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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..
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).
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.
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.
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 X²-T²
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 X²-T² 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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