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1.
The 2-D shallow velocity structure along the north-south Palashi-Kandi profile in the West Bengal sedimentary basin has been updated by travel-time inversion of seismic refraction, wide-angle reflection and gravity data. A six-layer shallow model up to a depth of about 7 km has been derived. The first layer, which has an average velocity of 2.0 kms?1, represents the alluvium deposit, which rests over the shale formation with average velocity of 3.0 kms?1. The thin (200 m) Sylhet limestone, observed at a nearby Palashi well, remains hidden in the present data set. Hence a 200-m thin layer with a velocity of 3.7 kms?1, corresponding to the Sylhet limestone, has been assumed to be present throughout the profile. The fourth layer with a velocity of 4.5–4.7 kms?1 at a depth of 1.7–2.4 km represents the Rajmahal traps. The ‘skip’ phenomenon and rapid amplitude decay of first arrivals indicate a low-velocity zone (LVZ) in the study area. Using the ‘skip’ phenomena and wide-angle reflection data, identified on seismograms, the LVZ with a velocity of 4.0 kms?1, indicating the Gondwana sediments, has been delineated below the Rajmahal traps. The next layer with a velocity 5.4–5.6 kms?1 overlying the crystalline basement (5.8–6.25 kms?1) may be associated with the Singhbhum group of meta volcanic rock that has been exposed in the western part of the basin. The basement lies at a variable depth of 4.9 to 6.8 km. The overall uncertainties of various velocity and boundary nodes are ± 0.15 kms?1 and ± 0.5 km, respectively. The elevated basement feature in the north might have acted as a structural barrier for the deposition of Sylhet limestone during the Eocene epoch. The seismically derived shallow structure correctly explains the observed Bouguer gravity anomaly along the profile. The addition of reflections in the present analysis provides a stronger control on the depths and velocities of basement and overlying sedimentary formations, compared to the earlier model derived mainly by the first arrival seismic data.  相似文献   

2.
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3.
The transitional area between the northeastern margin of the Qinghai-Tibetan Plateau, Ordos Block and Alxa Block,also being the northern segment of the North-South Seismic Belt, is characterized by considerably high seismicity level and high risk of strong earthquakes. In view of the special tectonic environment and deep tectonic setting in this area, this study used two seismic wide-angle reflection/refraction cross profiles for double constraining, so as to more reliably obtain the fine-scale velocity structure characteristics in both the shallow and deep crust of individual blocks and their boundaries in the study area,and further discuss the seismogenic environment in seismic zones with strong historical earthquakes. In this paper, the P-wave data from the two profiles are processed and interpreted, and two-dimensional crustal velocity structure models along the two profiles are constructed by travel time forward modeling. The results show that there are great differences in velocity structure,shape of intra-crustal interfaces and crustal thickness among different blocks sampled by the two seismic profiles. The crustal thickness along the Lanzhou-Huianbu-Yulin seismic sounding profile(L1) increases from ~43 km in the western margin of Ordos Block to ~56 km in the Qilian Block to the west. In the Ordos Block, the velocity contours vary gently, and the average velocity of the crust is about 6.30 km s-1; On the other hand, the velocity structures in the crust of the Qilian Block and the arclike tectonic zone vary dramatically, and the average crustal velocities in these areas are about 0.10 km s-1 lower than that of the Ordos Block. In addition, discontinuous low-velocity bodies(LVZ1 and LVZ2) are identified in the crust of the Qilian Block and the arc-like tectonic zone, the velocity of which is 0.10–0.20 km s-1 lower than that of the surroundings. The average crustal thickness of the Ordos Block is consistently estimated to be around 43 km along both Profile L2(Tongchuan-Huianbu-Alashan left banner seismic sounding profile) and Profile L1. In contrast to the gently varying intra-crustal interfaces and velocity contours in the Ordos Block along Profile L1, which is a typical structure characteristic of stable cratons, the crustal structure in the Ordos Block along Profile L2 exhibits rather complex variations. This indicates the presence of significant structural differences in the crust within the Ordos Block. The crustal structure of the Helan Mountain Qilian Block and the Yinchuan Basin is featured by "uplift and depression" undulations, showing the characteristics of localized compressional deformation.Moreover, there are low-velocity zones with alternative high and low velocities in the middle and lower crust beneath the Helan Mountain, where the velocity is about 0.15–0.25 km s-1 lower than that of the surrounding areas. The crustal thickness of the Alxa Block is about 49 km, and the velocity contours in the upper and middle-lower crust of the block vary significantly. The complex crustal velocity structure images along the two seismic sounding profiles L1 and L2 reveal considerable structural differences among different tectonic blocks, their coupling relationships and velocity structural features in the seismic zones where strong historical earthquakes occurred. The imaging result of this study provides fine-scale crustal structure information for further understanding the seismogenic environment and mechanism in the study area.  相似文献   

