Fine Velocity Structure Research Articles

2-D P-wave velocity structure in the mideast segment of Zhangjiakou-Bohai tectonic zone: Anxin-Xianghe-Kuancheng DSS profile

In order to get the 3-D fine velocity structure in the Capital-circle area of China, 6 explosions, ranging from 1800 to 2500 kg, were conducted and recorded by an array of 240 seismographs. A reflection/refraction survey was carried out along the profile extending from Anxin county, Hebei Province northeastward to Yanshan Mountains, crossing the Zhangjiakou-Bohai tectonic zone. The 2-D velocity structure of P wave was imaging along the profile. The results show that abnormality exists in the deep structure of the Zhangjiakou-Bohai tectonic zone: The basement is significantly depressed, the interfaces and Moho are uplifted, and a strong velocity gradient layer is existed above the Moho that may be dislocated by deep fault. The crust of Huabei basin is thin and low velocity body exists in the crust. The Yanshan Mountains’ crust is thick, the layers in the crust are quite clear and the velocity in the layer is homogeneous. Huabei basin differs from Yanshan Mountains in structure.

Fine Velocity Structure of the Upper Mantle Beneath the Xizang Plateau from Tomography and Its Geological Interpretations

AbstractWe use 24241 rays from 926 distant events recorded at 164 portable stations along the line Yadong‐Golmud to perform teleseismic P wave tomography. Our aim is to obtain the velocity structure of the upper mantle beneath the Xizang Plateau with a high resolution. Our result shows that the thick lithospheric mantle of the India plate is divided into two layers beneath the High Himalayas during the underthrusting of the India plate below the plateau, representing the first time of its delamination. The lower layer underthrusts beneath the High Himalayas at a about 22‐ angle northward to depth 350km and the upper layer extends northward to Yanshiping, forming the thin lithospheric mantle below the plateau. North to Yanshiping (33.7‐N), the upper layer of the Xizang mantle fractures and sinks when it meets the lithosphere of the Asia continent. Our result also confirms the existence of a deep low‐velocity body beneath Wudaoliang (35.27‐N). We suggest that this low‐velocity body in the crust may be associated with upwelling of hot materials at depth.

3‐D Tomography of Velocity Structure in the Upper Crust beneath the Tianchi Volcanic Area of Changbaishan

AbstractWe have obtained tomographic images of P‐ and S‐wave velocity structures and VP/VS perturbation at different depths benenth the Changbaishan Tianchi volcanic region by inversion of three‐dimensional (3‐D) fine velocity structure in the upper crust of the region. The inversion is based on reflected waves from Moho obtained by a 3‐D seismic transmission experiment in the study area. The result shows that the heterogeneity of the velocity structure in the Tianchi volcanic region and its surrounding areas is closely related to the geological structure and existence of the magma system in the area. The distribution of obvious P‐wave low‐velocity anomalous bodies and high VP/VS perturbation image suggests that the media are in a “soft and high‐temperature” state. This conclusion is consistent with the results of wide‐spread geothermal anomaly and low Q‐value structure obtained by other studies.

A Study on Velocity Distribution in Upper Crust and Tectonics of Basement and Cover in South China

AbstractFive representative wide‐angle seismic profiles in South China are analyzed to study the fine velocity structure. The interpolation was made by fitting the travel times of first arrive Pg waves, and combining with other geophysical data and geological data, the synthesis explanations for the tectonics of basement and cover are performed. It is proved that some fault basins infilled with Mesozoic volcanic rock above the basement are distributed in the eastern region of Zhejiang, Fujian, and Guangdong provinces and the thickness of the volcanic rock layer is 1 000–4 000 m. In the region of western Zhejiang, southern Anhui, Hunan, and Jiangxi Provinces, the basement surges obviously and the cover is mainly Palaeozoic group, and some Mesozoic Cretaceous smaller basins controlled by boundary faults develop there. The Chuxiong basin in western South China is a deep depression with complex structure, in which the thickness of Jurassic‐Cretaceous systems is over 10km. This basin is divided into two parts of eastern ascent and western depression, and the western depression is farther divided by a center uplift into two smaller deep depressions. Some Main faults in the researched region are reflected clearly in the contour velocity diagrams. Jiangshan‐Shaoxing fault, Lianhuashan fault and Wuchuan‐Sihui fault in the eastern region, and Chenghai fault, Luzhijiang fault in the western region are all the important tectonic boundaries, also flock basement faults are primarily the boundaries of the basins and the uplifts.

