首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 82 毫秒
1.
In young suduction zones we observe steady uplift of island arcs. The steady uplift of island arcs is always accompanied by surface erosion. The long duration of uplift and erosion effectively transports heat at depth to shallower parts by advection. If the rates of uplift and erosion are sufficiently large, such a process of heat transportation will strongly affect thermal structure in subduction zones. First, we quantitatively examine the effects of uplift and erosion on thermal structure by using a simple 1-D heat conduction model, based on the assumption that the initial thermal state is in equilibrium. The results show that temperature increase, Δ T  , due to uplift and erosion can be approximately evaluated by Δ T  = ν e tβ at depth, where ν e is the rate of uplift (erosion), t is the duration of uplift (erosion), and β is the gradient of the geotherm in the initial state. Next, considering the effects of vertical crustal movements such as uplift and erosion in island arcs and subsidence and sedimentation in ocean trenches, in addition to the effects of radioactive heat generation in the crust, frictional heating at plate boundaries and accretion of oceanic sediments to overriding continental plates, we numerically simulate the evolution process of the thermal structure in subduction zones. The result shows that the temperature beneath the island arc gradually increases as a result of uplift and erosion as plate subduction progresses. Near the ocean trench, on the other hand, the low-temperature region gradually expands as a result of sedimentation and accretion in addition to direct cooling by the cold descending slab. The surface heat flow expected from this model is low in fore-arc basins, high in island arcs and moderately high in back-arc regions.  相似文献   

2.
Abstract Simple elastic plate models have been used to determine the stratigraphic patterns that result from prograding sediment loads. The predicted patterns, which include coastal offlap/onlap and downlap in a basinward direction, are generally similar to observations of stratal geometry from Cenozoic sequences of the U.S. Atlantic and Gulf Coast margins. Coastal offlap is a feature of all models in which the water depth and elastic thickness of the lithosphere, T e (which is a measure of the long-term strength of the lithosphere), are held constant, and is caused by a seaward shift in the sediment load and its compensation as progradation proceeds. The coastal offlap pattern is reduced if sediments prograde into a subsiding basin, since subsidence causes an increase in the accommodation space and loading landward of a prograding wedge. The stratal geometry that results is complex, however, and depends on the sediment supply, the amount of subsidence, and T e. If the sediment supply to a subsiding basin proceeds in distinct 'pulses' (due, say, to different tectonic events in a source region) then it is possible to determine the relationship between stratal geometry and T e. Coastal offlap and downlap are features of most models where the lithosphere either has a constant T e slowly increases Te with time, or changes T e laterally; however, in the case where sediments prograde onto lithosphere that rapidly increases T e with rime, the offlap can be replaced by onlap. Lithospheric flexure due to prograding sediment loads is capable of producing a wide variety of stratal geometries and may therefore be an important factor to take into account when evaluating the relative role of tectonics and eustatic sea-level changes in controlling the stratigraphic record.  相似文献   

3.
Summary. The thermal effect of a rapid injection of hot magmas into the lower part of the lithosphere is modelled as an increase in heat production through the invaded region. The change in surface heat flow and the uplift resulting from the thermal expansion are determined in three-dimensional axially symmetric geometry: they are expressed as the space time convolutions of a Green's function with the anomalous heat production.
The anomalies with shorter wavelength (compared to the lithospheric thickness) are attenuated. This filtering affects the surface uplift more than the heat flow anomaly; the attenuation effect is larger when only the lower part of the lithosphere is invaded.
The uplift time constant is of the same order as the heat conduction time if the lower lithosphere is invaded by magmas at a moderate rate (i.e. the rate of injection does not exceed the equivalent of 0.1 per cent of the lithospheric volume in 106yr). Fifty per cent of the total uplift takes place in about 80 × 106yr for a lithosphere 100 km thick. The uplift is slightly faster when the whole lithosphere is invaded. The heat flow anomaly is delayed when the lower part of the lithosphere is invaded.
The spatial extent and the timing of the uplift and heat flow anomalies are critical in determining the mechanism's feasibility. Magma injections explain rapid uplifts [> 100 m (106 yr)−1] only if the magma is supplied at a very high rate (i.e. at least 10 per cent of the lithosphere volume per 106yr). It is a feasible mechanism for uplifts that occur over longer periods of time (≊ 30 × 106yr) such as those that seem to have occurred when the African plate came to rest with respect to the mantle.  相似文献   

