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CRUSTAL DEFORMATION STUDIES

Crustal deformationstudies using GPS Geodesy and Earthquake Data

C. D. Reddy, P. S. Sunil, K. Vijaykumar, S. H. Mahajan, M. Ponraj, S. Amirthraj, and Sandeep Sathian

 

Introduction

The Global Positioning System (GPS) is a space based navigation system, consisting of a constellation of 24 satellites, in six orbital planes with 55° inclination to the equator. The satellites are placed at a height of about 20,200 km with 12 hours orbital period and operated by the United States Department of Defense (DOD) for accurate determination of position, velocity and time. All the GPS satellites are controlled by system tracking stations, ground antennae and the master control station. The distance to GPS satellite is estimated by measuring the time a radio signal takes to reach us from the satellite. While the use of the GPS is extensive in defense, navigation and surveying applications, it is being used in geo-science, ionospheric & atmospheric studies, global climate changes, observing polar motion & earth rotation rate, mapping the gravity field, detecting seismo ionospheirc effects, transport and communications, environment management, for accurate time and frequency etc. At present it is able to achieve much better accuracies due to processing techniques which circumvent the purposeful degradation of the GPS signals. This motivated earth scientists to use GPS for monitoring the slow and relentless crustal deformation due to plate motion, earthquake, volcanic activities etc. by employing a technique called carrier tracking which allows determining baseline length within a few millimetres. Changes in deformational rates have intrinsic value in understanding the physics of the above mentioned processes. This in turn provides the earth scientists with the knowledge required for forecasting earthquakes and provide warnings for impending seismic risk.

Recent studies by the GPS Geodesy Group at IIG

 

Co and postseismic characteristics in response to the 2004 Sumatra earthquake:

 
 

The Sumatra earthquake on December 26, 2004 provided an excellent opportunity to investigate the post-seismic crustal deformation and thereby understand the rheology of the crust and mantle. Subsequent to this earthquake, we monitored the post-seismic deformation at strategically located five continuous GPS sites in Andaman and Nicobar region. The post-seismic transients are obtained and the viscoelastic modeling has been carried out. Post-seismic flow below a depth of 55–60 km with low viscosity of the order of 1019 Pa S can explain observed far field motion. There is also a contribution from upper mantle to post-seismic deformation which follows power law rheology. These results lead us to infer that the Sumatra-Andaman mechanical lithosphere is about at ~55 km depth (Reddy et al., 2009).

A rheological model of postseismic response due to 2004 Sumatra-Andaman earthquake

The Sumatra earthquake on December 26, 2004 provided an excellent opportunity to investigate the post-seismic crustal deformation and thereby understand the rheology of the crust and mantle. Subsequent to this earthquake, we monitored the post-seismic deformation at strategically located five continuous GPS sites in Andaman and Nicobar region. The post-seismic transients are obtained and the viscoelastic modeling has been carried out. Post-seismic flow below a depth of 55–60 km with low viscosity of the order of 1019 Pa S can explain observed far field motion. There is also a contribution from upper mantle to post-seismic deformation which follows power law rheology. These results lead us to infer that the Sumatra-Andaman mechanical lithosphere is about at ~55 km depth (Reddy et al., 2009).

Weak mantle in northwest India probed by geodetic measurements:

Far-reaching transient surface deformation following the 2001 Mw 7.6 Bhuj intraplate earthquake in NW India reveals visco-elastic flow in the mantle with only modest contributions from crustal relaxation processes. The relatively rapid decay of GPS-measured deformation rates indicates increasing effective viscosities of the mantle from 3×1018 Pa s in the first 6 months to 2×1019 Pa s during the 6-year observation period, consistent with a time and stress-dependent rheology, such as power-law flow by dislocation creep (Chandrasekhar et al., 2009).

Post-seismic crustal deformation and strain rate in Bhuj region:

The Mw 7.6 Bhuj earthquake in Gujarat, western India, which occurred on 2001 January 26 was a major intraplate event in the Indian subcontinent. To study the characteristics of transient post-seismic deformation at the earth’s surface, five GPS campaigns were conducted at 14 sites, in Bhuj region during 2001–2002. The daily variations in the site position coordinates and the baselines during the early after-shock period showed no short-term post-seismic crustal deformation. The estimated principal strain from the velocity field gives average compression and extension rates of 0.07 and 0.04 micro-strain yr−1, respectively. A zone of maximum compressive strain rate of 0.30 micro-strain yr−1 with azimuth of 11◦ delineated north of the epicentral region shows good agreement with seismic deformation along the blind fault derived from earthquake focal mechanisms (Reddy & Sunil, 2008).

Velocity strain-rate fields on Schirmacher glacier, east Antarctica:

GPS data were collected at 21 sites and analyzed to estimate the site coordinates, baselines and velocities. Horizontal velocities of the glacier sites lie between 1.89_0.01 and 10.88_0.01ma–1 to the northnortheast, with an average velocity of 6.21±0.01ma–1. The principal strain rates provide a quantitative measurement of extension rates, which range from (0.11±0.01)x10–3 to (1.48±0.85)x10–3 a–1, and shortening rates, which range from (0.04±0.02)x10–3 to (0.96±0.16)x10–3 a–1. The velocity and strain-rate distributions across the GPS network in Schirmacher Glacier are spatially correlated with topography, subsurface undulations, fracture zones/crevasses and the partial blockage of the flow by nunataks and the Schirmacher Oasis (Sunil et al., 2007).

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