4.
西沙地块地壳结构及其构造属性   总被引：4，自引：3，他引：1       下载免费PDF全文
西沙地块作为在南海形成演化过程中形成的微陆块,记录了南海演化历史的重要信息,其地壳结构、物质组成及构造属性是探讨南海形成演化的关键.基于采集到的OBS2013-3测线海底地震仪数据,用射线追踪和正演走时拟合方法,获得了西沙地块的二维纵波速度模型.模型显示沉积层速度为2.2~3.2km·s-1,厚度为0.8~3.0km,局部基底面起伏较大,上地壳顶部速度为5.0~5.5km·s-1,下地壳底部速度为6.9km·s-1,上地幔顶部速度为8.0km·s-1.西沙地块的地壳厚度平均为23km,上地壳厚度约为9km,下地壳厚度约为14km,莫霍面埋深为23~27km.从穿过西沙地块的纵、横两条大剖面推算,块体大小约为9.2×105 km3,与华南陆缘相比,表现为整体减薄的陆壳特征.西沙地块与南沙地块垂直于西南次海盆扩张脊分布,根据二者地壳结构的特征对比,二者互为共轭关系.  相似文献   

5.
The 2-D crustal velocity model along the Hirapur-Mandla DSS profile across the Narmada-Son lineament in central India (Murty et al., 1998) has been updated based on the analysis of some short and discontinuous seismic wide-angle reflection phases. Three layers, with seismic velocities of 6.5–6.7, 6.35–6.40 and 6.8 km s–1, and upper boundaries located approximately at 8, 17 and 22 km depth respectively, have been identified between the basement (velocity 5.9 km s–1) and the uppermost mantle (velocity 7.8 km s–1). The layer with 6.5–6.7 km s–1 velocity is thin and is less than 2-km deep between the Narmada north (at Katangi) and south (at Jabalpur) faults. The upper crust shows a horst feature between these faults, which indicates that the Narmada zone acts as a ridge between two pockets of mafic intrusion in the upper crust. The Moho boundary, at 40–44 km depth and the intra-crustal layers exhibit an upwarp suggesting that the Narmada faults have deep origins, involving deep-seated tectonics. A smaller intrusive thickness between the Narmada faults, as compared to those beyond these faults, suggests that the intrusive activities on the two sides are independent. This further suggests that the two Narmada faults may have been active at different geological times. The seismic model is constrained by 2-D gravity modeling. The gravity highs on either side of the Narmada zone are due to the effect of the high velocity/high density mafic intrusion at upper crustal level.  相似文献   

6.
The subsurface information gathered during exploration for oil and gas in the Cambay basin shows it as a deep graben with 5 km or more of Tertiary and Quaternary sediments resting on the Deccan Trap floor. The Trap floor of this graben extends from Lat. 24° N to about Lat. 19° N and possibly further south. The basin is divisible into separate morphotectonic blocks as a result of block differentiation in the Trap basement, reflected in the structural attitudes of the overlying sediments. This differentiation is believed to have originated in the Paleocene. The dominant structural grain of the area to south of the Narmada river is ENE-WSW with block faulting in the Traps along the older Satpura trend. North of the Narmada river, the trend is longitudinal upto the Meshwa river while further north the trends veer to a NNW-SSE alignment. These latter trends, in the greater part of the Cambay basin, were impressed early during its subsidence and are the result of reactivation along the old Dharwarian trends in post-Delhi times. Maximum thickness of the Traps penetrated so far is near Mechsana and Cambay where more than 1000 meters thickness has been drilled through. The drilling and gravity-magnetic evidence shows the thickness of Deccan Traps in this trough to be of the order of 2.5 km and points to the possibility of active subsidence of Cambay basin, concomitant with the outpouring of the basaltic lavas. The age of the Traps in the Cambay basin, as evidenced by the available data, is Upper Crealaceous. The influence of the structural grain of the basaltic floor on the overlying sedimentary sequence is evidenced during all the stages in the evolution of the Cambay Tertiary basin. Conglomerates, wackes and reddish brown clays of exclusive Trap derivation predominate in the sedimentary section in the initial stage of the basin evolution during Paleocene. General absence of well developed terrigenous reservoirs on a regional scale in the Paleogene section is due to predominance of Trap terrain as the provenance of clastic detritus, contributing essentially argillaceous matter.  相似文献   

