Dept of Civil Engineering, IIT Bombay
In recent times space geodetic techniques have undergone phenomenal advancements, and have made available modern instruments like the Global Positioning System (GPS), Very Long Baseline Interferometry(VLBI), Satellite Laser Ranging (SLR), Lunar Laser Ranging (LLR) etc. Today these techniques have become the sine qua non for monitoring the crustal dynamics and, therefore, form an important component of studies towards understand the earthquake mechanism. However, there is a need for combining data from these diverse techniques in an integrated analysis to understand this complex natural process, as none of these techniques alone can yield a complete insight into the earthquake generating mechanism.
d In India, an extensive high precision Geodetic & Geophysical control network has been established today by Survey of India (SOI), the national mapping agency of Govt. of India, for the primary purpose of national mapping. More recently, IIT Bombay and various other national organisations and institutions, including Geological Survey of India (GSI), National Geophysical Research Institute (NGRI), Indian Institute of Geomagnetism (IIG, Mumbai), Centre for Mathematical Modelling & Computer Simulation (CMMACS, Bangalore), and Wadia Institute of Himalayan Geology (WIHG, Dehradun), have taken up comprehensive geodetic, geophysical and geological surveys. The extensive control network established between these organisations, and the huge amount of valuable data thus generated, have contributed significantly towards monitoring the crustal dynamics of the Indian sub-continent.
Present Work by IIT Bombay
A collaborative work, involving the GPS teams from IIT Bombay,CMMACS, and IIG, Mumbai, has been taken up, to carry out immediate GPS observations in the earthquake-affected area of Gujarat near Bhuj, to understand the post-earthquake crustal deformation pattern, and to monitor any future crustal dynamics in this region. Teams from University of Colarado, USA, and University of Tokyo, Japan are also carrying out GPS observations in the area.
The GPS Network:
the fieldwork of Feb. 2001, the IITB team, with Prof Madhav Kulkarni, has
established a GPS station network consisting of a total of 12 stations, covering
an area of about 300 km in EW and 100 km in NS directions, around the epicenter
(see Figs. 1 and 2). Extended observations
have been carried out at each station, using geodetic dual-frequency GPS receivers.
Further, repeat observations will be carried out at these stations after a
gap of approximately 6 months. In addition, more GPS stations will be established
in the area, along with few reference stations outside the Bhuj region. The
aim of establishing these stations is to monitor the deformations of the epicentral
region from a reference point outside the region.These observations will be
useful in assesing the future crustal movements in the region,which may help
identify any critical incident in advance.
Map showing GPS Network
established for post-earthquake deformation studies
Map showing GPS Network established for post-earthquake deformation studies
A REMOTE SENSING APPROACH
Centre for Studies in Resource Engineering, IIT Bombay
Another team from the Center of Studies in Resources Engineering(CSRE), from IIT Bombay, headed by Dr K S Rao has taken up the study of the Bhuj region using remote sensing techniques. Remote sensing is a powerful tool for mapping surface deformations of the order of a few millimeters caused due to earthquakes. This specialized tool is known as Differential Synthetic Aperture Radar Interferometry (or simply D-InSAR). To exploit this important technique, the Department of Science & Technology ( Govt of India ) has constituted an Indian InSAR Group with CSRE, IIT Bombay as a nodal Institute to coordinate its activities at the national level.
Present Study Approach
Reconnaissance survey of Bhuj test site (100 x 100 km) has been carried out during February 8 - 14, 2001 with a team of Scientists/engineers of CSRE to gather preliminary ground truth on various aspects of earthquake devastation., regions of liquefaction etc. Indian Remote Sensing Satellite (IRS-1C) data before and after the earthquake were also acquired from National Remote Sensing Agency (NRSA) and processed to obtain information on the major changes due to earthquake such as surface deformation, liquefaction, disruption of transport and change of drainage network. As examples, two sets of photographs are included here, which indicates typical images obtained and their interpretation.
IRS PAN Images of Bhimsar
Before Earthquake After Earthquake
The two images shown in figures, after filtering, show sharp changes in the internal road networking of the city. In this image, a change in moisture content is also visible.
Before Earthquake After Earthquake
The comparison of the above two images, acquired before and after the earthquake, shows that water has surfaced in the drainage channel, and the salt pan patterns have also been disturbed ( at Gandhidham ). The road networking is not visible clearly on the image after the earthquake. The drainage pattern is changed and shows more water at the surface.