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1.
The conductivity structure of the Earth's mantle was estimated using the induction method down to 2100  km depth for the Europe–Asia region. For this purpose, the responses obtained at seven geomagnetic observatories (IRT, KIV, MOS, NVS, HLP, WIT and NGK) were analysed, together with reliable published results for 11  yr variations. 1-D spherical modelling has shown that, beneath the mid-mantle conductive layer (600–800  km), the conductivity increases slowly from about 1  S  m−1 at 1000  km depth to 10  S  m−1 at 1900  km, while further down (1900–2100  km) this increase is faster. Published models of the lower mantle conductivity obtained using the secular, 30–60  yr variations were also considered, in order to estimate the conductivity at depths down to the core. The new regional model of the lower mantle conductivity does not contradict most modern geoelectrical sounding results. This model supports the idea that the mantle base, situated below 2100  km depth, has a very high conductivity.  相似文献   

2.
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3.
In order to investigate the velocity structure, and hence shed light on the related tectonics, across the Narmada–Son lineament, traveltimes of wide-angle seismic data along the 240 km long Hirapur–Mandla profile in central India have been inverted. A blocky, laterally heterogeneous, three-layer velocity model down to a depth of 10 km has been derived. The first layer shows a maximum thickness of the upper Vindhyans (4.5 km s−1 ) of about 1.35 km and rests on top of normal crystalline basement, represented by the 5.9 km s−1 velocity layer. The anomalous feature of the study is the absence of normal granitic basement in the great Vindhyan Graben, where lower Vindhyan sediments (5.3 km s−1 ) were deposited during the Precambrian on high-velocity (6.3 km s−1 ) metamorphic rock. The block beneath the Narmada–Son lineament represents a horst feature in which high-velocity (6.5 km s−1 ) lower crustal material has risen to a depth of less than 2 km. South of the lineament, the Deccan Traps were deposited on normal basement during the upper Cretaceous period and attained a maximum thickness of about 800 m.  相似文献   

4.
Three heat flow values for south-west England are presented. Two of the sites, Geevor and South Crofty, are operating tin mines on the northern contacts of the Land's End and Carnmenellis Granites, respectively, while the third, Wilsey Down, is a stratigraphical borehole 5 km north of the Bodmin Moor Granite. After applying topographic corrections values of 128·6 mW m-2 (3·07 μ cal cm-2 s-1) for Geevor, 128·9 mW m-2 (3·08 (A cal cm-2 s-1) for South Crofty and 67·3 mW cm-2 (1·61 cal cm-2 s-1) for Wilsey Down, were determined. The value at Wilsey Down is shown to be consistent with that for an environment in which the Hercynian orogeny was the last significant thermal event. An additional heat source term must clearly be involved at Geevor and South Crofty to account for the unusually high values at these sites. Radiogenic heat production has been determined on granites from these sites and in spite of the fact that it is high it does not fully account for the measured heat flow. A compilation of underground temperature measurements made in the nineteenth century suggests that high heat flow is a general feature of the mineralized belt. At least part of this can be explained in terms of hot spring activity recorded widely throughout the area but the ultimate cause remains to be evaluated.  相似文献   

5.
Joint inversion of receiver function and surface wave dispersion observations   总被引：16，自引：0，他引：16  
We implement a method to invert jointly teleseismic P wave receiver functions and surface wave group and phase velocities for a mutually consistent estimate of earth structure. Receiver functions are primarily sensitive to shear wave velocity contrasts and vertical traveltimes, and surface wave dispersion measurements are sensitive to vertical shear wave velocity averages. Their combination may bridge resolution gaps associated with each individual data set. We formulate a linearized shear velocity inversion that is solved using a damped leastsquares scheme that incorporates a priori smoothness constraints for velocities in adjacent layers. The data sets are equalized for the number of data points and physical units in the inversion process. The combination of information produces a relatively simple model with a minimal number of sharp velocity contrasts. We illustrate the approach using noisefree and realistic noise simulations and conclude with an inversion of observations from the Saudi Arabian Shield. Inversion results for station SODA, located in the Arabian Shield, include a crust with a sharp gradient near the surface (shear velocity changing from 1.8 to 3.5 km s1 in 3 km) underlain by a 5kmthick layer with a shear velocity of 3.5 km s1 and a 27kmthick layer with a shear velocity of 3.8 km s1, and an upper mantle with an average shear velocity of 4.7 km s1. The crustmantle transition has a significant gradient, with velocity values varying from 3.8 to 4.7 km s1 between 35 and 40 km depth. Our results are compatible with independent inversions for crustal structure using refraction data.  相似文献   