An occluded coastal oceanic front

Field observations, including hydrographic, microwave imaging radar, and HF radar measurements, reveal the evolution of a complicated frontal interaction between three water masses on the continental shelf near Cape Hatteras, North Carolina, during a period of incursion of water from the Gulf Stream. The water masses were found to be separated by intersecting frontal lines configured in a manner analogous to an occluded atmospheric front. The densest water lay between inshore and offshore fronts that gradually merged or occluded in the generally downstream direction, leaving a single surface front. The overall frontal structure appeared as a distinct Y‐shaped feature in the radar imagery, similar to historical imagery of the study area. The interpretation of the observations is aided by the use of a two‐dimensional numerical model. The model is initialized with two fronts idealized from the ocean measurements. The model fronts quickly sharpen and begin to move together, eventually occluding into a single surface front. As a result of the occlusion, the water mass having intermediate density subducts and intrudes under the most buoyant water, carrying with it strong horizontal and vertical shears, and a frontal band of diverging currents is created in the densest water mass. The model thus suggests that in the ocean there will be an increase in hydrographic and velocity fine structure downstream of the frontal occlusion point.

The fine velocity structure of sediment-basement layer in the Three-Gorges Region of the Changjiang River (Yangtze River)

In this paper, the study on the fine velocity structure of sedimental and basement layers along 4 deep seismic sounding profiles in the Three Gorges Region of the Changjiang River (Yangtze River) are presented.

Magma Distribution in Island Arc Mantle in Three Dimensions.

Distribution of partial melt (magma) in the upper mantle is investigated in detail from the seismic tomography of the mantle wedge beneath northeastern Japan. The comparison of the fine velocity structure with the laboratory velocity data on a partiall-ymolten mantle rock yields estimates of three-dimensional distribution of melt. The results indicate that the cause of island arc volcanism in northeastern Japan is the upwelling of hot mantle materials (volcanic plumes) from beneath. The source of volcanic magma is diapir-like melting regions localized within the volcanic plumes. Extensive volcanic activity at the volcanic front is due to the presence of the vast melting zones right beneath the Moho discontinuity. Those melting zones may cause melting of lower crustal materials and produce felsic magma. Melt stays underneath the Moho, where crystallization fractionation may proceed. Melt exists at greater depths in the back-arc region, which may cause across-arc variations of chemical compositions of the volcanic rocks in northeastern Japan. We suggest that magma migration in the ductile lower crust may cause low-frequency microearthquakes, and magma penetration into the brittle upper crust may produce mid-crustal S-wave reflectors.

Line-averaged measurement of velocity fine structure in the ocean using acoustical reciprocal transmission

Abstract Reciprocal acoustical travel time measurements over paths of several hundred metres can be used to probe the statistical behaviour of turbulent velocity fine structure in the ocean. For homogeneous isotropic flows, and for long measuring baselines, an analytic expression relating line-averaged and point measurements of velocity is derived. Anisotropic and inhomogeneous flows are also considered. Correction formulas for the spatial and temporal variability of advection velocity along the measuring baseline are obtained. Practical limitations are established and experimental data from the arctic boundary layer beneath ice is compared with the theory

