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1.
About 50 000 P and S arrival times and 25 000 values of t * recorded at seismic arrays operated in the Central Andes between 20°S and 25°S in the time period from 1994 to 1997 have been used for locating more than 1500 deep and crustal earthquakes and creating 3-D P , S velocity and Qp models. The study volume in the reference model is subdivided into three domains: slab, continental crust and mantle wedge. A starting velocity distribution in each domain is set from a priori information: in the crust it is based on the controlled sources seismic studies; in slab and mantle wedge it is defined using relations between P and S velocities, temperature and composition given by mineral physics. Each iteration of tomographic inversion consists of the following steps: (1) absolute location of sources in 3-D velocity model using P and S arrival times; (2) double-difference relocation of the sources and (3) simultaneous determination of P and S velocity anomalies, P and S station corrections and source parameters by inverting one matrix. Velocity parameters are computed in a mesh with the density of nodes proportional to the ray density with double-sided nodes at the domain boundaries. The next iteration is repeated with the updated velocity model and source parameters obtained at the previous step. Different tests aimed at checking the reliability of the obtained velocity models are presented. In addition, we present the results of inversion for Vp and Vp/Vs parameters, which appear to be practically equivalent to Vp and Vs inversion. A separate inversion for Qp has been performed using the ray paths and source locations in the final velocity model. The resulting Vp , Vs and Qp distributions show complicated, essentially 3-D structure in the lithosphere and asthenosphere. P and S velocities appear to be well correlated, suggesting the important role of variations of composition, temperature, water content and degree of partial melting.  相似文献   

2.
b
We present expressions in a spherical harmonic framework for the gravitational potential of discrete point, surface, and volume mass elements located at any depth within a sphere. Through analysis of the spherical harmonic spectrum, insight is gained into the properties of the potentials arising from a variety of mass distributions. A point mass at the surface of a sphere displays the richest harmonic spectrum in all degrees; spectra become increasingly reddened as the source mass is distributed through larger elements of area or volume, or is located at greater depths below the surface of the reference sphere. The spectra of dipolar distributions, useful in representing compensated masses, are depressed, especially in the low harmonic degrees, relative to the spectra of monopole elements.  相似文献   

3.
Summary. A new-technology, broad-band seismic land source based on the principle of magnetic levitation is under development. The new source is designed to overcome limitations experienced with controlled-signal seismic sources at the present time, and to create new possibilities. In particular, She new source extends the constant peak force frequency band down to 2 Hz, and is capable of at least several weeks continuous, coherent transmission. It will enable three-dimensional long-range (>200 km) controlled-signal seismology with the potential for active earth tomography down into the mantle and systematic earthquake precursor measurement, and should improve shallow and deep seismic profiling technology. The optimum transmission frequencies depend on Q, transmission range, and noise spectrum.  相似文献   

4.
We provide textural, geochemical, and mineralogical data on a thin, silty deposit that unconformably mantles glaciated uplands in the eastern Upper Peninsula of Michigan. Previous research on this deposit, which we hypothesize to be loess, is nonexistent. The uplands were islands or narrow peninsulas within one or more glacial lakes. We compare the distribution, likely source and nature of the 20–60 cm thick silty mantle by using the loess formation model of Mason et al. [Mason, J.A., Nater, E.A., Zanner, C.W., Bell, J.C., 1999. A new model of topographic effects on the distribution of loess. Geomorphology 28, 223–236], which focuses on the generation of eolian silt by saltating sand across upwind, barren surfaces. Parabolic dunes, with arms open to the NW, are common on former lake floors upwind of the silt-mantled uplands, attesting to the strength and direction of paleowinds. The abrupt termination of the dunes at the footslopes of the uplands, associated with silt deposition on upland soil surfaces in downwind locations, are both consistent with the model of Mason et al. [Mason, J.A., Nater, E.A., Zanner, C.W., Bell, J.C., 1999. A new model of topographic effects on the distribution of loess. Geomorphology 28, 223−236]. Sediments on former lake floors contain abundant strata of fine/medium sand and silt, and thus are likely sources for the silt and dune sand. The cap, dune and lake sediments are similar along many different geochemical axes, whereas the substrate sediment, i.e., the drift below the cap, is unique. Cap sediments, normally containing roughly 30% silt, are enriched in quartz and depleted in Ti and Zr, relative to dune sediment. The dune sediment, a more residual eolian deposit, is enriched in Ti and Zr, relative to the cap, probably due to its greater abundance of heavy minerals. Therefore, we conclude that the silty cap is loess that was deflated from abandoned lake floors after nearby glacial lakes drained, probably contemporaneously with dune migration across the former lake floors.  相似文献   