7.
Seismic refraction and near earthquake data of the U.S. Geological Survey for central California have been compiled into record sections along profiles and interpreted in terms of crustal structure. The profiles are located northeast of the San Andreas fault of central California and run parallel to the general structures. For the explosion seismic line through the centre of the Diablo Range, an uppermost layer (Franciscan formation) with P velocities of 3.6–5.0 km s?1 decreases in thickness towards the northwest. The lower boundaries of layers with constant velocities of 5.75 and 6.8 km s?1 are found at almost constant depths of 12 and 21 km, respectively. Between 21 and 26 km depth a well-defined low-velocity zone appears whose velocity is estimated as ～ 5.3 km s?1 with the aid of a hedgehog inversion and the calculation of amplitudes. This zone is underlain by a layer 3–5 km thick with a velocity of 7.6 km s?1. The upper-mantle velocity beneath the Moho at 29–30 km depth is 8.2 km s?1. The near earthquake profiles, located ～ 20 km southwest and parallel to the explosion seismic line, follow more or less the Hayward and Calaveras fault systems. The velocity-depth distribution derived for the earthquake data is very similar to that found beneath the Diablo Range. However, the low-velocity zone at 21–26 km depth does not seem to exist everywhere along the line. The Moho is not disturbed beneath the Calaveras, Hayward and Silver Creek faults; it rises slightly from the Diablo Range towards the southwest.  相似文献   

8.
Long-period PKP amplitudes from 16 earthquakes in the distance range 110– 170° are compared with theoretical amplitudes which are derived from synthetic seismograms calculated for 56 systematic modifications of Earth model 1066B in the inner core. A suitable normalization procedure allows for the common representation of all observed amplitudes as a function of epicentral distance. Using the theoretical amplitude distributions it can be shown that the parameters of a regression line through the logarithmic and normalized amplitudes between 110 and 134° are related to the velocity and density jump at the inner-core boundary (ICB). The analysis shows that the dominant influence on the PKP amplitudes is the P-velocity jump which can be restricted to 0.64 ± 0.05 km s?1. There exists a trade-off between the S-velocity jump and the density jump. Restricting the latter to the reasonable range 0–1.2 g cm?3 the S-velocity jump at the ICB can be inferred to be 2.5–3.0 km s?1. A rather strong S-velocity gradient below the ICB follows from the condition that the S-wave travel-time through the inner core agrees with that implied by free oscillation observations. This leads to central S-velocities between 3.81 and 4.15 km s?1, assuming a parabolic velocity law.  相似文献   

