more typically PKiKP is not observed, even when clear PcP arrivals are seen. We use these data to place upper bounds on PKiKPIPcP amplitude ratios for 100 event-station pairs. These bounds indicate that most measurements of PKiKP amplitudes are biased toward large values and predict reflection coefficients at the inner core boundary (ICB) which are too high. Our upper limits on PKiKP amplitudes roughly constrain the density jump at the ICB to be less than l.Og~m-~ and the shear velocity at the top of the inner core to be greater than 2.5 km s-l, assuming a sharp discontinuity at the ICB. Upper bounds on PKiKPIP amplitude ratios at ranges between 70" and 90" are consistent with these results but are less reliable due to take-off angle differences between P and PKiKP. Approximately 50 observed free oscillations of the Earth are sensitive to the structure of the inner core. Modem models derived from these and other mode data typically have a density jump at the ICB of 0.5-0.6 g ~m-~. An experiment in which we varied the mean density of the inner core indicates that the mode frequencies are roughly linear functionals of this parameter. The fit to the data is seriously degraded if the density jump is significantly different from 0.55 g ~m-~. Many of the modes are also strongly sensitive to the shear velocity in the inner core, and forward modelling indicates that the average inner-core shear velocity is probably 3.45 f 0.1 km s-'. These results are compatible with the short-period PKiKP amplitude bounds, indicating that there is no inconsistency between PKiKP and normal mode data regarding the density and shear velocity structure at the inner core boundary.

Non-linear arrival time inversion: constraining velocity anomalies by seeking smooth models in 3-D

Abstract: SUMMARY The problem of constraining 3-D seismic anomalies using arrival times from a regional network is examined. The non-linear dependence of arrival times on the hypocentral parameters of the earthquakes and the 3-D velocity field leads to a multiparameter-type non-linear inverse problem, and the distribution of sources and receivers from a typical regional network results in an enormous 3-D variation in data constraint. To ensure computational feasibility, authors have tended to neglect the non-linearity of the problem by linearizing about some best-guess discretized earth model. One must be careful in interpreting 3-D structure from linearized inversions because the inadequacy of the data window may combine with non-linear effects to produce artificial or phantom ‘structure’. To avoid the generation of artificial velocity gradients we must determine only those velocity variations which are necessary to fit the data rather than merely estimating local velocities in different parts of the model, which is the more common practice. We present a series of inversion algorithms which seek to inhibit the generation of unnecessary structure while performing efficiently within the framework of a large-scale inversion. This is achieved by extending the subspace method of Kennett, Sambridge & Williamson (1988) and incorporating the smoothing strategy proposed by Constable, Parker & Constable (1987). A flexible model parametrization involving Cardinal spline functions is used, and full 3-D ray tracing performed. A comparison between linear and non-linear inversions shows that if a breakdown in the linearizing approximation occurs spurious velocity models may be obtained which would appear acceptable in a linear inversion. Application of the techniques to a SE Australian data set show that unnecessary structure can be suppressed. As the smoothing power of the algorithm is improved a robust low-velocity anomaly dipping to the north becomes the most dominant feature of the P-wave model and much of the complex structure of pure data-fitting models is removed.

The strain pattern in the western Hellenic arc deduced from a microearthquake survey