5.
The thermal evolution of the Earth is controlled by radioactive elements whose heat production rate decays with time and whose spatial distribution depends on chemical segregation processes.
We present a 2-D and finite-difference Boussinesq convection model with temperature-dependent viscosity and time- and space-dependent radioactive heat sources. We used Newtonian rheology, boxes of aspect ratio 3, and heating from within. Starting from the geochemical results of Hofmann (1988), it is assumed that the radioactive heat sources of the mantle were initially distributed homogeneously. In a number of calculations, however, higher starting abundances of radioactive sources were assumed in the upper mantle. For the present geological situation, this also results in a depleted upper mantle. It was assumed that, if the viscosity falls below a certain critical value, chemical segregation will take place. In this way, model continental crust develops, leaving behind areas of a depleted mantle. We obtained the heat source, flow line, temperature, viscosity and heat-flow distribution as a function of time with realistic values, especially for the present time. The present viscosity of the upper mantle is approximately at the standard value obtained for postglacial uplift modelling; the deeper-mantle viscosity is considerably higher. The time dependence of the computed mean of the kinetic energy of mantle convection bears a resemblance to that of the magmatic and orogenetic activity of the Earth. We assumed that the 670 km discontinuity cannot be penetrated by the flow.  相似文献   

6.
Summary. Spectral analysis of P and P coda from NTS explosions recorded at NORSAR shows that magnitudes based on spectral integration of P coda provide a significantly more precise measure of yield than those based on the initial P . Coda amplitudes and spectra are considerably less sensitive to near-source variations than the direct P . A plot of P -coda magnitude, after correcting for the effects of attenuation in the upper mantle and the source spectrum based on knowledge of approximate yield or mb , versus log yield appears to have a slope of unity. This implies that P coda from several stations (with different but known t * values) can be combined to improve the yield determinations further. Single station P coda from Soviet explosions can also be used to determine relative yields that appear to be at least as precise as those from Lg based on a network of stations.  相似文献   

7.
Summary. The crustal structure beneath the exposed terranes of southern Alaska has been explored using coincident seismic refraction and reflection profiling. A wide-angle reflector at 8–9 km depth, at the base of an inferred low-velocity zone, underlies the Peninsular and Chugach terranes, appears to truncate their boundary, and may represent a horizontal decollement beneath the terranes. The crust beneath the Chugach terrane is characterized by a series of north-dipping paired layers having low and high velocities that may represent subducted slices of oceanic crust and mantle. This layered series may continue northward under the Peninsular terrane. Earthquake locations in the Wrangell Benioff zone indicate that at least the upper two low-high velocity layer pairs are tectonically inactive and that they appear to have been accreted to the base of the continental crust. The refraction data suggest that the Contact fault between two similar terranes, the Chugach and Prince William terranes, is a deeply penetrating feature that separates lower crust (deeper than 10 km) with paired dipping reflectors, from crust without such reflectors.  相似文献   