9.
The Narmada–Son Lineament (NSL) Zone is the second most important tectonic feature after Himalayas, in the Indian geology. Magnetotelluric (MT) studies were carried out in the NSL zone along a 130 km long NNE-SSW trending profile. The area of investigation extends from Edlabad (20°46′16″; 75°59′05″) in the South to Khandwa (21°53′51″; 76°18′05″) in the North. The data shows in general the validity of a two-dimensional (2D) approach. Besides providing details on the shallow crustal section, the 2D modeling results resolved four high conductive zones extending from the middle to deep crust, spatially coinciding with the major structural features in the area namely the Gavligarh, Tapti, Barwani-Sukta and Narmada South faults. The model for the shallow section has brought out a moderately resistive layer (30–150 Ω m) representing the exposed Deccan trap layer, overlying a conductive layer (10–30 Ω m) inferred to be the subtrappean Gondwana sediments, the latter resting on a high resistive basement/upper crust. The Deccan trap thickness varies from around a few hundred meters to as much as 1.5 km along the traverse. A subtrappean sedimentary basin like feature is delineated in the northern half of the traverse where a sudden thickening of subtrappean sediments amounting to as much as 2 km is noticed. The high resistive upper crust is relatively thick towards the southern end and tends to become thinner towards the middle and northern part of the traverse. The lower crustal segment is conductive over a major part of the profile. Considering the generally enhanced heat flow values in the NSL region, coupled with characteristic gravity highs and enhanced seismic velocities coinciding with the mid to lower crustal conductors delineated from MT, presence of zones of high density mafic bodies/intrusives with fluids, presumably associated with magmatic underplating of the crust in the zone of major tectonic faults in NSL region are inferred.  相似文献   

10.
We have studied the lateral velocity variations along a partly buried inverted paleo–rift in Central Lapland, Northern Europe with a 2D wide-angle reflection and refraction experiment, HUKKA 2007. The experiment was designed to use seven chemical explosions from commercial and military sites as sources of seismic energy. The shots were recorded by 102 stations with an average spacing of 3.45 km. Two-dimensional crustal models of variations in P-wave velocity and Vp/Vs-ratio were calculated using the ray tracing forward modeling technique. The HUKKA 2007 experiment comprises a 455 km long profile that runs NNW–SSE parallel to the Kittilä Shear Zone, a major deformation zone hosting gold deposits in the area. The profile crosses Paleoproterozoic and reactivated Archean terranes of Central Lapland. The velocity model shows a significant difference in crustal velocity structure between the northern (distances 0–120 km) and southern parts of the profile. The difference in P-wave velocities and Vp/Vs ratio can be followed through the whole crust down to the Moho boundary indicating major tectonic boundaries. Upper crustal velocities seem to vary with the terranes/compositional differences mapped at the surface. The lower layer of the upper crust displays velocities of 6.0–6.1 km/s. Both Paleoproterozoic and Archean terranes are associated with high velocity bodies (6.30–6.35 km/s) at 100 and 200–350 km distances. The Central Lapland greenstone belt and Central Lapland Granitoid complex are associated with a 4 km-thick zone of unusually low velocities (<6.0 km/s) at distances between 120 and 220 km. We interpret the HUKKA 2007 profile to image an old, partly buried, inverted continental rift zone that has been closed and modified by younger tectonic events. It has structural features typical of rifts: inward dipping rift shoulders, undulating thickness of the middle crust, high velocity lower crust and a rather uniform crustal thickness of 48 km.  相似文献   

11.
During the Pamir Himalayan project in the year 1975 seismic refraction and wide-angle reflection data were recorded along a 270 km long Lawrencepur-Astor (Sango Sar) profile in the northwest Himalayas. The profile starts in the Indus plains and crosses the Main Central Thrust (MCT), the Hazara Syntaxis, the Main Mantle Thrust (MMT) and ends to the east of Nanga Parbat. The seismic data, as published by Guerra et al. (1983), are reinterpreted using the travel-time ray inversion method of Zelt and Smith (1992) and the results of inversion are constrained in terms of parameter resolution and uncertainty estimation. The present model shows that the High Himalayan Crystallines (HHC, velocity 5.4 km s−1) overlie the Indian basement (velocity 5.8–6.0 km s−1). The crust consists of four layers of velocity 5.8–6.0, 6.2, 6.4 and 6.8 km s−1 followed by the upper mantle velocity of 8.2 km s−1 at a depth of about 60 km.  相似文献   

12.
The paper presents a review and analysis of new seismic data related to the structure of the mantle beneath the East European platform. Analysis of observations of long-range profiles revealed pronounced differences in the structure of the lower lithosphere beneath the Russian plate and the North Caspian coastal depression. The highest P-velocities found at depths around 100 km are in the range 8.4–8.5 km s?1. Deep structure of the Baltic shield is different from the structures of both these regions. No evidence of azimuthal anisotropy in the upper mantle was found. A distribution of P-velocity in the upper mantle and in the transition zone consistent with accurate travel-time data was determined. The model involves several zones of small and large positive velocity gradients in the upper mantle, rapid increases of velocity near 400 and 640 km depths and an almost constant positive velocity gradient between the 400 and 640 km discontinuities. The depth of the 640 km discontinuity was determined from observations of waves converted from P to SV in the mantle.  相似文献   

