Contents of JULY 2005 - Vol. XXVI No.3

CRUSTAL EVALUATION AND BASEMENT TOPOGRAPHY BY GRAVITY SURVEY IN CENTRAL PART OF DELHI FOLD BELT, RAJASTHAN
- A.K.Mukherjee, S.L.Singh, R.Sharma, Babu Lal, R.L.Regar, Vinod Kumar, Om Prakash and M.K.Pandit

IDENTIFICATION OF EW TRENDING FAULTS AND DETERMINATION OF SEDIMENTARY THICKNESS IN SHILLONG-MAWLONG AREA BY ANALYSIS OF MAGNETIC DATA
- Pallabee Choudhury, A.Kumar and S.K.Laskar

A NEW EMPIRICAL ATTENUATION RELATIONSHIP FOR PEAK GROUND HORIZONTAL ACCELERATION FOR HIMALAYA REGION USING INDIAN AND WORLDWIDE DATA SET
- M.L.Sharma

SCENARIO HAZARD MAPS DUE TO A MODERATE SIZE EARTHQUAKE IN THE HIMALAYA
- Dinesh Kumar

P-WAVE SIGNATURES IN VTI SOLID UNDERLYINGA DIPPING PLANE
- K.P.Singh


CRUSTAL EVALUATION AND BASEMENT TOPOGRAPHY BY GRAVITY SURVEY IN CENTRAL PART OF DELHI FOLD BELT, RAJASTHAN

A.K.Mukherjee, S.L.Singh, R.Sharma, Babu Lal, R.L.Regar, Vinod Kumar, Om Prakash and M.K.Pandit

* Geological Survey of India, Jaipur, * University of Rajasthan, Jaipur

Abstract

The Delhi Fold Belt (DFB) of Central Rajasthan comprises of Proterozoic rocks disposed in a NE-SW direction. Major part of the present area is covered by alluvial tracts. Intrusive granites, pegmatite and ultramafic bodies are also reported from this area. Geophysical surveys comprising gravity method was carried out to establish the structural features of the crust and their continuity in the covered area. Gravity map of the area brought out low amplitude gravity high anomalies, trending in NE-SW direction over the DFB in the northern part. A significant E-W trending gravity discontnuity along Mangliawas – Nasirabad separates the southern gravity low anomaly from the northern gravity high. The discontinuity has been identified by a change in the trend of the contour pattern. The continuity of the northern gravity high could be traced in the SE part, near Masuda. The spectral analysis of the gravity data brought out three density interfaces at depths of 10.8, 6.13 and 1.33 km. The study suggests an eastward rise in basement, which slopes down towards west. 2D gravity model across the South Delhi Fold Belt (SDFB) indicated the Moho depth at 40 km and upwarp in the central part. One high-density body (+0.23 gm/cc) in the upper crust and another (+0.13 gm/cc) at 3 km depth were interpreted..

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IDENTIFICATION OF EW TRENDING FAULTS AND DETERMINATION OF SEDIMENTARY THICKNESS IN SHILLONG-MAWLONG AREA BY ANALYSIS OF MAGNETIC DATA

Pallabee Choudhury, A.Kumar and S.K.Laskar
Tezpur University, Tezpur, Assam

Abstract

Gradient analysis and downward continuation of aeromagnetic data and harmonic analysis of a ground magnetic profile are carried out for delineation of EW-trending basement faults and determination of sedimentary thickness in the Proterozoic basin of Shillong-Nongpoh area of Meghalaya. The analysis reveals that there exists a 40km long E-W trending hidden basement fault at about 7km north of Umsning under about 2.5km thick pile of apparently non-magnetic Shillong group of sediments. Starting from north of Umsaw reserve forest, the fault runs eastward along upstream of the west flowing tributary of river Umsaw and meets the NE-SW trending Barapani shear at a distance of about 30km east of the reserve forest. Thereafter it extends further east taking a SEW trend. The analysis further reveals that there exists a SEW trending fault at Umsning meeting the Barapani shear at about 5km east of Umsning. In all probability, the hidden 40km long E-W trending fault appears to be the Oldham fault predicted by Bilham and England (2001).