Abstract: SUMMARY than loo0 earthquakes recorded during 7 weeks in 1986 in the Peloponnese and surrounding areas show shallow seismicity spread over a wide area bounded by the Hellenic trench. The highest level in the energy release is for clusters located along the Hellenic trench, where changes in the morphology are seen. A few clusters are also observed, as at the intersection between the Gulf of Corinth and the Gulf of Patras, near the Lake Trikhonis, or between Kythira and Crete. In the Peloponnese, the shallow earthquakes do not define single faults, but are diffusely distributed. A higher concentration and a deepening of the foci (within the whole crust) towards the west seem to reflect a higher strain rate there. Fault plane solutions exhibit a scattered pattern for earthquakes shallower than 11 km, but for earthquakes deeper than 11 km, they show some consistency in the orientation of P-, T-, and B-axes. Gently dipping nodal planes are seen all over the Peloponnese, with the conjugate vertical plane striking in various directions, and with no consistency in the sense of motion. These earthquakes are located between 8 and 18 km deep and could reflect a decoupling between the lower and the upper crust. Reverse faulting is seen in the Gulf of Kefallinia and the western Peloponnese. The P-axes trend NE-SW to E-W. Normal faulting is seen in the Gulf of Corinth and in central Peloponnese with the T-axes trending N-S, and in the southern Peloponnese, where the T-axes trend NW-SE. The deformation over the western Hellenic arc, revealed by fault plane solutions of earthquakes with depth between 11 and 40 km, appears to be the superposition of two phenomena. First, throughout the Aegean, crustal extension with orientation roughly parallel to the trench dominates, with crustal shortening and subduction along the Aegean arc. Superimposed on this is a more local effect of the collision between the Aegea and Apulia which seems to induce horizontal compression roughly perpendicular to the Aegean arc west of the Peloponnese and Epirus. The compression due to this decays eastward, at a distance comparable with the width of the collision zone. The deepening of the brittleductile transition is likely to be due to an increase of the strain field towards the Ionian islands.

Asthenospheric Viscosity and Stress Diffusion: A Mechanism to Explain Correlated Earthquakes and Surface Deformations In Ne Japan

Abstract: SUMMARY A significant correlation is found, in both space and time, between the intraplate (land) and interplate (sea, thrust zone only) earthquakes in Tohoku, NE Japan that has persisted since the times of reliably reported events in AD 1600. the correlation peaks at a land-lead of about 36 yr with an average correlation distance of 200km, with the implication of an average strain migration rate of 5.6 km yr-1. the correlation is highly significant (> 99 per cent), both from formal statistics and from tests of random shuffles of the data. Additional analysis of the data, as a point process, confirms the results of the correlation analysis. the sharpness of the correlation peak, when compared to the individual times of occurrence of the land and sea events suggests a trigger mechanism. To explain the correlation, the general model ofsubduction-rupture-rebound is extended to include additional features; the buckling of the land plate from the force of the subducting slab, and the viscoelastic coupling of the plate to the underlying asthenosphere. A buckle produces a high-stress region in the continental plate where earthquakes are more prone to occur, thus producing the spatial correlation in the data. This may also explain the preferred location on land for the smaller modern-day seismic events in NE Japan. the viscoelastic coupling controls the interaction between the land and sea events, resulting in the temporal correlation in the data. Because of viscosity, the model equations are diffuse-like with strain pulses as solutions; thus from the inferred strain migration rate it is possible to estimate asthenospheric viscosity (η=7 × 1018Pa s) using this model. A large land shock generates a strain pulse that affects the locked fault at the thrust zone several decades later. As the continental plate tends to pull away from the subducting slab, the frictional force arising from the overburden pressure is reduced, thus unlocking the fault and triggering a sea earthquake. The viscoelastic model is also used to explain surface deformations measured by triangulation surveys in Japan in 1904 and 1964. Horizontal displacements, which we believe are surface manifestations of the strain pulse from the large 1896 Riku-U land shock (M= 7.5) in NE Japan, are fit well by the model and provide a viscosity estimate η= 13 × 1018 Pa s.

Overall elastic properties of isotropic materials with arbitrary distribution of circular cracks

Abstract: SUMMARY The transmission properties of the mean field in elastic material with a random distribution of circular cracks of small aspect ratio are presented here for the general case where the crack normals are distributed in any pre-determined way in space. Special distributions, where the crack normals lie in one direction only, or lie at a fixed angle to a fixed direction, reproduce established results. Expressions are given for the case of crack normals lying close to a given direction in a Gaussian distribution. All results are valid to second order in the crack number density.