13.
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14.
In order to investigate crustal structure beneath the eastern Marmara region, a seismic refraction survey was conducted across the North Anatolian Fault (NAF) zone in north west Turkey. Two reversed profiles across two strands of the NAF zone were recorded in the Armutlu Highland where a tectonically active region was formed by different continents. We used land explosions in boreholes and quarry blasts as seismic sources. A reliable crustal velocity and depth model is obtained from the inversion of first arrival travel times. The velocity-depth model will improve the positioning of the earthquake activities in this active portion of the NAF. A high velocity anomaly (5.6–5.8 km s−1) in the central highland of Armutlu block and the low velocity (4.90 km s−1) pattern north of Iznik Lake are the two dominant features. The crustal thickness is about 26 ± 2 km in the north and increases to about 32 ± 2 km beneath the central Armutlu block in the south. P-wave velocities are about 3.95 km s−1 to 4.70 km s−1 for the depth range between about 1 km and 5 km in the upper crust. The eastern Marmara region has different units of upper crust with velocities varying with depth to almost 8 km. The high upper crust velocities are associated with Armutlu metamorphic rocks, while the low velocity anomalies are due to unconsolidated sedimentary sequences. The western side of Armutlu block has complex tectonics and is well known for geothermal sources. If these sources are continuous throughout the portions of the crust, it may be associated with a granitic intrusion and deformation along the NAF zone. That is, the geothermal sources associated with the low velocity may be due to the occurrence of widespread shear heating, even shear melting. The presence of shear melting may indicate the presence of crustal fluid imposed by two blocks of the NAF system.  相似文献   

15.
Long-range seismic sounding carried out during the last few years on the territory of the U.S.S.R. has shown a basic inhomogeneity of the uppermost mantle, as well as evidence of regularities in the distribution of its seismic parameters. The following data were used: times and apparent velocities of P- and S-waves for investigation of mantle velocities, converted waves for seismic discontinuity model studies and wave attenuation for Q-factor estimation. Strong regularities were distinguished in the distribution of average seismic velocities for the uppermost mantle, in their dependence on the age and type of geostructure and on their position relative to the central part of the continent. Old platforms and the inner part of the continent are marked by velocities under the Mohorovi?i? discontinuity of more than 8.2–8.3 km s?1, young platforms and outer parts of the continent by 8.0–8.2 km s?1, and orogenic and rift zones by 7.8–8.0 km s?1. The difference becomes more pronounced at a depth of about 100–200 km: for the old platform mantle velocities of 8.5–8.6 km s?1 are typical; beneath the orogenic and rift areas, inversion zones with velocities less than 7.8 km s?1 are observed.The converted waves show fine inhomogeneities of the crust and uppermost mantle, the presence of many discontinuities with positive and negative changes of velocity, and anisotropy of seismic waves in some of the layers. Wave attenuation allowed the determination of the Q-factor in the mantle. It varied from one region to another but a close relation between Q and P-wave velocity is the main cause of its variation.  相似文献   

16.
The Sanjiang area in southwest China is considered as a tectonic intersection belt between the Tethys-Alps and the western Pacific, and has endured three-phase evolution processes: Proto-Tethys,Paleo-Tethys and Meso-Tethys[1―4]. In this area, its tectonics and struc- ture are extremely complicated, and intensively extru-sive deformation and faults are widely developed[1―3]. For that, the area is considered as the ideal na- ture-laboratory to study the evolution of Paleo-Tethys and also …  相似文献   

