• Volume 98, Issue 4

      December 1989,   pages  309-373

    • On self-similarity of premonitory patterns in the regions of natural and induced seismicity

      V G Kossobokov B K Rastogi V K Gaur

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      Anticipating the scale invariance of rock fracturing processes, we applied Keilis-Borok’s algorithm M8, originally designed for identifying times of increased probability (TIPS) of occurrence of strong earthquakes (M < 8.0), retrospectively to Koyna earthquakes which occurred in the region after the impoundment of the Shivaji Sagar reservoir in 1962. The algorithm which enables diagnosis of TIPS from the 7th year onwards after the commencement of the earliest available data set showed that the 5.3 magnitude earthquake of 20 September 1980 indeed occurred within a time of increased probability. This result, apart from its potential application to recognizing future TIPS in the region, points to selfsimilarity between the premonitory patterns of natural and induced earthquakes and to scale-invariant nature of their processes. Further, a typical precursory rise in seismicity followed by a relative quiescence was also found to precede all the three larger earthquakes of the sequence.

    • Anomalous geomagnetic variations and possible causative structures along foothills of Himachal Pradesh-Kumaun-Himalaya

      M V Mahashabde S Y Waghmare B R Arora

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      This paper presents the results of a magnetometer array study covering Kangra region to supplement information already available on the nature of geoelectrical structure at the foothills of Himachal Pradesh-Kumaun-Himalaya. The observed induction pattern across the Himalayan frontal fault is found to be the expression of a psuedo-geomagnetic coast effect associated with the edge of sheet current flowing in the sediments of the IndoGangetic Plains. An additional conductive sheet that is required beneath the frontal folded belt may represent increased hydration of certain sedimentary layers.

    • Effect of filtering on drift and anisotropy parameters determined by full correlation analysis

      H Chandra

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      The effect of low-pass filtering on the various drift and anisotropy parameters obtained by full correlation analysis of spaced receiver records is studied theoretically, assuming Gaussian correlation functions. A decrease in the characteristic velocityVcis shown with increasing filtering, unlessV ≫ Vv, whenVcincreases with filtering. The ratioVc/V however shows an increase as the decrease in the true drift velocityV is steeper thanVc. An increase in the size of the ground pattern with increasing filtering is also shown, while the axial ratio for anisotropic patterns shows a decrease. The results are verified with experimental data recorded earlier at Tiruchirapalli.

    • Stratification and convection over Arabian Sea during monsoon 1979 from satellite data

      M S Narayanan B M Rao

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      From the temperature and moisture retrievals from satellites, two types of indices were derived: one indicating suppression of convection and the other indicating organized deep convection. Sea surface skin temperature and equivalent potential temperatures up to 500 mbar level of the atmosphere, derived from TIROS-N satellite products, are the basis of the two indices. The maps of these indices for various phases of 1979 monsoon are compared with percentage cloudiness, a product also available from TIROS-N satellite observations. Despite the various limitations of satellite soundings, it is shown that these satellite-derived indices can be used to indicate the strengths of atmospheric convection and inversion over the oceans.

    • Cyclone track prediction using INSAT data

      P K Pal B M Rao C M Kishtawal M S Narayanan G Rajkumar

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      INSAT visible and infrared imageries of three cyclones in the Bay of Bengal during the period 1984–1987 were analysed with a view to improve the cyclone track prediction in this region. It was observed that the rotation in the major structural cloud features (as seen from the cloud-top temperature maps) associated with these cyclones in the Bay of Bengal is followed with a change in direction of their movement. This method is seen to be particularly effective when the cyclone is severe and when the major cloud features persist for a reasonably longer time. In the present study, only the direction of movement is forecast assuming a uniform speed of the cyclone.

    • Harmonic analysis of summer mean wind at 200 mbar level during contrasting monsoon years over India

      S M Bawiskar S T Awade S S Singh

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      Summer (June–August) mean zonal and meridional wind components at 200 mbar level are subjected to harmonic analysis for the years 1970, 1971, 1972 and 1979. It is found that the small scale disturbances are intense during normal monsoon years. The westerlies in the belt 10°S to 30°S are stronger during drought years. During normal monsoon years (1970, 1971) the northward transport of westerly momentum by wave number 1 at 19.6°N is large as compared to drought years (1972, 1979). The transport of westerly momentum by standing eddies is northward for all the years between 5°S and 28.7°N but large during the normal monsoon years.

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