M B Ananda
Articles written in Journal of Earth System Science
Volume 104 Issue 1 March 1995 pp 131-146
We report the results of the South Indian Strain Measuring Experiment (SISME) designed to determine whether strain related to microseismicity in the past century may have deformed the networks of the 19th century Great Trigonometrical Survey of India (GTS). More than a dozen GTS points were measured between Mangalore, Madras, and Kanyakumari in southernmost India using GPS geodesy to determine regional deformation. Detailed measurements were made near two of the original baselines of the survey to determine the reliability of dilatational strain data for the network. The regional measurements revealed negligible regional dilatational (+ 11.2 + 10 microstrain) and shear strain changes (0.66± 1.2μradians) in the southernmost 530 km of India. In addition to these measurements, we determined the rate of northward and eastward motion of a point in Bangalore (1991–1994) in the ITRF92 reference frame to be 39 ± 3.5 mm/year, and 51 ± 11 mm/year respectively. This is consistent with NUVEL-1A plate motion estimate for India. Simultaneous measurements to a point near Kathmandu reveal that the Indian plate and the Southern Himalaya are moving approximately in unison, placing an upper limit on the rate of creep processes beneath the lesser Himalaya of ≈6 mm/year, and suggesting relatively rigid behavior of the Indian plate north of Bangalore. The stability of the Indian plate is confirmed by the absence of significant changes in the lengths of the two baselines at Bangalore and Cape Comorin, which, within the limits of experimental error have not changed since 1869. The measurements place an upper limit for recent deformation in the southern peninsula, and hence a lower limit for the renewal time for intraplate earthquakes in the region of approximately 10,000 years, assuming shear failure strain of approximately 100 μradians. This, in turn, implies that recurrence intervals for Peninsular Earthquakes far exceed the length of the written historic record, suggesting that the characterisation of seismic recurrence intervals from historical studies is likely to be fruitless. In contrast, the SISME experiment demonstrates that the noise level of geodetic studies based on 19th century GTS data is less than 0.02 μstrain/year, providing considerable scope for delineating regions of anomalously high seismogenic strain, by GPS measurements at all available trig points of the 19th century GTS survey.
Volume 112 Issue 3 September 2003 pp 331-345
The 26th January 2001 Bhuj earthquake occurred in the Kachchh Rift Basin which has a long history of major earthquakes. Great Triangulation Survey points (GTS) were first installed in the area in 1856–60 and some of these were measured using Global Positioning System (GPS) in the months of February and July 2001. Despite uncertainties associated with repairs and possible reconstruction of points in the past century, the re-measurements reveal pre-seismic, co-seismic and post-seismic deformation related to Bhuj earthquake. More than 25 Μ-strain contraction north of the epicenter appears to have occurred in the past 140 years corresponding to a linear convergence rate of approximately 10 mm/yr across the Rann of Kachchh. Motion of a single point at Jamnagar 150 km south of the epicenter in the 4 years prior to the earthquake, and GTS-GPS displacements in Kathiawar suggests that pre-seismic strain south of the epicenter was small and differs insignificantly from that measured elsewhere in India. Of the 20 points measured within 150 km of the epicenter, 12 were made at existing GTS points which revealed epicentral displacements of up to 1 m, and strain changes exceeding 30 Μ-strain. Observed displacements are consistent with reverse co-seismic slip. Re-measurements in July 2001 of one GTS point (Hathria) and eight new points established in February reveal post-seismic deformation consistent with continued slip on the Bhuj rupture zone.