4.
The first detailed deep seismic refraction study in the Bransfield Strait, West Antarctica, using sensitive OBSs (ocean bottom seismographs) was carried out successfully during the Antarctic summer of 1990/1991. The experiment focused on the deep crustal structure beneath the axis of the Bransfield Rift. Seismic profile DSS-20 was located exactly in the Bransfield Trough, which is suspected to be a young rift system. Along the profile, five OBSs were deployed at spacings of 50-70 km. 51 shots were fired along the 310 km profile. This paper gives the first presentation of the results. A detailed model of the crustal structure was obtained by modelling the observed traveltimes and amplitudes using a 2-D ray-tracing technique. The uppermost (sedimentary?) cover, with velocities of 2.0-5.5 km s−1, reaches a depth of up to 8 km. Below this, a complex with velocities of 6.4-6.8 km s−1 is observed. The presence of a high-velocity body, with V p= 7.3-7.7 km s−1, was detected in the 14-32 km depth range in the central part of the profile. These inhomogeneities can be interpreted as a stage of back-arc spreading and stretching of the continental crust, coinciding with the Deception-Bridgeman volcanic line. Velocities of 8.1 km s−1, characteristic of the Moho, are observed along the profile at a depth of 30-32 km.  相似文献   

5.
We present a 3-D radially anisotropic S velocity model of the whole mantle (SAW642AN), obtained using a large three component surface and body waveform data set and an iterative inversion for structure and source parameters based on Non-linear Asymptotic Coupling Theory (NACT). The model is parametrized in level 4 spherical splines, which have a spacing of ∼ 8°. The model shows a link between mantle flow and anisotropy in a variety of depth ranges. In the uppermost mantle, we confirm observations of regions with   VSH > VSV   starting at ∼80 km under oceanic regions and ∼200 km under stable continental lithosphere, suggesting horizontal flow beneath the lithosphere. We also observe a   VSV > VSH   signature at ∼150–300 km depth beneath major ridge systems with amplitude correlated with spreading rate for fast-spreading segments. In the transition zone (400–700 km depth), regions of subducted slab material are associated with   VSV > VSH   , while the ridge signal decreases. While the mid-mantle has lower amplitude anisotropy (<1 per cent), we also confirm the observation of radially symmetric   VSH > VSV   in the lowermost 300 km, which appears to be a robust conclusion, despite an error in our previous paper which has been corrected here. The 3-D deviations from this signature are associated with the large-scale low-velocity superplumes under the central Pacific and Africa, suggesting that   VSH > VSV   is generated in the predominant horizontal flow of a mechanical boundary layer, with a change in signature related to transition to upwelling at the superplumes.  相似文献   

6.
A simple new model for sudden lithospheric thinning that considers the crust to be stretched and the lower layer of the lithosphere to be partially stretched and partially mechanically eroded is proposed. This model allows calculation of the thermal field of the lithosphere during the initial warming phase and the surface uplift.
Application of this model to the Tuscan Basin explains the high regional heat flux density values (>100 mW m−2 ), the tectonic subsidence (about 1 km) and the average uplift (>400 m) observed in this region well.  相似文献   

7.
We develop a method for spatio-spectral localization of harmonic data on a sphere and use it to interpret recent high-resolution global estimates of the gravity and topography of Venus in the context of geodynamical models. Our approach applies equally to the simple spatial windowing of harmonic data and to variable-length-scale analyses, which are analogous to a wavelet transform in the Cartesian domain. Using the variable-length-scale approach, we calculate the localized RMS amplitudes of gravity and topography, as well as the spectral admittance between the two fields, as functions of position and wavelength. The observed admittances over 10 per cent of the surface of Venus (highland plateaus and tessera regions) are consistent with isostatic compensation of topography by variations in crustal thickness, while admittances over the remaining 90 per cent of the surface (rises, plains and lowlands) indicate that long-wavelength topography is dominantly the result of vertical convective tractions at the base of the lithosphere. The global average crustal thickness is less than 30 km, but can reach values as large as 40 km beneath tesserae and highland plateaus. We also note that an Earth-like radial viscosity structure cannot be rejected by the gravity and topography data and that, without a mechanical model of the lithosphere, admittance values cannot constrain the thickness of the thermal boundary layer of Venus. Modelling the lithosphere as a thin elastic plate indicates that at the time of formation of relief in highland plateaus and tesserae, the effective elastic plate thickness, Te , was less than 20 km. Estimates of Te at highland rises are consistently less than 30 km. Our inability to find regions with Te > 30 km is inconsistent with predictions made by a class of catastrophic resurfacing models.  相似文献   