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A NEW EMPIRICAL ATTENUATION RELATIONSHIP FOR PEAK GROUND HORIZONTAL ACCELERATION FOR HIMALAYAN REGION USING INDIAN AND WORLDWIDE DATA SET

M.L.Sharma

Department of Earthquake Engineering, IIT Roorkee, Roorkee

Abstract

In absence of recorded strong ground motion, the site specific earthquake parameter studies in India are based on a few attenuation relationships developed elsewhere in similar prevailing tectonic environments. An endeavor has been made to develop attenuation relationship for Himalayan region in India by collating worldwide data selected from those relationships which are in use for seismic hazard computation in Indian region. The data base consisting of 666 recordings of peak ground accelerations from 82 earthquakes with magnitude ranging between 5 to 8.1 and epicentral distances upto 200 km has been used. The closest distance to the zone of energy release is used as the distance measure. The present analysis uses a two step stratified regression model. The comparison is made of the proposed relationship with the earlier developed attenuation relationships using Indian data for the Himalayas . The relationship is considered to be applicable for earthquake of magnitude ranging between 6.0 and 7.5 with least distance to the zone of energy release in the range of 10 to 200 km. Though the equation gives good results between magnitude 6.0 and 7.5, a larger data set from Indian side is required for accurate prediction of acceleration for all magnitudes to represent the prevailing tectonic environment for the region.

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SCENARIO HAZARD MAPS DUE TO A MODERATE SIZE EARTHQUAKE IN THE HIMALAYA

Dinesh Kumar

Department of Earth Sciences, Kurukshetra University, Kurukshetra

Abstract

The scenario hazard maps due to a moderate size earthquake (Ms 7) in the Himalaya have been presented. For this purpose, the accelerograms have been synthesized at large number of points using a fast semi-emperical technique. These scenario hazard maps show the spatial distribution of peak ground acceleration values, modified Mercallli Intensity and spectral accelerations for the periods T= 0.4s, 0.75s, 1.25s. These maps may be used to augment the information available in the probabilistic seismic hazard maps of the region. These maps are useful for the agencies involved in the planning of mitigation of seismic risk in the region.

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P-WAVE SIGNATURES IN VTI SOLID UNDERLYING A DIPPING PLANE

K.P.Singh

National Geophysical Research Institute, Hyderabad

Abstract

P-Wave phase, ray and CMP stacking velocities in transversely isotropic solid underlying a dipping plane depend on elastic parameters of solid, direction of propagation of wave from axis of symmetry, dip and depth of the reflector. The dependence of P-wave CMP stacking velocity on these parameters in transversely isotropic solids having vertical axis of symmetry (VTI) becomes very complex. To study the behavior of P-wave phase and ray velocities in VTI solids, mathematical expressions are also approximated in terms of Thomsen parameters e and d neglecting higher order terms involving e and d. Similar conditions and preassigned accuracy are applied to conventional and approximated (weak anisotropic) formulations to obtain P-wave reflection in common midpoint (CMP) array in all computations. Elastic parameters are also kept unchanged for various values of dip of the plane and depth of reflecting point. In case of weak anisotropic approximations, reflected phase and ray velocities increase with increase in angles and have no unique values corresponding to reflected phase and ray angles, both X-T and X-T plots are found highly nonlinear and fit with a polynomial of order greater than four. CMP stacking velocities abnormally increase with dip, and are almost twice of the velocities obtained without using any approximation. On the other hand CMP stacking velocities, computed without any approximation gradually increase with depth of reflection point and dip of the plane, phase and ray velocities smoothly increase with phase and ray angles, and X-T and X-T plots are hyperbolic and straight lines respectively. To obtain P-wave reflections under predefined conditions required maximum spread decreases and minimum spread increases with dip, and both increases with increase in depth. Thus, behavior of solids of small extent is not completely characterized in weak anisotropic approximations.

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