Internal waves and velocity fine structure in the Arctic Ocean

A series of closely spaced velocity profiles made in the ice‐covered Canada Basin of the Arctic Ocean during April 1985 are used to investigate the spatial scales and dynamics of small‐scale motions. The velocity fluctuations are very different from those found in mid‐latitudes, containing about 40–50 times less WKB normalized energy at low wave numbers and about 10 times less WKB normalized shear variance. The vertical wave number spectrum is much flatter, with the deficit in energy occurring primarily for wavelengths greater than 30 m. The horizontal wave number magnitude spectrum of horizontal kinetic energy can be estimated only for velocity fluctuations with vertical wavelengths near 40 m. Combined with estimates of kinetic and potential energy, this determines the distribution of energy between motions with various types of dynamics in this band. Near‐inertial frequency internal waves account for about two‐thirds of the energy. The data suggest such waves are generated during storms. Velocity fine structure accounts for about a quarter of the kinetic energy and has an absolute energy level comparable to that found at mid‐latitudes. It is dynamically distinct from internal waves and has a Rossby number near or greater than unity. High frequency internal waves account for a small fraction of the energy but carry most of the superinertial potential energy.

Three‐dimensional fine velocity structure and hypocentral distribution of earthquakes beneath the Kanto‐Tokai District, Japan

We determine the three‐dimensional P wave velocity structure beneath the Kanto‐Tokai district, central Japan, and simultaneously relocated by inverting travel time data for 91 events that were observed at 61 stations operated by the National Research Center for Disaster Prevention. The variance of the travel time data is reduced by 75%, as compared to the variance for the routinely used laterally homogeneous model. In general, our velocity model is consistent with tectonic models of the Kanto‐Tokai district. Our velocity model is also in good agreement with seismic explosion data and Bouguer gravity anomalies. The uppermost layer (from 0 to 16 km depth) is characterized by lateral velocity differences of more than 20%. We find low‐velocity zones beneath Tokyo Bay, the middle of the Boso Peninsula, and Omae‐zaki, where there are thick sedimentary layers. A marked 30‐km‐thick high‐velocity zone extends to the northeast and the west of the Izu Peninsula. This zone dips gradually from the surface to a depth of 70 km in the eastern region and to a depth of 40 km in the western region and is interpreted as the Philippine Sea (PHS) plate, which is being subducted at the Sagami and the Suruga troughs. The data unquestionably require a high‐velocity zone beneath Mount Tsukuba that cannot be explained in terms of present tectonic models. We use our three‐dimensional velocity model to relocate about 23,000 earthquakes that took place from 1983 through 1985. The accuracy of the hypocentral parameters improved considerably. As a result, the double seismic zone within the subducting Pacific plate is more clearly delineated than in previous studies. The inclined seismic zone for the subducting PHS plate coincides fairly well with the high‐velocity zone of our three‐dimensional velocity model.

Crustal block of the western Ganga Basin: A fragment of oceanic affinity?

Abstract Three-component, long-period New Delhi records of two earthquakes occurring near Multan, central Pakistan, are used to determine group velocities of multimode surface waves in the period range of 5 to 57 sec. Inversion of these data has provided insight regarding the crustal shear velocity structure in the western Ganga Basin, in the northwestern wedge of the Indian subcontinent. The shear velocity is less than 3.10 km/sec within the topmost 10 km of its 40-km-thick crust. The estimated velocity in the 10- to 20-km depth range is 3.80 ± 0.09 km/sec. In the lower half crust, the velocity is as high as 3.98 ± 0.08 km/sec. Unlike the continental shield crust in the New Delhi-Shillong region, the crustal structure in this western part of the Ganga Basin is strongly reminiscent of certain oceanic plateaus. This suggests that the crustal block underlying the sediments is not part of the stable core, or the craton of the Indian Shield. The available data do not allow the determination of fine velocity structure in the upper mantle. The average velocity in the uppermost 80 km of the mantle is 4.52 km/sec. This value appears to be characteristic of the entire northern Indian subcontinent and may be indicative of active tectonism in the mantle south of the Himalayas.