8.
A 3-D P -velocity map of the crust and upper mantle beneath the southeastern part of India has been reconstructed through the inversion of teleseismic traveltimes. Salient geological features in the study region include the Archean Dharwar Craton and Eastern Ghat metamorphic belt (EGMB), and the Proterozoic Cuddapah and Godavari basins. The Krishna–Godavari basin, on the eastern coastal margin, evolved in response to the Indo–Antarctica breakup. A 24-station temporary network provided 1161 traveltimes, which were used to model 3-D P -velocity variation. The velocity model accounts of 80 per cent of the observed data variance. The velocity picture to a depth of 120 km shows two patterns: a high velocity beneath the interior domain (Dharwar craton and Cuddapah basin), and a lower velocity beneath the eastern margin region (EGMB and coastal basin). Across the array velocity variations of 7–10 per cent in the crust (0–40 km) and 3–5 per cent in the uppermost mantle (40–120 km) are observed. At deeper levels (120–210 km) the upper-mantle velocity differences are insignificant among different geological units. The presence of such a low velocity along the eastern margin suggests significantly thin lithosphere (<100 km) beneath it compared to a thick lithosphere (>200 km) beneath the eastern Dharwar craton. Such lithospheric thinning could be a consequence of Indo–Antarctica break-up.  相似文献   

9.
The Canary Islands swell: a coherence analysis of bathymetry and gravity   总被引：2，自引：0，他引：2  
The Canary Archipelago is an intraplate volcanic chain, located near the West African continental margin, emplaced on old oceanic lithosphere of Jurassic age, with an extended volcanic activity since Middle Miocene. The adjacent seafloor does not show the broad oceanic swell usually observed in hotspot-generated oceanic islands. However, the observation of a noticeable depth anomaly in the basement west of the Canaries might indicate that the swell is masked by a thick sedimentary cover and the influence of the Canarian volcanism. We use a spectral approach, based on coherence analysis, to determine the swell and its compensation mechanism. The coherence between gravity and topography indicates that the swell is caused by a subsurface load correlated with the surface volcanic load. The residual gravity/geoid anomaly indicates that the subsurface load extends 600 km SSW and 800 km N and NNE of the islands. We used computed depth anomalies from available deep seismic profiles to constrain the extent and amplitude of the basement uplift caused by a relatively low-density anomaly within the lithospheric mantle, and coherence analysis to constrain the elastic thickness of the lithosphere ( Te ) and the compensation depth of the swell. Depth anomalies and coherence are well simulated with Te =28–36 km, compensation depth of 40–65 km, and a negative density contrast within the lithosphere of ∼33 kg m−3. The density contrast corresponds to a temperature increment of ∼325°C, which we interpret to be partially maintained by a low-viscosity convective layer in the lowermost lithosphere, and which probably involves the shallower parts of the asthenosphere. This interpretation does not require a significant rejuvenation of the mechanical properties of the lithosphere.  相似文献   

10.
The inversion of high-resolution geoid anomaly maps derived from satellite altimetry should allow one to retrieve the lithospheric elastic thickness, T e , and crustal density, c . Indeed, the bending of a lithospheric plate under the load of a seamount depends on both parameters, and the associated geoid anomaly is correspondingly dependent on the two parameters. The difference between the observed and modelled geoid signatures is estimated by a cost function, J , of the two variables, T e and c . We show that this cost function forms a valley structure along which many local minima appear, the global minimum of J corresponding to the true values of the lithospheric parameters. Classical gradient methods fail to find this global minimum because they converge to the first local minimum of J encountered, so that the final parameter estimate strongly depends on the starting pair of values ( T e ,   c ). We here implement a non-linear optimization algorithm to recover these two parameters from altimetry data. We demonstrate from the inversion of synthetic data that this approach ensures robust estimates of T e and c by activating two search phases alternately: a gradient phase to find a local minimum of J , and a tunnelling phase through high values of the cost function. The accuracy of the solution can be improved by a search in an iteratively restricted parameter subspace. Applying our non-linear inversion to the Great Meteor Seamount geoid data, we further show that the inverse problem is intrinsically ill-posed. As a consequence, minute geoid (or gravity) data errors can induce large changes in any recovery of lithospheric elastic thickness and crustal density.  相似文献   

