1,590 research outputs found
A review of crust and upper mantle structure beneath the Indian subcontinent
This review presents an account of the variations in crustal and upper mantle structure beneath the Indian subcontinent and its environs, with emphasis on passive seismic results supplemented by results using controlled seismic sources. Receiver function results from more than 600 seismic stations, and over 10,000 km of deep seismic profiles have been exploited to produce maps of average crustal velocities and thickness across the region. The crustal thickness varies from 29 km at the southern tip of India to 88 km under the Himalayan collision zone, and the patterns of variation show significant deviations from the predictions of global models. The average crustal shear velocity (Vs) is low in the Himalaya–Tibet collision zone compared to Indian shield. Major crustal features are as follows: (a) the Eastern Dharwar Craton has a thinner and simpler crustal structure crust than the Western Dharwar Craton, (b) Himalayan crustal thickness picks clearly follow a trend with elevation, (c) the rift zones of the Godavari graben and Narmada–Son Lineament show deeper depths of crust than their surroundings, and (d) most of the Indian cratonic fragments, Bundelkhand, Bhandara and Singhbhum, show thick crust in comparison to the Eastern Dharwar Craton. Heat flow and crustal thickness estimates do not show any positive correlations for India. Estimates of the thickness of the lithosphere show large inconsistencies among various techniques not only in terms of thickness but also in the nature of the transition to the asthenosphere (gradual or sharp). The lithosphere beneath India shows signs of attrition and preservation in different regions, with a highly heterogeneous nature, and does not appear to have been thinned on broader scale during India's rapid motion north towards Asia. The mantle transition zone beneath India is predominantly normal with some clear variations in the Himalayan region (early arrivals) and Southwest Deccan Volcanic Province and Southern Granulite Terrain (delayed arrivals). No clear patterns on influence on the mantle transition zone discontinuities can be associated with lithospheric thickness. Over 1000 anisotropic splitting parameters from SKS/SKKS phases and 139 using direct S waves are available from various studies. The shear-wave splitting results clearly show the dominance of absolute-plate-motion related strain of a highly anisotropic Indian lithospheric mantle with delay times between the split S phases close to 1 s. There are still many parts of India where there is, at best, limited information on the character of the crust and the mantle beneath. It is to be hoped that further installations of permanent and temporary stations will fill these gaps and improve understanding of the geodynamic environment of the Indian subcontinent.This study has been supported by a grant
from the Ministry of Earth Sciences (MoES), IITKGP/CKH
Simulation of underground gravity gradients from stochastic seismic fields
We present results obtained from a finite-element simulation of seismic
displacement fields and of gravity gradients generated by those fields. The
displacement field is constructed by a plane wave model with a 3D isotropic
stochastic field and a 2D fundamental Rayleigh field. The plane wave model
provides an accurate representation of stationary fields from distant sources.
Underground gravity gradients are calculated as acceleration of a free test
mass inside a cavity. The results are discussed in the context of
gravity-gradient noise subtraction in third generation gravitational-wave
detectors. Error analysis with respect to the density of the simulated grid
leads to a derivation of an improved seismometer placement inside a 3D array
which would be used in practice to monitor the seismic field.Comment: 24 pages, 12 figure
Magnetic field induced Coulomb blockade in small disordered delta-doped heterostructures
At low densities, electrons confined to two dimensions in a delta-doped
heterostructure can arrange themselves into self-consistent droplets due to
disorder and screening effects. We use this observation to show that at low
temperatures, there should be resistance oscillations in low density two
dimensional electron gases as a function of the gate voltage, that are greatly
enhanced in a magnetic field. These oscillations are intrinsic to small samples
and give way to variable range hopping resistivity at low temperatures in
larger samples. We place our analysis in the context of recent experiments
where similar physical effects have been discussed from the point of view of a
Wigner crystal or charge density wave picture.Comment: 6 pages RevTeX, 2 figures, published versio
Interactions of multi-scale heterogeneity in the lithosphere: Australia
Understanding the complex heterogeneity of the continental lithosphere involves a wide variety of spatial scales and the synthesis of multiple classes of information. Seismic surface waves and multiply reflected body waves provide the main constraints on broad-scale structure, and bounds on the extent of the lithosphere-asthenosphere transition (LAT) can be found from the vertical gradients of S wavespeed. Information on finer-scale structures comes through body wave studies, including detailed seismic tomography and P-wave reflectivity extracted from stacked autocorrelograms of continuous component records. With the inclusion of deterministic large-scale structure and realistic medium-scale stochastic features fine-scale variations are subdued. The resulting multi-scale heterogeneity model for the Australian region gives a good representation of the character of observed seismograms and their geographic variations and matches the observations of P-wave reflectivity. P reflections in the 0.5–3.0 Hz band in the uppermost mantle suggest variations on vertical scales of a few hundred metres with amplitudes of the order of 1%. Interference of waves reflected or converted at sequences of such modest variations in physical properties produce relatively simple behaviour for lower frequencies, which can suggest simpler structures than are actually present. Vertical changes in the character of fine-scale heterogeneity can produce apparent discontinuities. In Central Australia a ‘mid-lithospheric discontinuity’ can be tracked via changes in frequency content of station reflectivity, with links to the broad-scale pattern of wavespeed gradients and, in particular, the gradients of radial anisotropy. Comparisons with xenolith results from southeastern Australia indicate a strong tie between geochemical stratification and P-wave reflectivity.Part of this study was supported by JSPS KAKENHI Grant Number
26400443 to K.Y., with additional support from the AuScope AuSREM
project. The Earth Simulator Center of JAMSTEC is thanked for providing
CPU time on the Earth Simulator
Magnetotransport in disordered delta-doped heterostructures
We discuss theoretically how electrons confined to two dimensions in a
delta-doped heterostructure can arrange themselves in a droplet-like spatial
distribution due to disorder and screening effects when their density is low.