Density and velocity fine structure enhancement in oceanic eddies

Advection‐diffusion models of the oceanic thermocline require a global ocean, mean vertical eddy diffusivity of about 1 cm2 s−1; however, maximum values estimated from microstructure measurements at mid‐gyre locations are generally smaller and, occasionally, 1–2 orders of magnitude less, depending on the particular assumptions made by individual analysts. Mesoscale features are high kinetic energy sources, which may fuel vertical mixing mechanisms on fine structure scales, resulting in local enhancements of eddy diffusivity above the canonical value of 1 cm2 s−1. The effects of one such mechanism, the Kelvin‐Helmholtz instability, are assessed for a Gulf Stream ring and a mid‐thermocline eddy. The necessary Richardson numbers are computed from Yvette profiles obtained in these features and are accurate to within about 10% for Ri = 1, and more accurate at the critical value Ri = 0.25. A plausible extension of the Miles‐Howard theorem for a nonparallel shear flow is formulated for a two‐dimensional perturbation. The shear appropriate for this Richardson number calculation is well approximated by the total shear for slowly depth‐varying direction, as is the case for the vast majority of the data. Upper and lower bounds on the vertical scale for the Richardson number calculation are set by the shear spectra and the Ozmidov scale. Median Richardson number was lower toward the center of the eddy because of a large decrease in Brunt‐Vaisala frequency (N2) and was depressed on the fringes by high shear. Lower median Ri is also found at the center of the ring as a result of low N2. Lower bounds on vertical eddy diffusivity are estimated based on the assumptions of a simple, mixing‐length model and of complete mixing of each region where Ri is less than 0.25. The spatial distribution of this quantity within the eddy and the ring mirrors that of median Ri. Values range from Az ∼ 1.1 cm2 s−1 near the center of the eddy and 0.90 cm2 s−1 near the center of the ring to zero in regions outside of the features where no Ri less than 0.25 were observed.

Observations of Near-Inertial Waves in a Front

Abstract Near-inertial with horizontal scales ∼O(10 km) dominate profiles of velocity finestructure collected in the North Pacific Subtropical Front during January 1980. Considerable spatial variability is observed. Two features in particular contain most of the energy: a 20 cm s−1 amplitude (λz = 100 m) wave on the warm edge of the front propagating downward and away from the front, and a low wavenumber (λz = 500 m) wave reflecting off the surface. The propagating wavegroup is four times as energetic as the local downgoing near-inertial wave field. Its spatial structure is not consistent with propagation in a homogeneous medium, which suggests that it may be interacting with the front. Possible mechanisms for the existence and properties of the wavegroup are discussed, including baroclinic/barotropic instability, wind-forcing and enhancement by wave-mean flow interaction. a wave-mean flow interaction model that predicts trapping and amplification of near-inertial flow interaction. A wave-mean flow intera...

Vertical fine structure observations in the eastern equatorial Pacific

Measurements of vertical displacement and horizontal velocity finestructure near the equator at 110°W in the eastern Pacific Ocean are reported. Profiles were scaled to a constant Brünt‐Väisälä frequency ocean (No = 1 cph) in accordance with a WKBJ approximation. A total of 57 CTD casts between 3°N and 3°S taken during five cruises in 1979 were analyzed. Results show an equatorial enhancement of vertical displacement variance for vertical wavelengths longer than 50 sdbar (stretched decibars). This enhancement is similar to that which has been reported at 125°W and 179°E. Differences between locations can be accounted for by the observed temporal variability at 110°W. Coherence between vertical displacement profiles separated in time by delays of 2 hour to 120 hour indicate that the high wave number structures were largely associated with time scales of 4 days and less. Meridionally, vertical structures longer than 300 sdbar were coherent within 50 km of the equator. We interpret this vertical displacement fine structure enhancement as high wave number equatorially trapped inertial‐gravity waves. The velocity fine structure measurements in July 1979 also indicate equatorially enhanced horizontal kinetic energy for vertical wave lengths longer than 100 sdbar. The velocity structures persisted over the 56 hour of measurement and appeared to have longer time scales than the vertical displacements. Meridional energy exceeded zonal energy; however, the two components were coherent. We interpret these velocity structures as inertial‐gravity waves which were produced off the equator and are propagating through the equatorial region.