17.
Several long-range explosion seismology experiments have been conducted in the northwestern Pacific basin, where one of the oldest oceanic lithospheres is postulated to exist. The experiments were conducted from 1974 to 1980. Highly sensitive ocean-bottom seismographs which had been developed for longshot experiments were used. The lengths of the profiles ranged from 1000 to 1800 km, and the directions were chosen to provide wide azimuthal coverage. One of the aims of this series of experiments was to test the existence of velocity anisotropy on a large, regional scale.The results show that the oceanic lithosphere has anisotropy wherein the velocity changes by 4–7%. The anisotropy extends from a depth of at least 40 to 140 km beneath the sea bottom; however, the magnitude of the anisotropy may vary with depth. The azimuth of the maximum velocity is 150–160° clockwise from north, and coincides with the “fossil” direction of spreading of the Pacific plate, whereas it differs from the present direction of plate motion by ～ 30°. The azimuth does not seem to depend on depth. In the direction of maximum velocity, the lithosphere is basically two-layered: 8.0–8.2 and 8.6 km s?1. The depth of the interface is 50–60 km beneath the sea floor.  相似文献   

18.
The eastern Pontides orogenic belt is one of the most complex geodynamic settings in the Alpine–Himalayan belt due to the lack of systematical geological and geophysical data. In this study, 1D crustal structure and P-wave velocity distribution obtained from gravity modeling and seismological data in the area has been used for the development of the thermal model of the eastern Pontides orogenic belt. The computed temperature-depth profiles suggest a temperature of 590?±?60°C at a Moho depth of 35?km indicates the presence of a brittle-ductile transition zone. This temperature value might be related to water in the subducted crust of the Tethys oceanic lithosphere. The Curie temperature depth value of 29?km, which may correspond to the crustal magma chambers, is found 5–7?km below the Moho depth. Surface heat flow density values vary from 66.5 and 104.7?mW?m?2. High mantle heat flow density value of 48?mW?m?2 is obtained for the area should be related to melting of the lithospheric mantle caused by upwelling of asthenosphere.  相似文献   

19.
We report on an experimental study conducted to investigate the influence of small-scale wind waves on the airflow structure in the immediate vicinity of the air–water interface. PIV technique was used to measure the two-dimensional velocity fields at wind speeds of 3.7 and 4.4 m?s?1 and at a fetch of 2.1 m. The flow structure was analyzed as a function of wave phase. In the near-surface region, significant variations were observed in the flow structure over the waveform. The phase-averaged profiles of velocity, vorticity, and Reynolds stress showed different behavior on the windward and leeward sides of the wave in the near-surface region. The influence of wave-induced velocity was restricted within a distance of three significant wave heights from the surface, which also showed opposite trends on the windward and leeward sides of the crest. The results also show that the turbulent Reynolds stress mainly supports downward momentum transfer whereas the wave-induced Reynolds stress is responsible for the upward momentum transfer from wave to wind. In the immediate vicinity of the air–water interface, the momentum is transferred from waves to wind along the windward side, whereas, the momentum transfer is from wind to waves along the leeward side.  相似文献   

20.
P-wave travel-time residuals at the Warramunga Seismic Array (WRA) in the Northern Territory, Australia, have been studied from 49 earthquakes with epicenters south of 19°S in the Fiji-Tonga region. Focal depths are between 42 and 679 km as determined from pP-P. Using the Jeffreys-Bullen and the Herrin travel-time tables the epicentral parameters have been redetermined by considering only “normal” seismic stations in the location procedure. These are those stations where P-wave travel times are probably not affected by lateral heterogeneities caused by the lithosphere descending beneath the Tonga trench. Epicenters of deep earthquakes below 300 km have been relocated by using stations at Δ > 25° only. Epicenters from shallower-depth earthquakes have been recalculated without using stations between 35 < Δ < 75° epicentral distance. In both cases focal depths were determined from pP-P times. The resulting pattern of P-residuals at WRA does not show any significant change with depth below 350 km. The residuals become more negative for shallower earthquakes above about 250 km. P-waves to WRA are advanced by approximately 2 s compared with those from deep earthquakes. The results do not essentially differ for the two different travel-time tables used. The observations can be interpreted by P-wave velocities that are higher in the sinking slab down to 350–400 km by 5±2% than in both the Jeffreys-Bullen and Herrin models. Without considering possible elevations of phase boundaries this estimate yields a temperature contrast of 1000±450°C between slab and normal mantle material in this depth range.  相似文献   

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