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Some Recent Results from GPS Studies for the January 2001 Bhuj Earthquake

Ami Shah
het_shah@yahoo.com and amishah@iitb.ac.in

M. N. Kulkarni, V.S. Tomar, S. Likhar
Department of Civil Engineering
Indian Institute of Technology Bombay
Mumbai-76, India

On January 26, 2001, one of the most destructive earthquakes with 6.9 Richter scale and epicenter at 23.4°N (Latitude) and 70.28°E (Longitude) ever to strike India occurred in the Kachchh region of Gujarat State in western India. The earthquake was felt in nearly all parts of India and surrounding regions. The Kachchh peninsula has undergone many stages of deformation in the geological past. This crustal deformation/re-adjustment is still continuing resulting in high seismic activity in the form of earthquakes of varying magnitudes.

Among the natural calamities, earthquakes are the most destructive, in terms of loss of life and destruction of property. The Earth is formed of several layers that have very different physical and chemical properties. The outer layer, which averages about 70 kilometers in thickness, consist of irregular shaped plates that slide over, under and past each other on top of the partly molten inner layer. “Six better-known plates are: the American, the African, the Eurasian, the Arabian the Indian and the Pacific” (Hemmady, 1996). Plates directly relevant to India are the Indian, Eurasian and Arabian plates. The rocky crust of the earth is not stable, but undergoes complex movements due to continental plate movements. There are slow vertical movements of uplift and depression, the rate of which is measurable only by lengthy precise observations; the effects of which are seen at many places. The earthquakes are sudden crustal movements that can be detected and measured by special instruments. The earthquake shocks are caused mainly by adjustments to strains in crustal rocks due to movements along faults and fracture surfaces where stresses accumulate locally, in the rocks until breaking point is reached, when slip along the fracture occurs.

Role of GPS for Crustal Deformation Studies
GPS is the satellite based surveying and navigation system for determination of precise position and time, using radio signals in both real-time and in post processing mode. GPS finds numerous applications in various fields, including navigation, surveying, mapping, remote sensing, and in earthquake hazard assessment because it gives very precise measurements related to the station location. For everyday surveying, GPS has become a highly competitive technique to the terrestrial surveying methods using theodolites and EDMs (Electronic Distance Measurements). It is highly advantages in use for determining precise horizontal positions of points more than a few tens of kms apart. Phase information in the GPS signal can be used to determine the position difference between the sites with an accuracy of a few millimeters in the horizontal and vertical directions. Thus, GPS provides an economic and efficient technique with sufficient accuracy to measure the mm-level crustal deformations produced by the earthquakes [Kulkarni, 1999]. With the high accuracy achieved by GPS in estimation of base line lengths, this relatively new geodetic positioning technique has assumed great importance in crustal dynamic studies. Precise GPS repeat measurements and data processing to achieve high accuracy yield the estimates of deformations of the Earth's crust over the period of the repeat observations, both in the horizontal and vertical directions. Thus, GPS data is valuable for understanding the complex process of Earthquakes (Kulkarni, 1999).

Kachchh Region
This region lies within 400 km of the active plate boundary zone between the Indian subcontinent and the Asian plate along the India-Pakistan border. The Kachchh basin is highly faulted. According to earth scientists, faults are composed of segments that may rupture individually or in groups of adjacent segments during the earthquake. The main faults in this region are: The Allah Bandh Fault, The Kachchh Mainland Fault, Vighodi Fault, Katrol Hill Fault, North Kathiawar Fault, Banni Fault, Island Belt Fault, Nagar Parker Fault (Sinvhal, 2001). The location of the epicenter and devastation indicate that the Kachchh Mainland fault or a part of it possibly was reactivated on 26th January 2001.

GPS Data Collection
Indian Institute of Technology (IIT) Bombay has been carrying out extensive GPS survey in the Bhuj region immediately after the devastating earthquake of 26th January 2001 initiated by the “Department of Science and Technology (DST) of the Government of India. Data used for this study has been collected by the GPS team from IIT Bombay for the geodetic control network consist stations at approximately 20-40 km spacing, of the series of the Great Triangulation (GT) Network of India. During three GPS field epochs of February 2001, 2002 and 2003, data has been collected for total 15 stations (see Table 1 and Figure 1).

Table 1: GPS Stations
Station No.  Station Name  Station ID  Status
1  Netra  NETR  Old GT Point
2  Nara  NARA  New GPS Point
3  Roha  ROHA  Old GT Point
4  Samdhan  SAMD  New GPS Point
5  Sumatra  SUMT  New GPS Point
6  Asaparamata  ASAP  New GPS Point
7  Charakda  CHAR  New GPS Point
8  Jhuran  JHUR  New GPS Point
9  Kakarawa  KAKA  New GPS Point
10  Sukhpur  SUKH  New GPS Point
11  Chitroad  CHIT  Old GT Point
12  Kanmer  KANM  New GPS Point
13  Pir-Pita-I-Shah  PIRP  Old GT Point
14  Rapar School  RAPS  New GPS Point
15  Kanduka  KNDK  New GPS Point

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