Measurements and models of fine structure, internal gravity waves, and wave breaking in the deep Ocean

Measurements of horizontal and vertical current by propeller cluster current meters and temperature by thermistors mounted on a rigid array 8 m high and 20 m long moored in the oceanic main thermocline near Bermuda are interpreted in terms of thermocline‐trapped internal wave modes in the presence of temperature and velocity fine structure. The array was deployed roughly 250 m off the sloping bottom in water roughly 900 m deep on three occasions. Two‐turning‐point uniformly valid asymptotic solutions to the internal wave equation are developed to describe the wave functions. Mode decay beyond the turning point in depth or frequency produces a sharp cutoff in vertical current spectra above the local buoyancy frequency N(z). An internal wave wave number‐frequency spectral model E( α,ω) = E(ω/N0)−2(α/α0)−2 describes vertical current spectra and ratios of potential energy to horizontal kinetic energy. The red wave number shape suppresses peaks in both these quantities at frequencies near N(z). A dip in the vertical current spectra at 0.5 cph not predicted by the model appears related to the bottom slope. Temperature fine structure is modeled as a passive vertical field advected by internal waves. Quasi‐permanent fine‐scale features of the stratification and vertically short internal waves are indistinguishable in this study. The model of McKean [1974] is generalized to include fine structure fields specified by their vertical wave number spectra. In addition to the trapped internal wave model, moored temperature spectra, temperature vertical difference spectra, and coherence over vertical separations are described by using a fine structure vertical wave number spectrum PT(k) ATk−5/2; which agrees with other spectra made by using vertical profiling instruments in the range 0.1–1.0 cpm. Horizontal current fine structure is also modeled as a passive field advected by vertically long internal waves. The model describes moored horizontal current spectra (least successfully at frequencies near N(z)) and finite difference vertical shear spectra. Contours of temperature in depth versus time indicate possible mixing events. These events appear concurrently with high shear and Richardson numbers 0.25 ≲ Ri ≲ 1.0. Over a 7‐m vertical separation a cutoff in Ri at 0.25 is observed, indicating possible saturation of the internal wave spectrum. Spectra of finite difference approximations to shear and buoyancy frequency are dominated by fine structure contributions over nearly the entire internal wave range, suggesting that breaking is enhanced by fine structure/Breaking appears equally likely at all frequencies in the internal wave range.

Experiment on the geometry of the fine-structure regions in fully turbulent fluid

An attempt has been made to identify the geometric character of the random fine-structure regions dispersed in fully turbulent fluid. The technique was measure-ment of two-position coincidence functions for the presence of velocity fine-structure. In this primitive approach, we tried to distinguish among only three possible categories for the random shapes: (a) ‘blobs’, (b) ‘rods’ and (c) 'slabs’, depending on whether three mean orthogonal dimensions of the domains were such that (a) all were of the same order, (b) one was an order larger than the other two, or (c) one was an order smaller than the other two.Highly idealized paradigms for these three categories were studied analytically: the two-position coincidence functions were computed for the cases of (a) spheres, (b) circular cylinders and (c) plane slabs, each field containing randomly sized elements distributed randomly in space with a homogeneous and isotropic distribution. Comparison of the measured coincidence functions with these three paradigms suggests that the fine-structure regions are more nearly of ‘rod-like’ geometry than like either of the other two. No attempt was made to distinguish shapes which might be called 'strips’.

