April, 2009

April, 2009   ||  Volume 13 No.2

Seismic a-value and the Spatial Stress-Level Variation in Northeast India


Prosanta Kumar Khan, Manoj Ghosh1 and V.K.Srivastava

Department of Applied Geophysics, Indian School of Mines University, Dhanbad
1Reliance Industries Limited, Seismic Data Acquisition and Processing Group, Mumbai
Email: pkkhan_india@yahoo.com, manoj.ghosh@zmail.ril.com

The present study aims at understanding the variation of stress level vis-à-vis crustal heterogeneity based on seismicity distribution and a-values in the northeast part of India. The study area lies between latitude 24.5° and 27.2°N and longitude 89° and 96°E, and bounded by major thrust sheets of the Himalaya and Indo-Burman Ranges towards north and east. A crustal scale transcurrent Dauki fault demarcates its southern boundary, while the Yamuna lineament and the tail end of the Brahmaputra and Ganga rivers encompass all along the eastern boundary. Regarding seismicity, the area recorded several moderate to large earthquakes during the historical past, and the most damaging well-known 1897 Shillong earthquake was famous for its own kind. In the present study, we have analysed a-values using a comprehensive database recorded by the network jointly run by RRL, Jorhat and NGRI, Hyderabad. A total of 3655 events were used under the present study.

Seismicity distribution shows three major clusters of higher concentration over the study area. Contours based on estimated a-values over 240 square grids of dimension 0.6°×0.6° show wide variation. However, the near uniform a-values over specific five zones allowed us for depth probing of a-values. The higher a-values in different layers towards the eastern part are correlated with the reactivation of fractures at lower stress level, whereas the minimum a-values with higher gradient towards the southwestern part of the study area can be associated with higher stress level and linked to the thinner crustal root, and uplifted Moho. The area between the Main Boundary Himalayan Thrust and the Shillong Plateau account higher a-values, and might be indicating brittle failure of the weaker crust at lower stress level around the Tura region. Finally, it may be inferred that the seismicity of the northeast India is due to tectonic adjustment of different geomorphologic features presumably caused by the orogenic processes in the Himalaya and Indo-Burman Ranges.


Fractal Nature of Earthquake Occurrence in Northwest Himalayan Region
P.N.S.Roy and S.K.Mondal
Department of Applied Geophysics, Indian School of Mines University, Dhanbad
Jharkhand - 826 004
E-mail: pnsmay1@gmail.com

The Himalaya has seen many earthquakes in past ranging from low to very high magnitude causing massive losses. Earthquakes in Himalaya are mainly caused due to release of elastic strain energy created and replenished by persistent collision of the Indian plate with the Eurasian plate. In this paper the fractal analysis were done for earthquakes (mb³3) occurred during 1973 - 2008, which led to the detection of a clustering events in three consecutive fifty events window having low Dc value ranging from 0.932 to 0.533 during the period between 12.9.2004 to 28.2.2005.Spatio-temporal clustering of events apparently indicates a highly stressed region, leading to increase of shear strain causing weak zone from where the rupture propagation may eventually nucleate causing large earthquake. This kind of clustering pattern study using well-constrained catalogue data for the Himalayan fault systems of seismically active region can eventually help in the preparedness and mitigation of earthquake hazard.

Mono-spectral (greyscale) imaging for Nanoscale physical character analysis of Silica from lake deposition: An experimental study

Sudip Dey, Chandrani Debbarma and Prasamita Sarkar
Department of Geography and Disaster Management, Tripura University, Suryamaninagar - 799 130
E.mail : sudip_geo@rediffmail.com

Digital microphotography and image analysis is considered as an important tool in sedimentology and mineralogy for the assessment of physical characteristics at micro and nano level. The authors attempted to analyse the physical characteristic like surface condition of a silica grain of a lake deposition layer from mono-spectral imaging at nanometre scale. For that purpose a silica grain was picked up from microphotography of a thin section of sediment layer and 1mm X 1mm base image in greyscale was prepared for operation with sophisticated software. Reflectance capacity of the particle has been considered to measure the topographic condition. Contour plot from the base image was prepared along with a topographic image prepared on the basis of blackbody law. A complete topography map of silica grain was prepared on the basis of two previous plots. A 3D image and four profiles were also consulted for understanding the geometry of the surface of silica. Roughness analysis was also done from the mono-spectral image.

Geotechnical Studies in Relation to Seismic Microzonation of Union Territory of Chandigarh
G.C.Kandpal, Biju John1 and K.C.Joshi
Earthquake Geology Division, Geological Survey of India, Northern Region, Aliganj, Lucknow – 226 024
1National Institute of Rock Mechanics, Kolar.
E.mail : gckandpal@yahoo.co.in

The paper highlights the outcome of site response studies undertaken around Union Territory of Chandigarh, based on geotechnical approach. Twenty-four boreholes were drilled down to 30 m depth in order to determine the variation in the geotechnical properties of the geological units present around Chandigarh. Standard Penetration Test (SPT), as per BIS Code, was conducted in all the boreholes for determination of N values of the subsurface geological units at 1 to 3 m intervals. Shear Wave Velocities of the subsurface layers were computed based on the empirical relation. The average shear wave velocity of the sediments down to 30 m depth at different sites around Chandigarh Urban Centre varies from less than 216 to 305 m/s.
The predominant frequencies at which the seismic waves are expected to resonate in the soil column of 30 m depth were also computed using frequency-shear wave velocity relationship.
Based on the analysis of the geotechnical properties of subsurface soil samples, their disposition with respect to each other and also with ground water table conditions liquefaction potential around Union Territory has also been assessed.



Long-term Temperature Trends at Four Largest Cities of India during the Twentieth Century
Amit Dhorde, Anargha Dhorde1 and Alaka S.Gadgil

Department of Geography, University of Pune, Pune - 411 007
1Post Graduate Department of Geography, Nowrosjee Wadia College, Pune - 411 001

Investigations have been carried out by climatologists to find a possible link of climate change with anthropogenic activities by studying trends in different climatic parameters, particularly surface air temperature of densely populated cities. The present research aimed at quantifying the change in surface air temperature at India's four most populated cities - Delhi, Kolkata, Mumbai and Chennai. The cities taken up for study have experienced rapid urbanization in the recent past. Therefore, the main objective of the research was to find the impact of urbanization on temperature trends at these cities. Trends in annual and seasonal temperature series were analyzed using linear trend and Mann-Kendall test. From the population trends Less Urbanized Period (LUP) and More Urbanized Period (MUP) were identified. The average population and surface air temperature of these two periods were used to further assess the urbanization effect. Most of the trends showed positive change in temperature with different rates in different seasons. In some cases, the trends showed asymmetry. For example, the maximum temperature at Mumbai during winter and monsoon is significantly increasing whereas minimum temperature shows significant decrease. On the other hand remaining cities recorded significant increase in minimum temperature during winter. The analysis based on LUP and MUP indicates that, on seasonal scale the difference in temperature of these two periods is significant for post-monsoon and winter seasons. Further the relationship indicates a negative change in temperature with increase in population. This indicates that the effect of urbanization is more pronounced during these seasons, as far as warming is concerned. Thus, an inconsistent climatic response to urbanization is observed at these cities.