8.
The two-point correlation tensor provides complete information on mantle convection accurate up to second-order statistics. Unfortunately, the two-point spatial correlation tensor is in general a data-intensive quantity. In the case of mantle convection, a simplified representation of the two-point spatial correlation tensor can be obtained by using spherical symmetry. The two-point correlation can be expressed in terms of a planar correlation tensor, which reduces the correlation's dependence to only three independent variables: the radial locations of the two points and their angular separation. The eigendecomposition of the planar correlation tensor provides a rational methodology for further representing the second-order statistics contained within the two-point correlation in a compact manner. As an illustration, results on the planar correlation are presented for the thermal anomaly obtained from the tomographic model of Su, Woodward & Dziewonski (1994 ) and the corresponding velocity field obtained from a simple constant-viscosity convection model Zhang & Christensen 1993 ). The first 10 most energetic eigensolutions of the planar correlation, which constitute an almost three orders of magnitude reduction in the data, capture the two-point correlation to 97 per cent accuracy. Furthermore, the energetic eigenfunctions efficiently characterize the thermal and flow structures of the mantle. The signature of the transition zone is clearly evident in the most energetic temperature eigenfunction, which clearly shows a reversal of thermal fluctuations at a depth of around 830  km. In addition, a local peak in the thermal fluctuations can be observed around a depth of 600  km. In contrast, due to the simplicity of the convection model employed, the velocity eigenfunctions exhibit a simple cellular structure that extends over the entire depth of the mantle and do not exhibit transition-zone signatures.  相似文献   

9.
An analysis of the field scaling power spectrum yields useful information about the source distribution, but it is uncertain whether deterministic, random, fractal or mixed approaches have to be used for the interpretation. To this end, the scaling properties of potential field spectra are analysed for a number of different source models of geological interest. Besides the models of Naidu (purely random sources) and Spector and Grant (gross block statistical ensembles) we consider other types of density and magnetization distributions with spectral exponents in the fractal range, such as a single homogeneous body with a random white source distribution. Spectral slopes in the fractal range are obtained.
We also study the effects of important natural sources, such as salt domes and sedimentary basins, representing them with simple Gaussians or combinations of Gaussian signals. The same spectral slopes as for gravity signals generated by 3-D fractal source distributions are found for them. Hence the power law decay of the field is not a characteristic only of fractal source models.
If a 3-D fractal source distribution is assumed a priori , a way of verifying the goodness of the model is to examine the whitened field at source level. The probability that the whitened field derives from a random white population is estimated for synthetic and real anomalies by applying the usual statistical tests.  相似文献   

10.
Shear-coupled PL     
Summary. Observed teleseismic shear-coupled PL -waves ( SPL ) display a variety of waveforms depending on factors such as source depth, source type and velocity structure. Using a WKBJ spectral method for SV -wave propagation, synthetic seismograms of SPL are produced to examine the factors important in SV and SPL excitation. Results show that SPL is preferentially excited by shallow sources compared to deep sources. This is due to large source area reverberations which consequently leak as SV -waves into the mantle. Interaction at the receiver area then sets up the classic prograde elliptical motion by which SPL can be identified. This is in accordance with the teleseismic observations and indicates that most previous models for the propagation of SPL were not appropriate for shallow source since emphasis was placed on wave interactions occurring only near the receiver.  相似文献   

11.
Summary. This paper concerns the calculation and analysis of admittance functions from large and uniform data sets of gravity and topography in four regions of the northern and western Pacific Ocean. The purpose is to separate and describe possible differences in isostatic compensation between several 'type' regions of oceanic crust: a mid-ocean ridge (Juan de Fuca), a mid-plate seamount chain (Hawaiian Ridge), fracture zone topography on old crust (north of Hawaii) and a marginal basin (Philippine Sea). Results suggest that there are significant differences in the degree to which long wavelength topography has been compensated which can be distinguished between regions. These differences are set in the perspective of three simple compensation mechanisms. Two of these consider local Airy models in which raised topography is compensated at depth either by crustal roots or low density mantle. A third considers the effects of an elastic plate of variable thickness supporting crustal variations. Conclusions are that: (a) a thick plate possibly in excess of 30 km supports the Hawaiian Ridge; (b) a much thinner plate of 5 to 15 km existed when the fracture zone topography was formed; (c) the Juan de Fuca Ridge is compensated either regionally by a plate 5 to 10 km thick or locally by sub-crustal low densities at depths of 15 to 20 km; and (d) the Philippine Sea shows no evidence for regional support: ridges are compensated locally by differences in crustal thickness whereas the basins are underlain by density variations at depths comparable to those of the much younger Juan de Fuca Ridge. The major difference between admittance functions for the Philippine Sea and comparably aged regions of the north Pacific Ocean adds further new evidence of possible evolutionary differences between it and normal ocean basins.  相似文献   