11.
We have analysed the fundamental mode of Love and Rayleigh waves generated by 12 earthquakes located in the mid-Atlantic ridge and Jan Mayen fracture zone. Using the multiple filter analysis technique, we isolated the Rayleigh and Love wave group velocities for periods between 10 and 50  s. The surface wave propagation paths were divided into five groups, and average group velocities calculated for each group. The average group velocities were inverted and produced shear wave velocity models that correspond to a quasi-continental oceanic structure in the Greenland–Norwegian Sea region. Although resolution is poor at shallow depth, we obtained crustal thickness values of about 18  km in the Norwegian Sea area and 9  km in the region between Svalbard and Iceland. The abnormally thick crust in the Norwegian Sea area is ascribed to magmatic underplating and the thermal blanketing effect of sedimentary layers. Maximum crustal shear velocities vary between 3.5 and 3.9  km  s−1 for most paths. An average lithospheric thickness of 60  km was observed, which is lower than expected for oceanic-type structure of similar age. We also observed low shear wave velocities in the lower crust and upper mantle. We suggest that high heat flow extending to depths of about 30  km beneath the surface can account for the thin lithosphere and observed low velocities. Anisotropy coefficients of 1–5 per cent in the shallow layers and >7 per cent in the upper mantle point to the existence of polarization anisotropy in the region.  相似文献   

12.
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   

13.
The McKenzie model proposed in 1978, which is widely used in calculating the thermal history of rift basins and other extensional basins, incorrectly assumes that all heat passing through the lithosphere originates below the lithosphere. In reality, heat from radiogenic sources within the lithosphere, especially in the upper crust, may represent more than half the heat flow at the top of basement. Thinning of the lithosphere during extension does indeed result in an increase of heat flowing from the asthenosphere, but this thinning also reduces the radiogenic heat from within the lithosphere. Because these two effects cancel to a large degree, the direct effects of lithospheric extension on heat flow at the top of basement are smaller than those predicted by the McKenzie model. Because of permanent loss of radiogenic material by lithospheric thinning, the heat flow at the top of basement long after rifting will be lower than the pre-rift heat flow.The McKenzie model predicts an instantaneous increase in heat flow during rifting. The Morgan model proposed in 1983, however, predicts a substantial time delay in the arrival of the higher heat flow from the asthenosphere at the top of basement or within sediments. Using the Morgan model, heat flow during the early stages of rifting will actually be lower than prior to rifting, because the time delay in the loss of radiogenic heat is less than the time delay in arrival of new heat from the asthenosphere.  相似文献   

14.
Classical models of lithosphere thinning predict deep synrift basins covered by wider and thinner post‐rift deposits. However, synextensional uplift and/or erosion of the crust are widely documented in nature (e.g. the Base Cretaceous unconformity of the NE Atlantic), and generally the post‐rift deposits dominate basins fills. Accordingly, several basin models focus on this discrepancy between observations and the classical approach. These models either involve differential thinning, where the mantle thins more than the crust thereby increasing average temperature of the lithosphere, or focus on the effect of metamorphic reactions, showing that such reactions decrease the density of lithospheric rocks. Both approaches result in less synrift subsidence and increased post‐rift subsidence. The synextensional uplift in these two approaches happens only for special cases, that is for a case of initially thin crust, specific mineral assemblage of the lithospheric mantle or extensive differential thinning of the lithosphere. Here, we analyse the effects of shear heating and tectonic underpressure on the evolution of sedimentary basins. In simple 1D models, we test the implications of various mechanisms in regard to uplift, subsidence, density variations and thermal history. Our numerical experiments show that tectonic underpressure during lithospheric thinning combined with pressure‐dependent density is a widely applicable mechanism for synextensional uplift. Mineral phase transitions in the subcrustal lithosphere amplify the effect of underpressure and may result in more than 1 km of synextensional erosion. Additional heat from shear heating, especially combined with mineral phase transitions and differential thinning of the lithosphere, greatly decreases the amount of synrift deposits.  相似文献   

