Premonitory crustal deformations, strains and seismotectonic features (b-values) preceding Koyna earthquakes |
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| Authors: | SK Guha |
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| Abstract: | There have been instances of premonitory variations in tilts, displacements, strains, telluric current, seismomagnetic effects, seismic velocities ( Vp, Vs) and their ratio (Vp/Vs), b-values, radon emission, etc. preceding large and moderate earthquakes, especially in areas near epicentres and along faults and other weak zones. Intensity and duration (T) of these premonitory quantities are very much dependent on magnitude (M) of the seismic event. Hence, these quantities may be utilised for prediction of an incoming seismic event well in advance of the actual earthquake. In the recent past, tilts, strain in deep underground rock and crustal displacements have been observed in the Koyna earthquake region over a decade covering pre- and postearthquake periods; and these observations confirm their reliability for qualitative as well as quantitative premonitory indices. Tilt began to change significantly one to two years before the Koyna earthquake of December 10, 1967, of magnitude 7.0. Sudden changes in ground tilt measured in a watertube tiltmeter accompanied an earthquake of magnitude 5.2 on October 17, 1973 and in other smaller earthquakes in the Koyna region, though premonitory changes in tilt preceding smaller earthquakes were not so much in evidence. However, changes in strains in deep underground rock were observed in smaller earthquakes of magnitude 4.0 and above. Furthermore, as a very large number of earthquakes (M = 1–7.0) were recorded in the extensive seismic net in the Koyna earthquake region during 1963–1975, precise b-value variations as computed from the above data, could reveal indirectly the state of crustal (tectonic) strain variations in the earthquake focal region and consequently act as a powerful premonitory index, especially for the significant Koyna earthquakes of December 10, 1967 (M = 7.0) and October 17, 1973 (M = 5.2). The widespread geodetic and magnetic levelling observations covering the pre- and postearthquake periods indicate significant vertical and horizontal crustal displacements, possibly accompanied by large-scale migration of underground magma during the large seismic event of December 10, 1967 in the Koyna region (M = 7.0). Duration (T) of premonitory changes in tilt, strains, etc., is generally governed by the equation of the type logT = A + BM (A and B are statistically determined coefficients). Similar other instances of premonitory evidences are also observed in micro-earthquakes (M = − 1 to 2) due to activation of a fault caused by nearby reservoir water-level fluctuations. |
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