12.
The short-period seismic phase Sn has been interpreted by Stephens & Isacks as a lid wave' in which the seismic energy is constrained to the uppermost few tens of kilometres of the mantle. We have extended their normal-mode interpretation for structures both with and without low-velocity zones (LVZ) in the upper mantle. We have used spherical, anelastic models of the Earth. For a model of an oceanic mantle with a LVZ, we agree that Sn is a lid wave for sources above 200–250 km, if only the onset of Sn is considered. The later portions of the Sn wave train sample the structure as deeply as the 420-km discontinuity. For deeper foci, the pseudo-lid wave does not appear to be generated; even the onset of Sn samples the deeper mantle structure. For a model of a continental mantle without a LVZ, in general, sources at all depths above the 420-km discontinuity appear to generate teleseismic Sn which samples the entire mantle as deeply as the discontinuity and which travels with a velocity significantly greater than the lid velocity. Thus the velocity of Sn may be an important diagnostic to determine whether or not a LVZ exists in the upper mantle.  相似文献   

13.
Dissolved organic matter(DOM) is an important component of ice cores but is currently poorly characterized. DOM from one Holocene sample(HS, aged at 1600–4500 B.P.) and one Last Glacial Maximum sample(LS, aged at 21000–25000 B.P.) from the North Greenland Eemian Ice Drilling(NEEM) ice core were analyzed by ultra-high resolution Fourier transform ion cyclotron resonance mass spectrometry(FT-ICR-MS). CHO compounds contributed 50% of the compounds identified in negative-ionization mode in these two samples, with significant contributions from organic N, S, and P compounds, likely suggesting that marine DOM was an important source in these samples. Overall, the chemical compositions are similar between these two samples, suggesting their consistent DOM sources. However, subtle differences in the DOM between these two samples are apparent and could indicate differences in source strength or chemistry occurring through both pre-and post-depositional processes. For example, higher relative amounts of condensed carbon compounds in the HS DOM(5%), compared to the LS DOM(2%), suggest potentially important contributions from terrestrial sources. Greater incorporation of P in the observed DOM in the LS DOM(22%), compared to the HS DOM(13%), indicate more active microbiological processes that likely contribute to phosphorus incorporation into the DOM pool. Although these two samples present only a preliminary analysis of DOM in glacial/interglacial periods, the data indicate a need to expand the analysis into a broader range of ice-core samples, geographical locations, and glacial/interglacial periods.  相似文献   

14.
Summary. It is the supposed presence of intermediate depth earthquakes in areas of continental collision which supports the existence of subducting slabs in regions such as the Zagros mountains of Iran. Mounting field evidence from that region suggests that intermediate focal depths allocated by tele-seismic locations are wrong. No pP depth control exists for earthquakes in the Zagros and all teleseismic locations use P phases alone.
This paper examines the effect of random noise in arrival-time data on the variances calculated for origin time and hypocentral depth. These can be simply related to the distribution of recording stations and it can be shown that in any one region the smaller shocks will tend to have greater errors in depth than the larger. However, this effect cannot alone account for all the probable mislocation in depth of some small shocks in the Zagros. The discrepancy may, however, be explained by the difference in the quality of arrival-time data between large and small events. It is also shown that crustal earthquakes will have greater errors in depth than earthquakes of equivalent size in the mantle. If reliably read, PKP phases can help contribute to the accuracy of hypocentral depth. The conclusion is that although errors in origin time and depth are well correlated for teleseismic locations of all earthquakes, the value of the errors themselves may be small for the bigger shocks, and this may explain why standard bulletin locations seem to give very reasonable focal depths for the biggest events in Iran. Thus the focal depths of the smaller earthquakes (which include all the published deeper ones in the Zagros) are unreliable, while the focal depths for the largest events are likely to be much better.  相似文献   