Crust and upper mantle structure along the DSS Baltic profile in SE Finland

Abstract: SUMMARY The 430 km long DSS Baltic profile in SE Finland runs through the Rapakivi intrusion, the Svecokarelian geosynclinal complex, including the Ladoga-Bothnian Bay zone, and the Archean basement complex. A brief description of field operations in 1982 is presented. Record sections of P- and S-waves were used for the interpretations. A starting model was constructed by inverse methods and improved by dynamic ray tracing modelling. Three blocks are indentified on the cross-section from SW to NE. These are the southern, associated with the Rapakivi Massif, depth to Moho (M) about 40 km; the central, Ladoga-Bothnian Bay zone, with M of approximately 55-60 km, and the northern, Archean crystalline basement, with M about 40-45 km. The average crustal velocity is 6.6-6.7 kms-’. The crustal structure along the Baltic profile is compared with the surrounding DSS profiles, Finnish Sveka and Soviet Kern-Tulos and Ladoga profiles. The eastern border of the Ladoga-Bothnian Bay zone was located and specific features of the crust and uppermost mantle along the profile area are discussed.

Deep seismic sounding studies in the north Cambay and Sanchor basins, India

Abstract: SUMMARY Deep seismic sounding (DSS) studies have been carried out in the north Cambay and Sanchor sedimentary basins in western India along three lines covering about 350km. Seismic refraction and wide angle reflection data, pertinent to the sedimentary basin as well as the deep crustal section, have been recorded from 41 shot points using a 60 channel DFS-V digital recording system with 200 m geophone spacing and 4 ms data sampling. Extensive modelling and interpretation of a large number of seismic record sections reveal four sub-basins in the sedimentary section along these lines. Maximum depth to the granitic/Proterozoic basement (P-wave velocity 5.9–6.0 km s-1) is about 5000 m in the north Sanchor and the Patan sub-basins and about 5600 m in the south Sanchor sub-basin. The deepest part of the sedimentary basin is delineated within the Gandhinagar sub-basin where the basement depth reaches 7700 m. The Deccan Traps (P-wave velocity 4.3–4.8 km s-1) form the base of the Tertiary sediments, almost in the entire study area except the extreme northern part. There is also some indication of the presence of sub-Trappean Mesozoic sediments along this profile. Within the sedimentary basin two horst features, one near Diyodar (the Diyodar ridge) and the other northwest of Mehsana (the Unhawa ridge), are indicated by the seismic data consistent with the tectonics of the region. The thickness of the upper crust in this region does not exceed 15 km (P-wave velocity reaching 6.3km s-1). A prominent low-velocity zone (velocity 5.5 km s-1) occurs in the depth range from 10.5 to 12.5 km. The lower crust consists of two layers of velocities 6.6–6.9 km s-1 and 7.3–7.4 km s-1, the discontinuity between them occurring at 23–25 km depth. The Moho discontinuity (PM velocity 8.0 km s-1) lies at a depth of 31–33 km. The high-velocity (7.3–7.4 km s-1) lower crustal layer represents underplating of the crust due to mantle upwelling and rifting with large-scale extrusion of the Deccan volcanics. The large thickness of the Tertiary sediments in the Cambay basin and a relatively thin crust in the region suggest further rifting during the Tertiary.

Resolution of ferrimagnetic and paramagnetic anisotropies in rocks, using combined low-field and high-field measurements

Abstract: SUMMARY A method of separation of ferrimagnetic and paramagnetic components of magnetic anisotropy from the total anisotropy is developed. From the anisotropy measurement of a specimen in a low field (susceptibility anisotropy) and from measurement in two high fields stronger than the saturation field of the ferrimagnetic fraction present, the following anisotropy components can be calculated: susceptibility anisotropy (field-independent) of the paramagnetic fraction, low-field (susceptibility) anisotropy of the ferrimagnetic fraction and high-field anisotropy of the ferrimagnetic fraction. Errors possibly arising from imperfect saturation of the ferrimagnetic fraction in the field practically available are analysed.