15.
We report source parameters for eight earthquakes in East Africa obtained using a number of techniques, including (1) inversion of long-period P and SH waves for moment tensors and source-time functions, (2) forward modelling of first-motion polarities and P and pP amplitudes on short-period seismograms, and (3) determination of pP-P and sP-P differential traveltimes from short-period records. The foci of these earthquakes lie between depths of 24 and 34 km in Archean and Proterozoic lithosphere, and all but one fault-plane solution indicates normal faulting (primarily E-W extension), consistent with the regional stress regime in East Africa. Because many of these earthquakes occurred in areas where the crust may have been thinned by rifting, it is difficult to ascertain whether or not their foci lie within the lower crust or upper mantle. Some of them, however, occurred away from rift structures in Proterozoic crust that is possibly 35–40 km thick or thicker, and thus they probably nucleated within the lower crust. Strength profile calculations suggest that in order to account for seismogenic (i.e. brittle) behaviour at sufficient depths to explain lower crustal earthquakes in East Africa, the lower crust must not only be composed of mafic lithologies, as suggested by previous investigators, but also that significantly more heat (∼100 per cent) must come from the upper crust than predicted by the crustal heat source distribution obtained from a 1-D interpretation of the linear relationship between heat flow and heat production observed in Proterozoic terrains within eastern and southern Africa. Precambrian mafic dike swarms throughout East Africa provide evidence for magmatic events which could have delivered large amounts of mafic material to the lower crust over a very broad area, thus explaining why the lower crust in East Africa might be mafic away from the volcanogenic rift valleys.  相似文献   

16.
Summary. The deep structure of the Faeroe–Shetland Channel has been investigated as part of the North Atlantic Seismic Project. Shot lines were fired along and across the axis of the Channel, with recording stations both at sea and on adjacent land areas. At 61°N, 1.7 km of Tertiary sediments overlies a 3.9–4.5 km s-1 basement interpreted as the top of early Tertiary volcanics. A main 6.0–6.6 km s-1 crustal refractor interpreted as old oceanic crust occurs at about 9 km depth. The Moho (8.0 ° 0.2 km s-1) is at about 15–17 km depth. There is evidence that P n may be anisotropic beneath the Faeroe–Shetland Channel. Arrivals recorded at land stations show characteristics best explained by scattering at an intervening boundary which may be the continent–ocean crustal contact or the edge of the volcanics.
The Moho delay times at the shot points, determined by time-term analysis, show considerable variation along the axis of the Channel. They correlate with the basement topography, and the greatest delays occur over the buried extension of the Faeroe Ridge at about 60° 15'N, where they are nearly 1 s more than the delays at 61°N after correction for the sediments. The large delays are attributed to thickening of the early Tertiary volcanic layer with isostatic downsagging of the underlying crust and uppermost mantle in response to the load, rather than to thickening of the main crustal ayer.
The new evidence is consistent with deeply buried oceanic crust beneath the Faeroe–Shetland Channel, forming a northern extension of Rockall Trough. The seabed morphology has been grossly modified by the thick and laterally variable pile of early Tertiary volcanic rocks which swamped the region, accounting for the anomalous shallow bathymetry, the transverse ridges and the present narrowness of the Channel.  相似文献   