15.
Summary. There is evidence that the equivalent seismic sources of the Amchitka Island explosions — Longshot, Milrow and Cannikin — depart significantly from the simple model of a point compressional-source in a layered elastic-medium. Consequently modelling the observed seismograms using standard source-models may not be the most efficient method of determining source properties. Here an alternative to modelling is used to obtain information on the seismic sources due to the explosions. Broad-band (BB) estimates of the P signals are obtained from the short-period (SP) seismograms, corrected for attenuation, and interpreted in terms of P, pP and radiation from secondary sources. the main conclusions are:
(i) BB estimates of the radiated displacement from the explosions can be obtained with only a small reduction in the signal-to-noise ratio seen on SP seismograms;
(ii) observations of differences in pulse amplitudes and spectra are not necessarily due to differences in anelastic attenuation;
(iii) P and pP at a given station may differ in shape so that notches in the signal spectrum may not be related to source depth;
(iv) there is evidence of arrivals that others have identified as due to slap-down but which could be interpreted as an overshoot to pP;
(v) direct interpretation of the estimated ground displacement is a better procedure for determining the seismic source properties of explosions than modelling SP seismograms using idealised models as a starting point.  相似文献   

16.
Delineation of detailed mantle structure frequently requires the separation of source signature and structural response from seismograms recorded at teleseismic distances. This deconvolution problem can be posed in a log-spectral domain where the operation of time-domain convolution is reduced to an additive form. The introduction of multiple events recorded at many stations leads to a system of consistency equations that must be honoured by both the source time functions and the impulse responses associated with propagation paths between sources and receivers. The system is inherently singular, and stabilization is accomplished through the supply of an initial estimate of the source time function. Although alternative choices exist, an effective estimate is derived from the eigenimage associated with the largest eigenvalue in a singular-value decomposition of the suite of aligned seismograms corresponding to a given event. The relation of the deconvolution scheme to simultaneous least-squares deconvolution is examined. Application of the methodology to broadband teleseismic P waveforms recorded on the Canadian National Seismograph Network demonstrates the retrieval of effective Green's functions including secondary phases associated with upper-mantle structure.  相似文献   

17.
The flexural rigidity of the oceanic lithosphere is strongly dependent on its temperature structure at the time of loading. It is commonly assumed that the depth to the 450°C isotherm defines the effective elastic thickness Te of the lithosphere. However, recent gravity studies across the Baltimore Canyon and Nova Scotian margins suggest that temperature may play a more complicated role in controlling the mechanical strength of extended continental lithosphere. For example, the flexural strength of the Baltimore Canyon margin (with sediment thicknesses of ? 15 km) appears to be controlled by the depth to the 150°C isotherm whereas the strength of the Nova Scotian margin (with sediment thicknesses cf ? 10 km) is controlled by the depth to the 250°C isotherm. The apparent correlation between sediment thickness and controlling isotherm suggests that sediment blanketing may play a role in modifying the flexural strength of extended continental lithosphere. This hypothesis was investigated by simulating the sedimentation history of a margin as a Gaussian function in which sedimentation peak and rate are determined by the mean and standard deviation of the function. The temperature structure of the lithosphere is continually modified as sediments are deposited on, and incorporated into the temperature structure of, the underlying lithosphere. Given a ‘starting’ value of Te defined by the degree of extension of the lithosphere, the modification of Te appears to be directly proportional to the sedimentation rate and cumulative sediment thickness, and inversely proportional to the time at which the sedimentation rate is a maximum. The first-order consequence of sediment blanketing is to reduce the cooling rate of the lithosphere relative to cooling in the absence of sediments. At thermal equilibrium, the initial value of Te is reduced by the cumulative sediment thickness. Local isostatic conditions (i. e. Te? 0) can only be approached when the sedimentation rate is unrealistically high (> 1000 m/Myr) during the rift or early post-rift phase of basin development. However, while these early loads may be locally compensated, any subsequent loads will be regionally compensated. Thus, it is unlikely that the low present-day flexural strengths interpreted from the Baltimore Canyon and Nova Scotian passive continental margins are a consequence of sediment blanketing.  相似文献   