Crustal and upper mantle structure in the Ryukyu Island Arc deduced from deep seismic sounding

Abstract: SUMMARY In 1984 an extensive geophysical investigation was conducted in the northernmost part of the Ryukyu Island Arc, south of Kyushu Island, Japan. The aim of this investigation was to obtain new information on the nature of crust and upper mantle at the continental margin through various geophysical measurements (seismic profiling, gravity, magnetism, sea beam and heat flow). This paper presents the crust and upper mantle structure deduced from ocean bottom seismographic profiling conducted along two lines. One line, 190 km long, was taken behind the northernmost part of the Ryukyu Island Arc, parallel to the major tectonic units, i.e. the Okinawa Trough, Ryukyu Arc and Ryukyu Trench. The other, 295 km long, was perpendicular to the tectonic units. Along these profiles, we deployed ocean bottom seismograms spaced 10-20km apart. As a controlled source, we used both explosives and an airgun array. The experiment was successful and provided quite important information on the tectonics of a trenchisland arc-back arc system. The velocity structure obtained for the profile behind the Ryukyu Islands has continental properties. The sediment thickness is 3-4 km, although it shows lateral variations due to basement undulation. The velocities of the upper and lower crust are 5.8-6.2 and 6.6-6.8 km s-', respectively. The total crustal thickness decreases southwestward, from 27-30 to 23-24 km. This is direct evidence for crustal thinning associated with the process of back-arc spreading. The velocity structure along the profile from the trench to the island arc clearly shows the subduction, accretion and deformation at this margin. The crust beneath the trench has oceanic properties. The sediment thickness is 1.3-3.0 km, beneath which the igneous basement shows severe undulation. The total crustal thickness is 7-8 km. The P, velocity is slightly less than 8.0 km s-'. Our seismic data revealed a huge sedimentary wedge located 50-150 km landward of the trench. The maximum thickness of this wedge exceeds 12 km. The origin of this wedge may be oceanic because the P-wave velocity in its eastern half is almost comparable to that obtained for the trench area. The velocity structure landward of the wedge is continental and similar to that obtained for the profile behind the island arc.

Standard electrical conductivity of isotropic, homogeneous olivine in the temperature range 1200°‐1500°C

Abstract: In order to produce electrical conductivity-temperature plots suitable for interpreting mantle conductivity profiles, the authors have used several spatial averaging schemes to reduce the orthorhombic conductivity tensor of olivine to that appropriate for an isotropic material. The starting data were new measurements of electrical conductivity [sigma] in the three orthogonal principal directions of a San Carlos olivine (fayalite 9 per cent) to 1500[degrees]C at 1 atmosphere total pressure. These measurements were made in a CO[sub 2]/CO atmosphere that provided an oxygen partial pressure of 10[sup [minus]4] Pa (10[sup [minus]9] atmosphere) at 1200[degrees]C; this is slightly more reducing than the quartz-fayalite-magnetite (QFM) buffer curve. The highest [001] and lowest [010] conducting directions differ by a factor of 2.3. The next step was to obtain series and parallel bounds from the three principal directions; these absolute upper and lower bounds differ by 15 per cent. A standard conductivity curve comes from applying to the series and parallel curves various averaging schemes such as the Hashin-Shtrikman and Maxwell-Waff bounds or the effective medium, geometric mean, or (a new technique) self-similar methods; these all agree to within 3 per cent. The self-similar calculation demonstrates that the VRH method is a form of parallel averagemore » that is biased toward high values. The standard curve SO1 is given by [sigma] = 46.9 exp ([minus]1.38/kT) + 5.22 [times] 10[sup 8] exp ([minus]3.90/kT) where T is absolute temperature, k is the Boltzmann constant, and the activation energy is in eV. It is valid for the measurement interval of 1200[degrees]-1500[degrees]C and can be used for extrapolation on either side of this range. Uncertainties associated with applying SO1 to inferring mantle temperatures are discussed.« less