6.
Summary. We have implemented an algorithm which is based on Bailey's solution of the inverse problem of electromagnetic induction in the Earth. The study was motivated by recent determinations of very long period data and also benefited from recent redeterminations of high frequency data. The algorithm has been successfully tested to provide reliable estimates of conductivity down to a depth of 2000 km, using synthetic data in the period range from 4 days to 11 years. Smooth data sets, which are required for the inversion, were constructed from various sources. At a given depth, the range of inverted models is less than one order of magnitude. Due to the lack of high frequency data, the conductivity of the upper 600 km of the mantle, which is found to be of the order of 10−1Ω−1 m−1, may be overestimated. The algorithm performs well in the middle mantle, where conductivity rises steadily from 1 to 50 Ω−1 m−1. The lack of very low frequency data and limitations of the algorithm prevent one from obtaining meaningful estimates in the lower mantle. However, the study of the propagation of the late 1960s secular variation acceleration provides an estimate of the mean conductivity of the whole mantle. Thus, a complete mantle profile can be constructed. It is found that deep mantle conductivity probability does not exceed a few hundred Ω−1 m−1.  相似文献   

7.
By inversion analysis of the baseline changes and horizontal displacements observed with GPS (Global Positioning System) during 1990–1994, a high-angle reverse fault was detected in the Shikoku-Kinki region, southwest Japan. The active blind fault is characterized by reverse dip-slip (0.7±0.2  m yr−1 within a layer 17–26  km deep) with a length of 208±5  km, a (down-dip) width of 9±2  km, a dip-angle of 51°±2° and a strike direction of 40°±2° (NE). Evidence from the geological investigation of subfaults close to the southwestern portion of the fault, two historical earthquakes ( M L=7.0, 1789 and 6.4, 1955) near the centre of the fault, and an additional inversion analysis of the baseline changes recorded by the nationwide permanent GPS array from 18 January to 31 December 1995 partially demonstrates the existence of the fault, and suggests that it might be a reactivation of a pre-existing fault in this region. The fact that hardly any earthquakes ( M L>2.0) occurred at depth on the inferred fault plane suggests that the fault activity was largely aseismic. Based on the parameters of the blind fault estimated in this study, we evaluated stress changes in this region. It is found that shear stress concentrated and increased by up to 2.1 bar yr−1 at a depth of about 20  km around the epicentral area of the 1995 January 17  Kobe earthquake ( M L=7.2, Japan), and that the earthquake hypocentre received a Coulomb failure stress of about 5.6 bar yr−1 during 1990–1994. The results suggest that the 1995  Kobe earthquake could have been induced or triggered by aseismic fault movement.  相似文献   

8.
Summary. The paper gives the results of a study of the anisotropy of seismic wave velocities within the Ashkhabad test field in Central Asia. The anisotropy was studied by analysing variations in the values of apparent velocities of first arrivals for epicentral distances ranging from 30 to 130 km and by analysing the delays (Δ ts1-s2 ) between the arrival times of shear waves with different polarizations.
The velocities of P -waves vary with azimuth from 5.3 to 6.27 km s-1 and the velocities of S -waves vary from 3.15 to 3.5 km s-1.
The delay times Δ tS1 - S2 depend on the direction of the propagation. The character of the variation of the propagation velocity of the longitudinal wave, the presence of two differently polarized shear waves S 1 and S 2 propagating at different velocities, and the character of the distribution of Δ tS1 - S2 on the stereogram suggest that the symmetry of the anisotropic medium is close to hexagonal with a nearly horizontal symmetry axis coinciding with the direction of maximal velocity. The azimuth of the symmetry axis of the medium is 140° and coincides with the direction of geological faults.  相似文献   

9.
Magnetic fields originating from magnetized crustal rocks dominate the geomagnetic spectrum at wavelengths of 0.1–100 km. It is not known whether the magnetization is predominantly induced or remanent, and static surveys cannot discriminate between the two. Long‐running magnetic observatories offer a chance, in principle, of separating the two sources because secular variation leads to a change in the main inducing field, which in turn causes a change in the induced part of the short‐wavelength crustal field. We first argue that the induced crustal field, b I( t ), is linearly related to the local core field, B ( t ), through a symmetric, trace‐free matrix A : b I( t )= A B ( t ). We then subtract a core field model from the observatory annual means and invert the residuals for three components of the remanent field, b R( t ), and the five independent elements of A . Applying the method to 20 European observatories, all of which have recorded for more than 50 years, shows that the most difficult task is to distinguish b R from the steady part of b I. However, for nine observatories a time‐dependent induced field fits the data better than a steady remanent field at the 99 per cent confidence level, suggesting the presence of a significant induced component to the magnetization.  相似文献   