18.
Global mapping of upper mantle reflectors from long-period SS precursors   总被引：1，自引：0，他引：1  
Long-period precursors to SS resulting from underside reflections off upper mantle discontinuities ( SdS where d is the discontinuity depth) can be used to map the global distribution and depth of these reflectors. We analyse 5,884 long-period seismograms from the Global Digital Seismograph Network (1976-1987, shallow sources, transverse component) in order to identify SdS arrivals. Corrections for velocity dispersion, topography and crustal thickness at the SS bounce point, and lateral variation in mantle velocity are critical for obtaining accurate estimates of discontinuity depths. The 410 and 660 km discontinuities are observed at average depths of 413 and 653 km, and exhibit large-scale coherent patterns of topography with depth variations up to 40 km. These patterns are roughly correlated with recent tomographic models, with fast anomalies in the transition zone associated with highs in the 410 km discontinuity and lows in the 660 km discontinuity, a result consistent with laboratory measurements of Clapeyron slopes for the appropriate phase changes. The best resolved feature in these maps is a trough in the 660 km discontinuity in the northwest Pacific, which appears to be associated with the subduction zones in this region. Amplitude variations in SdS arrivals are not correlated with discontinuity depths and probably result from focusing and defocusing effects along the ray paths. The SdS arrivals suggest the presence of regional reflectors in the upper mantle above 400 km. However, only the strongest of these features are above probable noise levels due to sampling inadequacies.  相似文献   

19.
Summary. The rather abrupt changes in velocity gradient which have sometimes been proposed, notably in the upper mantle and near the base of the mantle, have an effect equivalent to that of one or more second-order discontinuities, where partial reflection occurs due to a change in curvature of the wavefront across these discontinuities. The effect is ignored in the classical WKBJ approximation to the wave functions, but it can be explicitly demonstrated by applying the extended WKBJ method (Langer's approximation) to a piecewise smooth layered model. For the purpose of this study it is convenient to represent the response of such a modelby a generalized reflection coefficient. For a model of one or a system of several second-order discontinuities (approximating a change in velocity gradient over a finite depth interval), the reflection coefficient can be perhaps surprisingly large for long-period waves near their turning point. It is shown that this effect can significantly alter the amplitude decay of SH waves diffracted around the core, in models where a change in velocity gradient near the core—mantle boundary constitutes a low-velocity zone at the base of the mantle; such models have recently been proposed. With the same velocity gradients, the effect on P diffraction is less important. The results for SH diffraction in these models support the conclusion that a small amplitude decay must be explained by a velocity decrease with depth, i.e. a low-velocity zone at the base of the mantle.  相似文献   

20.
Summary. Teleseismic P and S arrival times to North American stations are obtained from the ISC bulletins for the 10-yr period 1964–73, and relative travel-time delays are calculated with respect to standard tables. Station anomalies as well as variations of the delays with azimuth and epicentral distance from station are analysed, and the location of the velocity anomalies responsible for them is discussed. Inversion of the P delays to infer upper mantle velocity structure down to a depth of 700 km is obtained using three-dimensional blocks, as proposed by Aki, Christofferson & Husebye. Three layers can be resolved in this depth range. It is found that the heterogeneities responsible for the travel-time delays are primarily located in the first 250 km of the upper mantle, and that they correlate with surface features. Significant heterogeneities subsist to depths of at least 700 km and their broad scale pattern also correlates with the surface features: in the third layer (500 to 700 km depth) there is an increase of velocity from the West to the East of the United States, while the second layer (250 to 450 km depth) exhibits a reversed pattern. A tentative interpretation of these deeper anomalies is made, as being due mainly to topography of the major upper mantle discontinuities, near 400 and 650 km depth.  相似文献   

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