Site-Specific Modeling of SH and P-SV Waves for Microzonation Study of Kolkata Metropolitan City, India

Kolkata, one of the oldest cities of India, is situated over the thick alluvium of the Bengal Basin, where it lies at the boundary of the zone III and zone IV of the seismic zonation map of India. An example of the study of site effects of the metropolitan Kolkata is presented based on theoretical modeling. Full synthetic strong motion waveforms have been computed using a hybrid method that combines the modal summation and finite difference techniques. The 1964 Calcutta earthquake, which was located at the southern part of Kolkata, is taken as the source region, with the focal mechanism parameters of dip?=?32°, strike?=?232° and rake?=?56°. Four profiles are considered for the computation of the synthetic seismograms from which the maximum ground acceleration (A _MAX) is obtained. Response spectra ratios (RSR) are then computed using a bedrock reference model to estimate local amplifications effects. The A _MAX varies from 0.05 to 0.17?g and the comparison of the A _MAX with the different intensity scales (MM, MSK, RF and MCS) shows that the expected intensity is in the range from VII to X (MCS) for an earthquake of magnitude 6.5 at an epicentral distance of about 100?km. This theoretical result matches with the empirical (historical and recent) intensity observations in Kolkata. The RSR, as a function of frequency, reaches the largest values (largest amplification) in the frequency range from 1.0 to 2.0?Hz. The largest site amplification is observed at the top of loose soil.     

Recent seismicity in Northeast India and its adjoining region

Recent seismicity in the northeast India and its adjoining region exhibits different earthquake mechanisms – predominantly thrust faulting on the eastern boundary, normal faulting in the upper Himalaya, and strike slip in the remaining areas. A homogenized catalogue in moment magnitude, M _W, covering a period from 1906 to 2006 is derived from International Seismological Center (ISC) catalogue, and Global Centroid Moment Tensor (GCMT) database. Owing to significant and stable earthquake recordings as seen from 1964 onwards, the seismicity in the region is analyzed for the period with spatial distribution of magnitude of completeness m _t, b value, a value, and correlation fractal dimension D _C. The estimated value of m _t is found to vary between 4.0 and 4.8. The a value is seen to vary from 4.47 to 8.59 while b value ranges from 0.61 to 1.36. Thrust zones are seen to exhibit predominantly lower b value distribution while strike-slip and normal faulting regimes are associated with moderate to higher b value distribution. D _C is found to vary from 0.70 to 1.66. Although the correlation between spatial distribution of b value and D _C is seen predominantly negative, positive correlations can also be observed in some parts of this territory. A major observation is the strikingly negative correlation with low b value in the eastern boundary thrust region implying a possible case of extending asperity. Incidentally, application of box counting method on fault segments of the study region indicates comparatively higher fractal dimension, D, suggesting an inclination towards a planar geometrical coverage in the 2D spatial extent. Finally, four broad seismic source zones are demarcated based on the estimated spatial seismicity patterns in collaboration with the underlying active fault networks. The present work appraises the seismicity scenario in fulfillment of a basic groundwork for seismic hazard assessment in this earthquake province of the country.     

OnP F and other phases in the early part of seismogram

Summary It has been found that when seismic energy propagates along the surface of the homogeneous crust beside usual Rayleigh waves, it produces certain instability in layers through which it propagates. In the light of this instability, a type of motion corresponding to longitudinal wave will be prominent in horizontal component compared to the vertical component; while transverse wave will be prominent in the vertical component but weak in the horizontal component, a contradiction with the existing knowledge. This has been identified withP _F phase. On taking the medium of propagation as slightly heterogeneous which allows existence of low velocity layer, a few larger number of such instabilities have been found. Velocity equation for Rayleigh waves for such media reveals existence of different velocities corresponding to vertical and horizontal components. Table for these velocities has been furnished.     

Unification of periodic fluctuations in high latitude geophysical phenomena

The oscillatory nature of wave-particle interaction is proposed to be the source of non-local fluctuations in the electron energy. The energized electron flux when precipitated in the lower ionosphere gives rise to various geophysical phenomena. The time scale of energy oscillation is estimated and its validity is discussed with reference to observed time scales of various geophysical phenomena.     

Some possible wormhole solutions on the brane

In this paper some wormhole solutions have been presented on the brane which are distinct from the presently available wormhole solution. The matter on the brane is chosen as perfect fluid and all physical variables are functions of the radial co-ordinate r. The solutions are obtained considering the trace of the resulting matter i.e. the Ricci scalar on the brane to be zero.     

A novel finite element method for seepage analysis

A novel finite element method has been proposed in this paper for the solution of seepage problems economically and accurately. In this method the governing equation and the prescribed boundary conditions are transformed so that they refer to a suitable logarithmically condensed ‘image’ space; the physical problem domain is also mapped into the image space. The transformed equation is then solved in the image space using standard finite elements, subject to the transformed boundary conditions. Because physical space is logarithmically condensed in the image space, the proposed method is capable of dealing with large or very large aspect ratio seepage problems economically and accurately. The validity of the method has been demonstrated by means of a number of examples including anisotropy and non-linearity. In all cases an excellent degree of accuracy was achieved, efficiently and economically.     

Seismic waves due to storm over the sea: I

Summary Elastic wave propagation problem has been studied in the composite medium — a gravitating liquid layer overlying an elastic half-space. Assuming moving normal pressure load on the free surface, an attempt has been made to study the order of disturbance on the earth's surface. In the discussion asymptotic values of the displacement, components have been considered for small values of the parameter.     

Exact solution of a basic equation in finite atmosphere by the method of Laplace transform and linear singular operators

A finite atmosphere having distribution of intensity at both surfaces with definite form of scattering function and source function is considered here. The basic integro-differential equation for the intensity distribution at any optical depth is subjected to the finite Laplace transform to have linear integral equations for the surface quantities under interest. These linear integral equations are transformed into linear singular integral equations by use of the Plemelj's formulae. The solution of these linear singular integral equations are obtained in terms of theX-Y equations of Chandrasekhar by use of the theory of linear singular operators which is applied in Das (1978a).