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

Ami Shah
[email protected] and [email protected]

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

Introduction
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

Four Trimble 4000SSI and two Trimble 5700 dual frequency geodetic GPS receivers were used with the chock ring and Zephyr type of antennas respectively for this field work. The GPS data were organized into 24 hours segments with the combination of data from some of the surrounding IGS sites, namely IISC, BAHR, MALI, LHAS. to constrain the site co-ordinates. Coordinates for each IGS site for each year were calculated with the help co-ordinates and velocities in corresponding directions available in the year 2000 in Indian Terrestrial Reference Frame (ITRF).

Fig. 1 Location and Network of GPS Stations for 2003 Epoch

Data Processing with Bernese, Analysis and Results
The data for each epoch is processed with the scientific software Bernese 4.2, developed by the University of Bern. Network considered for the different campaign for the year 2003 with the reference to their Northing and Easting is shown graphically by figure 1.

After processing with the Bernese, out put file gives the position of the stations in terms of X, Y, Z and also in form of Height, Latitude and Longitude. Deviation in northing, easting and height separately for each GPS station was calculated by converting from latitude and longitude. This is graphically represented in figure 2 (A) and (B), which shows that most of the stations are moving in the North-East direction.

*** Inconsistent result, being analysed.
— RAPS station was not established in 2001 epoch so there is no data for the year 2001.

Fig. 2 Difference in Co-ordinates of the Stations

Table 3: Local Deformations for station

In figure 3, baseline length is plotted for the station NETR to NARA, ROHA and SAMD to scale in meter. Separately the movement of the stations NARA and NETR is shown for the three years with their directions through arrows (to the scale in cm from table 2). Change in baseline lengths of three years is achieved by joining these points for NARA and NETR (for values refer table 3). For the results in table 3, rms values are ranging from minimum 0.1 mm to maximum 0.5 mm with the mean value of 0.367 mm.

Fig. 3 Local Deformations for NETR to NARA, SAMD and ROHA

Table 4 gives the baseline length for some of the stations of the network w.r.t two IGS stations IISC, LHAS for 3 epochs and also gives the change in length for two consecutive years in mm. It is presented as a regional and global deformation in the figure 4 (A) and (B) respectively. Table 4: Difference in Baseline Length for w.r.t IISC, LHAS

Conclusions
By processing post-earthquake GPS data collected for February 2001, 2002 and 2003, changes in baseline length are calculated with 0.5mm RMS value. The estimation of horizontal regional and global deformation in baseline is done with the help of IGS sites IISC, MALI, BAHR and LHAS. Standard ionospheric and tropospheric models were taken into consideration during processing the data.

Local deformation shows an expansion for NETR to NARA and contraction for NETR to ROHA from 2001 to 2003.

Regional (Intra-Plate) deformation is estimated between different stations of Bhuj region and IISC; both on the Indian plate. The deformation is in the range of -54.2 mm to 40.8 mm with average of -4.65 mm. The baseline between IISC-SAMD, JHUR, KAKA, and SUMT expanded, IISC-NETR, NARA, ROHA, PIRP, CHAR, and KNDK have expansion and contraction, and IISC-KANM, RAPS, CHIT have contraction from 2001 to 2003.

The estimated horizontal global deformation in baseline length from IGS stations LHAS, BAHR, and MALI is -79.3 mm (minimum) to 40.8 mm (maximum) with a mean value of -1.98 mm. Global (Inter-Plate) deformation means the deformation studies w.r.t. different plates. The baseline lengths from LHAS to different stations of Bhuj region have expansion for 2001to 2002 and contraction for 2002 to 2003 in majority. The deformations are in the range of -59.1 mm to 24.1 mm.

Acknowledgements
The data used for this study was collected by the GPS team of IIT Bombay, under a research project with Prof. M.N. Kulkarni as the Principal Investigator. The data processing and the analysis has been carried out by the first author as a part of her M.Tech project under the guidance of second author with contribution from the members of IIT Bombay GPS team, at the Department of Civil Engineering, IIT Bombay. The funding and support for instrumentation and field and analysis work, provided by the DST, at the IIT Bombay are gratefully acknowledged.

References

• Bhatnagar, P. K., “Report on international seminar on Earthquake strategies with special reference to India”, Indian Surveyor, January 2002.
• 2. Champati Ray, P. K., “Appraisal of Kachchh Earthquake “, GIS development, Issue-3, Volume-5, March-2001.
• Champati Ray, P.K., Foreste Kenny, and Roy, P. S., “Bhuj Earthquake”, GIS development, August-2001.
• Chang, C. C., “Estimation of Horizontal Displacements Associated With the 1999 Taiwan Earthquake”, Survey Review, Volume-35, No.-278, October-2000.
• Department of Science and Technology, “Earthquake Research in India”, Note of DST, Earth System Science Division, Government of India, 1999.
• Herring, Thomas A., “Geodetic Application of GPS”, Proceedings of the IEEE, pp. 92-110, Volume-87, No.-1, January-1999.
• Hugentobler, U., S. Schaer, P. Fridez, “Bernese GPS Software Version 4.2 Documentation”, Astronomical Institute University of Bern, Switzerland, 2001.
• Kulkarni, M. N. “The GPS Data Processing for Seismic Hazard Assessment”, Indian Surveyor, pp. 28-29, 1999.
• Sinvhal, A., Bose, A. A., Prakash, V., Bose, A., Saraf, A. K., and Sinvhal, H., “Damage, Seismotectonics and Isoseismals for the Kutch Earthquake on 26th January, 2001”, Workshop on Recent Earthquake of Chamoli and Bhuj, May 24-26, 2001.