10.
Seismic anisotropy within the uppermost mantle of southern Germany   总被引：1，自引：0，他引：1  
This paper presents an updated interpretation of seismic anisotropy within the uppermost mantle of southern Germany. The dense network of reversed and crossing refraction profiles in this area made it possible to observe almost 900 traveltimes of the Pn phase that could be effectively used in a time-term analysis to determine horizontal velocity distribution immediately below the Moho. For 12 crossing profiles, amplitude ratios of the Pn phase compared to the dominant crustal phase were utilized to resolve azimuthally dependent velocity gradients with depth. A P -wave anisotropy of 3–4 per cent in a horizontal plane immediately below the Moho at a depth of 30 km, increasing to 11 per cent at a depth of 40 km, was determined. For the axis of the highest velocity of about 8.03 km s−1 at a depth of 30 km a direction of N31°F was obtained. The azimuthal dependence of the observed Pn amplitude is explained by an azimuth-dependent sub-Moho velocity gradient decreasing from 0.06 s−1 in the fast direction to 0 s−1 in the slow direction of horizontal P -wave velocity. From the seismic results in this study a petrological model suggesting a change of modal composition and percentage of oriented olivine with depth was derived.  相似文献   

11.
We use teleseismic three-component digital data from the Trabzon, Turkey broadband seismic station TBZ to model the crustal structure by the receiver function method. The station is located at a structural transition from continental northeastern Anatolia to the oceanic Black Sea basin. Rocks in the region are of volcanic origin covered by young sediments. By forward modelling the radial receiver functions, we construct 1-D crustal shear velocity models that include a lower crustal low-velocity zone, indicating a partial melt mechanism which may be the source of surfacing magmatic rocks and regional volcanism. Within the top 5 km, velocities increase sharply from about 1.5 to 3.5 km s−1. Such near-surface low velocities are caused by sedimentation, extending from the Black Sea basin. Velocities at around 20 km depth have mantle-like values (about 4.25 km s−1 ), which easily correlate to magmatic rocks cropping out on the surface. At 25 km depth there is a thin low-velocity layer of about 4.0 km s−1. The average Moho velocity is about 4.6 km s−1, and its depth changes from 32 to 40 km. Arrivals on the tangential components indicate that the Moho discontinuity dips approximately southwards, in agreement with the crustal thickening to the south. We searched for the solution of receiver functions around the regional surface wave group velocity inversion results, which helped alleviate the multiple solution problem frequently encountered in receiver function modelling.
Station TBZ is a recently deployed broadband seismic station, and the aim of this study is to report on the analysis of new receiver function data. The analysis of new data in such a structurally complex region provides constraining starting models for future structural studies in the region.  相似文献   

12.
The highest intermediate depth moment release rates in Indonesia occur in the slab beneath the largely submerged segment of the Banda arc in the Banda Sea to the east of Roma, termed the Damar Zone. The most active, western-part of this zone is characterized by downdip extension, with moment release rates (∼1018 Nm yr–1 per 50 km strike length) implying the slab is stretching at ∼10−14 s−1 consistent with near complete slab decoupling across the 100–200 km depth range. Differential vertical stretching along the length of the Damar Zone is consistent with a slab rupture front at ∼100–200 km depth beneath Roma propagating eastwards at ∼100 km Myr–1. Complexities in the slab deformation field are revealed by a narrow zone of anomalous in-plane P -axis trends beneath Damar, where subhorizontal constriction suggests extreme stress concentrations ∼100 km ahead of the slab rupture front. Such stress concentrations may explain the anomalously deep ocean gateways in this region, in which case ongoing slab rupture may have played a key role in modulating the Indonesian throughflow in the Banda Sea over the last few million years.  相似文献   