A simulation of heat flow in the lower troposphere by a laboratory model

A laboratory model of the troposphere has been constructed in which the heat flow above the ground in the lowest kilometer may be simulated. The scaling criteria, derived from the governing equations with a number of approximations, are 104 for length, 200 for velocity and 0.2 for temperature. Water is the simulating fluid, which moves through a closed-circuit water-tunnel. A set of electrically heated aluminum plates simulate the heating of the ground by insolation. Temperature and velocity fine structure are measured. The fluid isotherms are inferred from shadowgraph photography. All observations are confined to conditions of natural and free convection. A comparison between the temperature and velocity profiles within the prototype and the model, for identical boundary conditions, indicates that the model is a good representation of the prototype to heights of 1000 meters. It is found that the vertical transfer of warm air is along cylindrical columns, which take on different forms in natural and in free convection. In the former, the columns carry heat aloft into caps which rise at some 1.2 meter per second, or which may merge with neighbouring columns to continue upwards to heights of several hundred meters. If an elevated temperature-inversion layer is present, these convective columns perturb it, initiating horizontally-travelling waves. In free convection, the columns are inclined downwind, with their upper extremities extending almost horizontally; the cap is absent and the wave-like tail may separate and move across the fluid independently of its parent.

A Coherence Analysis of Fraunhofer Line Radial Velocity Fine Structure and Continuum Brightness Fluctuations near the Center of the Solar Disk.

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Coherence Analysis of Fraunhofer Line Radial Velocity Fine Structure and Continuum Brightness Fluctuations near the Center of the Solar Disk. Edmonds, Frank N., Jr. Abstract Cross correlation analyses between fluctuations in radial velocity V and continuum brightness I have been carried out for four sets of measurements made near the center of the solar disk; those obtained from a McMath-Hulbert spectrogram (w18), those of R. S. Richardson and M. Schwarzschild (Astrophys. J. 111, 351, 1950) (RS), those of H. H. Plaskett (Monthly Notices Roy. Astron. Soc. 114, 251, 1954) (P), and those of R. Servajean (Ann. Astrophys. 24, 1, 1961) (S). These analyses employed a coherence technique which gives the coherence or cross correlation between the Fourier components of the same wavelength for each set of fluctuations, and the phase difference (across the solar disk) between these Fourier components. The analyses also computed power spectra, RMS's and filtered and unfiltered cross correlations. A significant VI coherence with 180~ phase difference was found in the wavelength range 1200-5200 km (on the surface of the sun) for the RS measurements, no VI coherence was found for the w18 measurements, and the S and P measurements gave inconclusive results. These results cannot be explained in terms of differences in the solar atmosphere levels making an appreciable contribution to line formation and to continuum formation. The inconsistent picture presented by the four VI coherence analyses can be explained in terms of a negative VI cross correlation existing for wavelengths ~ 5000 km plus certain differences in instrumental and atmospheric resolution for the four sets of measurements. However, an explanation in terms of a heterogeneous convection pattern at the sun's surface seems more reasonable. Publication: The Astronomical Journal Pub Date: March 1962 DOI: 10.1086/108817 Bibcode: 1962AJ.....67Q.271E full text sources ADS |

Fine Structure in the Velocity Distributions of Slowed Fission Fragments

Fine structure is observed in the velocity distribution of ${\mathrm{U}}^{235}$ fission fragments that have been slowed by aluminum or nickel, but none is observed for unslowed fragments. Identification of this velocity fine structure with the fine structure in the fission mass yield confirms the influence of the 82-neutron shell in the fission act, as distinguished from its influence in post-fission neutron boil-off. The production of velocity fine structure by aluminum or nickel slowing is shown, by means of the measured range-velocity relation of fragments in these metals, to be the result of isobaric velocity bunching. The measured range-velocity relation of fragments in gold indicates no velocity bunching and therefore is consistent with the absence of velocity fine structure in measured velocity distributions of ${\mathrm{U}}^{235}$ fragments slowed in gold or platinum.The absence of velocity fine structure in the velocity distribution of ${\mathrm{U}}^{233}$ and ${\mathrm{Pu}}^{239}$ fragments slowed by aluminum is discussed.