ESTIMATION, OF STRONG GROUND MOTION PARAMETERS OF UTTARKASHI AND CHAMOLI EARTHQUAKES

Abstract

Strong ground motion parameters of two moderate earthquakes namely, Uttarkashi ,earthquake of 1991 (mb = 6.6) and Chamoli earthquake of 1999 (mb = 6.4) have been estimated from the available digital strong motion data. The data-set comprised of 23 accelerograms obtained from the strong motion network deployed in the Garhwal and Kumaon Himalaya. For Uttarkashi earthquake seven time domain, three frequency domains parameters and for Chamoli earthquake seven time domain parameters have been estimated. For two earthquakes durations of strong ground motion at various stations have been estimated employing two methods based on integrating squared accelerations and cumulative RMS function. Estimated strong motion parameters include RMS acceleration, arias intensity, cumulative absolute velocity, characteristic intensity, earthquake power, spectrum intensity, acceleration spectrum intensity, effective peak acceleration (Bolt and Abrahmson, 1982), EPA & EPV (ATC 3-6), and Vmax/Amax . For Uttarkashi earthquake most of the strong motion parameters vary exponentially with hypocentral distance except cumulative absolute velocity that exhibits a linear decay with hypocentral distance. For Chamoli earthquake no specific pattern of the variations of the parameters with hypocentral distance have been brought out. The relationship between PGA and various parameters is linear except for CAV which shows logarithmic variation. No specific pattern has appeared between PGA and spectrum intensity. Effective peak accelerations (EPA) computed at three threshold levels (0.01 g, 0.02g, and 0.04g), and at 90 and 95 percentiles have brought out that EPA computed at 0.02g threshold level and 95 percentile is a better measure of PGA. The equivalent harmonic period of wave motion computed at various stations from Vm /Amy ratios has shown that Tehri, Gopeshwar and Joshimath stations are located over deep stiff soil, whereas Ghansali and Ukhimath stations are located over stiff soil and rock respectively. Durations of strong ground motion estimated at various stations from integrating squared accelerations gave shorter duration as compared to durations computed using cumulative RMS function. The strong ground motion durations measured from cumulative RMS function are better representative of the duration of shaking at various stations. No correlation has been brought out between the durations obtained using two methods.