13.
We present velocity constraints for the upper-mantle transition zones beneath Central Siberia based on observations of the 1982 RIFT Deep Seismic Sounding (DSS) profile. The data consist of seismic recordings of a nuclear explosion in north-western Siberia along a 2600 km long seismic profile extending from the Yamal Peninsula to Lake Baikal. We invert seismic data from the mantle transition zones using a non-linear inversion scheme using a genetic algorithm for optimization and the WKBJ method to compute the synthetic seismograms. A statistical error analysis using a graph-binning technique was performed to provide uncertainty values in the velocity models.
Our best model for the upper-mantle velocity discontinuity near 410 km depth has a two-stage velocity-gradient structure, with velocities increasing from 8.70–9.25 km s−1 over a depth range of 400–415 km, a gradient of 0.0433 s−1, and from 9.25–9.60 km s−1 over a depth range of 415–435 km, a gradient of 0.0175 s−1. This derived model is consistent with other seismological observations and mineral-physics models. The model for the velocity discontinuity near 660 km depth is simple, sharp and includes velocities increasing from 10.15 km s−1 at 655 km depth to 10.70 km s−1 at 660 km depth, a gradient of 0.055 s−1.  相似文献   

14.
We present geological and morphological data, combined with an analysis of seismic reflection lines across the Ionian offshore zone and information on historical earthquakes, in order to yield new constraints on active faulting in southeastern Sicily. This region, one of the most seismically active of the Mediterranean, is affected by WNW–ESE regional extension producing normal faulting of the southern edge of the Siculo–Calabrian rift zone. Our data describe two systems of Quaternary normal faults, characterized by different ages and related to distinct tectonic processes. The older NW–SE-trending normal fault segments developed up to ≈400  kyr ago and, striking perpendicular to the main front of the Maghrebian thrust belt, bound the small basins occurring along the eastern coast of the Hyblean Plateau. The younger fault system is represented by prominent NNW–SSE-trending normal fault segments and extends along the Ionian offshore zone following the NE–SW-trending Avola and Rosolini–Ispica normal faults. These faults are characterized by vertical slip rates of 0.7–3.3  mm  yr −1 and might be associated with the large seismic events of January 1693. We suggest that the main shock of the January 1693 earthquakes ( M ~ 7) could be related to a 45  km long normal fault with a right-lateral component of motion. A long-term net slip rate of about 3.7  mm  yr −1 is calculated, and a recurrence interval of about 550 ± 50  yr is proposed for large events similar to that of January 1693.  相似文献   

15.
Inference of mantle viscosity from GRACE and relative sea level data   总被引：12，自引：0，他引：12  
Gravity Recovery And Climate Experiment (GRACE) satellite observations of secular changes in gravity near Hudson Bay, and geological measurements of relative sea level (RSL) changes over the last 10 000 yr in the same region, are used in a Monte Carlo inversion to infer-mantle viscosity structure. The GRACE secular change in gravity shows a significant positive anomaly over a broad region (>3000 km) near Hudson Bay with a maximum of ∼2.5 μGal yr−1 slightly west of Hudson Bay. The pattern of this anomaly is remarkably consistent with that predicted for postglacial rebound using the ICE-5G deglaciation history, strongly suggesting a postglacial rebound origin for the gravity change. We find that the GRACE and RSL data are insensitive to mantle viscosity below 1800 km depth, a conclusion similar to that from previous studies that used only RSL data. For a mantle with homogeneous viscosity, the GRACE and RSL data require a viscosity between  1.4 × 1021  and  2.3 × 1021  Pa s. An inversion for two mantle viscosity layers separated at a depth of 670 km, shows an ensemble of viscosity structures compatible with the data. While the lowest misfit occurs for upper- and lower-mantle viscosities of  5.3 × 1020  and  2.3 × 1021  Pa s, respectively, a weaker upper mantle may be compensated by a stronger lower mantle, such that there exist other models that also provide a reasonable fit to the data. We find that the GRACE and RSL data used in this study cannot resolve more than two layers in the upper 1800 km of the mantle.  相似文献   

16.
The anisotropy of heterogeneity scale lengths in the lower mantle is investigated by modelling its effect on the high-frequency precursors of PKIKP scattered by the heterogeneities. Although models having either an isotropic or an anisotropic distribution of scale lengths can fit the observed coda shapes of short-period precursors, the frequency content of broad-band PKIKP precursors favours a dominantly isotropic distribution of scale lengths. Precursor coda shapes are consistent with 1 per cent fluctuations in P velocity in the wavenumber band 0.05–0.5  km−1 extending to 1000  km above the core–mantle boundary, and with a D" region open to circulation throughout the lower mantle. The level of excitation of PKIKP precursors observed in the frequency band 0.02–2  Hz requires a power spectrum of heterogeneity that is nearly white or slowly increasing with wavenumber. Anisotropy of scale lengths may exist in a D" layer having larger horizontal than vertical scale lengths and produce little or no detectable effects on PKIKP precursors for P -velocity perturbations as high as 3 per cent when averaged over a vertical scale of several kilometres, and much higher when averaged over scales of hundreds of metres or less.  相似文献   

