October, 2008

October, 2008   ||  Volume 12 No.4

Site Amplification Factors in Koyna Region using Coda Waves

Babita Sharma, Dinesh Kumar1 and S.S.Teotia1

Institute of Seismological Research, Sector-18, Gandhinagar-382 018
1Department of Geophysics, Kurukshetra University, Kurukshetra -136 119.
E-mail : babita_s@rediffmail.com

The site amplification factors have been obtained at two sites of Koyna region using the reference site method. The coda waves of 37 local earthquakes have been used for this purpose. The site Warna (WRN) has been taken as reference site on the basis of local geology for the estimation of site amplification factors at Chikali (CKL) and Katwali (KTL). The coda decay curve has been prepared for the three sites in order to validate the spectral ratio method used here. The single back scattering model has been used to obtain the coda decay curve of the region. The coda decay curves have been found to be similar for all source-station pairs in Koyna region except at very low frequencies. The estimated site amplification values show the non-linear variation with frequency at all the sites considered. The amplification factors vary from 1.4 to 3.3 at CKL and 1.6 to 3.1 at KTL. The amplification factors at both the station increase with increase in frequency in the wide range 5 – 13 Hz. The station CKL shows the higher amplification factors as compared to KTL in this frequency range. This reflects the difference in the station site geology as the amplification factors decrease with in increase in geological age. The results presented here are the first order of estimation in this region.


Resistivity studies to delineate structural features near Abhishekapatti, Tirunelveli, Tamil Nadu, India
Y.Srinivas, D.Muthuraj1 and N.Chandrasekar
Center for Geo-Technology, Manonmaniam Sundaranar University, Tirunelveli - 627 012
1Department of physics, The M.D.T.Hindu college Tirunelveli - 627 010
E.mail: drysv@yahoo.co.in

Geophysical survey using electrical resistivity method was carried out near Abhishekapatti, Tirunelveli district, Tamilnadu in order to delineate the possible structural features of the subsurface. A detailed survey was carried out using an indigenously made resistivity meter. Electrical resistivity profiling using Wenner configuration and vertical electrical sounding with Schlumberger configuration were carried out. The present study area is covered of thin soil underlain by crystalline massive metamorphic rocks of high resistivity. The inferred resistivity structure from Wenner and Schlumberger method yield significant insight into the resistivity distribution of the area. Vertical and lateral extension of low resistivity features, resistivity pseudo section and apparent resistivity contour map indicates the existence of a contact zone approximately in east west direction.

Finite Difference Modeling of SH-Wave Propagation in Multilayered Porous Crust

Pallavika, V.K.Kalyani1, S.K.Chakraborty and Amalendu Sinha1
Department of Applied Mathematics, B.I.T., Mesra, Ranchi – 835 215
1Central Institute of Mining & Fuel Research, Dhanbad-826015, India
Email: vkkalyani@yahoo.com

The present article deals with the propagation of SH-waves in a multilayered porous media with Weiskopf type anisotropy. Using Biot’s theory of porous medium, the problem has been formulated. The finite difference method has been used here to model the wave propagation problem and also to analyze the effect of porosity and anisotropic factors on the phase and group velocities. Three-dimensional diagram has been developed to describe the displacement of the SH wave propagation as a function of two variables x and z. Also, the relation has been developed between the incremental displacement and the time, which is shown graphically. The convergence and stability criteria of the finite difference method has been established i) to minimize the exponential growing of the error; ii) to make the finite difference method stable; and iii) to decide the valid range of numerical values of the parameters.

Vertical velocity from HF Doppler measurements over Ahmedabad
H.Chandra, Som Sharma, S.R.Das and R.Raghava Rao1
Physical Research Laboratory, Navarangpura, Ahmedabad-380 009.
1 Physics Department, JNTU, Hyderabad.
E-mail: hchandra@prl.res.in

HF Doppler radar operating at 2.5 MHz was developed and operated at Ahmedabad for a number of nights during 1986-87. The system made use of the existing ionosonde, C-4 model, as transmitter and a crystal controlled frequency synthesiser provided the RF excitation. Quadrature outputs of the phase coherent receiver were recorded both in analogue form on paper charts and in digital form at a sampling rate of 50 Hz. The velocity could be determined with a resolution of 1 m/s. Measurements show vertical velocity ranging between 0 and 25 m/s with a mean value of 9.2 m/s. A comparison with the vertical velocity obtained from the time variation of the minimum virtual height of F-layer, h’F, from a co-located ionosonde shows good agreement.



Recent floods related natural hazards over West coast and Northeast India
Tongdi Jamir, Alaka.S Gadgil and U.S.De

Department of Environmental Science University of Pune – 411 007
E.mail :tongdi@unipune.ernet.in

The severity of the weather, which manifest in the form of floods and landslides on account of rainfall, has a substantial impact on life and properties. In view of these, an attempt is made in this paper to study the economic damage and human casualties associated with floods that occurred during the period 1971-2005 over two specific zones i.e. northeast and the west coast regions of India. For this study, data from Disastrous Weather Events, Weekly Weather Reports and “Weather in India” published by Indian Meteorology Department during the above mentioned period have been analyzed. The study reveals that flood frequencies and associated human casualties and economic losses have increased in the recent decade.