17.
An Mw 5.9 earthquake occurred in the Lake Rukwa rift, Tanzania, on 1994 August 18, and was well recorded by 20 broad-band seismic stations at distances of 160 to 800 km and 21 broad-band stations at teleseismic distances. The regional and teleseismic waveforms have been used to investigate the source characteristics of the main shock, and also to locate aftershocks that occurred within three weeks of the main shock. Teleseismic body-wave modelling yields the following source parameters for the main shock: source depth of 25 ± 2 km, a normal fault orientation, with a horizontal tension axis striking NE-SW and an almost vertical pressure axis (Nodal Plane I: strike 126°–142°, dip 63°–66°, and rake 280°–290°; Nodal Plane II: strike 273°–289°, dip 28°–31°, and rake 235°–245°), a scalar moment of 4.1 times 1017 N m, and a 2 s impulsive source time function. Four of the largest aftershocks also nucleated at depths of 25 km, as deduced from regional sPmp–Pmp times. The nodal planes are broadly consistent with the orientation of both the Lupa and Ufipa faults, which bound the Rukwa rift to the northeast and southwest, respectively. The rupture radius of the main shock, assuming a circular fault, is estimated to be 4 km with a corresponding stress drop of 6.5 MPa. Published estimates of crustal thickness beneath the Rukwa rift indicate that the foci of the main shock and aftershocks lie at least 10 km above the Moho. The presence of lower-crustal seismicity beneath the Rukwa rift suggests that the pre-rift thermal structure of the rifted crust has not been strongly modified by the rifting, at least to depths of 25 km.  相似文献   

18.
The onshore crustal and upper mantle velocity structure of the British Isles has been investigated by teleseismic receiver function analysis. The results of the study augment the dense offshore and sparse onshore models of the velocity structure beneath the area. In total almost 1500 receiver functions have been analysed, which have been calculated using teleseismic data from 34 broadband and short-period, three-component seismic recording instruments. The crustal structure has primarily been investigated using 1-D grid search and forward modelling techniques, returning crustal thicknesses, bulk crustal Vp / Vs ratio and velocity-depth models. H −κ stacking reveals crustal thicknesses between 25 and 36 km and Vp / Vs ratios between 1.6 and 1.9. The crustal thicknesses correlate with the results of previous seismic reflection and refraction profiles to within ±2 km. The significant exceptions are the stations close to the Iapetus Suture where the receiver function crustal thicknesses are up to 5 km less than the seismic refraction Moho. This mismatch could be linked to the presence of underplated magmatic material at the base of the crust. 1-D forward modelling has revealed subcrustal structures in northern Scotland. These correlate with results from other UK receiver function studies, and correspond with the Flannan and W-reflectors. The structures are truncated or pinch out before they reach the Midland Valley of Scotland. The isolated subcrustal structure at station GIM on the Isle of Man may be related to the closure of the Iapetus Ocean.  相似文献   

19.
Summary. Closely spaced refraction profiling across the Whipple Mountains metamorphic core complex in southeastern California yields a complex picture of crustal structure in this region of large continental extension. A NE-directed profile, parallel to the extension direction, reveals a high-velocity mid-crustal layer (6.6–6.8 km s−1) at 16-18 km depth, bounded above and below by laterally discontinuous low-velocity zones (<6.0 km s−1). In marked contrast, a NW-directed profile shows a more uniform 6.0 km s−1 crust down to the crust-mantle boundary. The apparent contrast between these two perpendicular profiles may be related not only to a more complex geologic structure in the NW-SE direction, but also to velocity anisotropy associated with mid-crustal mylonites. Despite the differences between the two refraction profiles, both define a flat Moho at 26-27 km depth with an associated upper mantle-velocity of 7.8 km s−1. This observation is significant as it suggests that, although the amount of extension has been highly variable regionally, the crust is no thinner beneath the Whipple Mountains (where extension has been extreme) than the surrounding mountain ranges. Such an observation requires either that the crust was considerably thicker prior to extension, or that lateral flow in the lower crust and/or inflation of the crust via magmatism occurred contemporaneous with extension.  相似文献   

20.
We investigate the particle orbits of long-period (about 20 s) P waves observed with the global seismic network. By analysing 84 three-component seismograms recorded at 25 stations from 60 earthquakes occurring beneath 300 km, we quantitatively evaluate the orbits by three sets of eigenvalues and eigenvectors, using a covariance matrix method. The eigenvalues for P waves recorded at stations located on continents are explained by the standard horizontal layered structure model (iasp91). On the other hand, the orbits observed at stations close to island arcs are affected not only by the horizontal layered structure but also by heterogeneity due to subducting plates, mantle diapirs and so on. On the basis of a single-scattering model for a plane P wave, we quantify the heterogeneities by an isotropic scattering coefficient g0. Fitting the theoretical eigenvalues to the observed ones, we estimate g0 for the crust and upper mantle beneath continents to be less than 0.0005 km-1, and the mean g0 for the structure beneath island arcs to be about 0.0015 to 0.003 km-1.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号