We apply this droplet picture to magnetotransport and derive the expected
dependence on electron density of several quantities relevant to this
transport, in the regimes of weak and moderate magnetic fields. We find good
qualitative and quantitative agreement between our calculations and recent
experiments on delta-doped heterostructures.Comment: 10 pages RevTeX, 2 figures, uses psfrag; published versio
Arnotts Blending Project
Established and supported under the Australian Government’s Cooperative Research Centre Progra
Spin Waves in Disordered III-V Diluted Magnetic Semiconductors
We propose a new scheme for numerically computing collective-mode spectra for
large-size systems, using a reformulation of the Random Phase Approximation. In
this study, we apply this method to investigate the spectrum and nature of the
spin-waves of a (III,Mn)V Diluted Magnetic Semiconductor. We use an impurity
band picture to describe the interaction of the charge carriers with the local
Mn spins. The spin-wave spectrum is shown to depend sensitively on the
positional disorder of the Mn atoms inside the host semiconductor. Both
localized and extended spin-wave modes are found. Unusual spin and charge
transport is implied.Comment: 14 pages, including 11 figure
Exchange anisotropy, disorder and frustration in diluted, predominantly ferromagnetic, Heisenberg spin systems
Motivated by the recent suggestion of anisotropic effective exchange
interactions between Mn spins in GaMnAs (arising as a result of
spin-orbit coupling), we study their effects in diluted Heisenberg spin
systems. We perform Monte Carlo simulations on several phenomenological model
spin Hamiltonians, and investigate the extent to which frustration induced by
anisotropic exchanges can reduce the low temperature magnetization in these
models and the interplay of this effect with disorder in the exchange. In a
model with low coordination number and purely ferromagnetic (FM) exchanges, we
find that the low temperature magnetization is gradually reduced as exchange
anisotropy is turned on. However, as the connectivity of the model is
increased, the effect of small-to-moderate anisotropy is suppressed, and the
magnetization regains its maximum saturation value at low temperatures unless
the distribution of exchanges is very wide. To obtain significant suppression
of the low temperature magnetization in a model with high connectivity, as is
found for long-range interactions, we find it necessary to have both
ferromagnetic and antiferromagnetic (AFM) exchanges (e.g. as in the RKKY
interaction). This implies that disorder in the sign of the exchange
interaction is much more effective in suppressing magnetization at low
temperatures than exchange anisotropy.Comment: 9 pages, 8 figure
Extending shear-wave tomography for the lower mantle using S and SKS arrival-time data
Seismic tomography using S wave travel times faces the difficulty imposed by the interference between S and SKS phases near 83° epicentral distance, as the SKS phase overtakes the S waves in the mantle. If the cross-over is avoided completely by excluding S data beyond 82° then no resolution is available below 2200 km in the lower mantle. A partial solution is to try to pick up the S phase beyond the cross-over which improves coverage and resolution in depth. However, a much larger improvement can be made by following the first arrival with S character and including SKS information with S.
Arrival times for both S and SKS phases and the event hypocentres have been taken from the reprocessing of data reported to international agencies. Each event has been relocated, including depth phase information, and later phases re-associated using the improved locations to provide a set of travel times whose variance is significantly reduced compared with the original data catalogues.
S travel-time tomography including SKS information out to 105°, provides tomographic images with improved rendition of heterogeneity in the lower mantle. The three-dimensional models of SV wavespeed relative to the ak135 reference velocity model show a significant increase in heterogeneity at the base of the mantle which matches the behaviour seen in results derived from waveform inversion.
For most of the mantle there is a considerable similarity between the patterns of heterogeneity in the S wave images and recent P wave tomographic results, but greater differences develop in the lowermost mantle. In the D″ region the SV wavespeed patterns also show some differences from recent SH wavespeed results which mostly correlate with regions of recognised structural complexity
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