17.
Using the viscoelastic correspondence principle, we utilize the surface coseismic spheroidal deformation fields (i.e. vertical displacements, potential perturbations and gravity changes) of SNREI earth models caused by four typical types of point dislocation, derived by Sun & Okubo (1993 ), to deduce the fundamental formulas for spheroidal fields relevant to viscoelastic earth models. In computations, we employ a strike-slip dislocation on a vertical plane buried at the bottom of the lithosphere to estimate the maximal viscous relaxation responses to this kind of source that possibly exist on the surface of the earth. We take the seismic moment as 1022  N  m, which is characteristic of an average large earthquake. The numerical results demonstrate that, if we take the viscosity as 1019  Pa  s in the asthenosphere, and 1021  Pa  s in the other mantle layers, the rates of surface vertical displacements and gravity changes within about 2.5° for the 10 postseismic years are respectively 1.5–8.1  cm  yr−1 and 4.0–14.9  μgal  yr−1 : the viscous relaxation for this mantle viscosity profile proceeds much faster than for a constant mantle viscosity of 1021  Pa  s.  相似文献   

18.
In the austral summer seasons 2001/02 and 2002/03, Global Positioning System (GPS) data were collected in the vicinity of Vostok Station to determine ice flow velocities over Lake Vostok. Ten GPS sites are located within a radius of 30 km around Vostok Station on floating ice as well as on grounded ice to the east and to the west of the lake. Additionally, a local deformation network around the ice core drilling site 5G-1 was installed.
The derived ice flow velocity for Vostok Station is  2.00 m a−1± 0.01 m a−1  . Along the flowline of Vostok Station an extension rate of about 10−5 a−1 (equivalent to 1 cm km−1 a−1) was determined. This significant velocity gradient results in a new estimate of 28 700 years for the transit time of an ice particle along the Vostok flowline from the bedrock ridge in the southwest of the lake to the eastern shoreline. With these lower velocities compared to earlier studies and, hence, larger transit times the basal accretion rate is estimated to be 4 mm a−1 along a portion of the Vostok flowline. An assessment of the local accretion rate at Vostok Station using the observed geodetic quantities yields an accretion rate in the same order of magnitude. Furthermore, the comparison of our geodetic observations with results inferred from ice-penetrating radar data indicates that the ice flow may not have changed significantly for several thousand years.  相似文献   

19.
We study the crustal structure of eastern Marmara region by applying the receiver function method to the data obtained from the 11 broad-band stations that have been in operation since the 1999 İzmit earthquake. The stacked single-event receiver functions were modelled by an inversion algorithm based on a five-layered crustal velocity model to reveal the first-order shear-velocity discontinuities with a minimum degree of trade-off. We observe crustal thickening from west (29–32 km) to east (34–35 km) along the North Anatolian Fault Zone (NAFZ), but we observe no obvious crustal thickness variation from north to south while crossing the NAFZ. The crust is thinnest beneath station TER (29 km), located near the Black Sea coast in the west and thickest beneath station TAR (35 km), located inland in the southeast. The average crustal thickness and S -wave velocity for the whole regions are  31 ± 2  km and  3.64 ± 0.15 km s−1  , respectively. The eastern Marmara region with its average crustal thickness, high heat flow value (101 ± 11 mW m−2) and with its remarkable extensional features seems to have a Basin and Range type characteristics, but the higher average shear velocities (∼3.64 km s−1) and crustal thickening from 29 to 35 km towards the easternmost stations indicate that the crustal structure shows a transitional tectonic regime. Therefore, we conclude that the eastern Marmara region seems to be a transition zone between the Marmara Sea extensional domain and the continental Anatolian inland region.  相似文献   

20.
Crust and upper mantle structure of the central Iberian Meseta (Spain)   总被引：2，自引：0，他引：2  
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 6.1 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 6.4 km s−1 and overlies a lower crust with a mean P -velocity of 6.9 km s−1 and a possible slight negative gradient; the mean v p/ v s ratio for the crust is about 1.75; the Moho is reached at about 31 km depth and consists of a transition zone at least 1.5 km thick. The P -velocity of the upper mantle is close to 8.1 km s−1 and the S -velocity about 4.5 km s−1, which gives a v p /v s ratio of 1.8 for the uppermost mantle. A tentative petrological interpretation of the velocities and composition of the layers is given.  相似文献   

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