Source Parameters of Uttarkashi Earthquake of 21 st Sept (original) (raw)
In the past four years, a permanent network of 35 strong motion accelerographs became operational in Garhwal Himalayas by Indian Institute of Technology, Roorkee (IITR). Another network of 12 short period stations is also being operated by IITR in Tehri region. These two networks recorded many earthquakes that occurred within this Garhwal Himalayan belt. One such earthquake is Uttarkashi earthquake of 21/9/2009 which occurred in vicinity of Uttarkashi earthquake of 20/10/1991 that took a toll of about 727 human lives, injured several thousand people and caused severe to partial damage to about 100,000 houses. This earthquake was widely recorded at 24 stations, 12 strong motion instruments and 12 short period instruments of Tehri network. This event occurred on 21 st Sept 2009 at 09:43:52.3 (GMT) at latitude 30.837N, longitude 78.984E, depth 53.3 km have magnitude of 4.7. Only a few events were occurred at a depth greater than 45 km in this region. This event occurred at the foot of main central thrust (MCT) and has (strike: 263.42, dip: 66.84, rake: 36.07) strike-slip trend with normal faulting component, however the Uttarkashi earthquake of 1991 has thrust faulting. In this study Brune's model that yield a fall-off of 2 beyond corner frequency is considered with high frequency dimunition factor, a Butterworth high-cut filter presented by Boore (1983) that fits well for frequencies greater than f max . The seismic moment for this event has been found to be of the order of (107 ± 0.19) × 10 23 dyne.cm and the moment magnitude has been calculated 4.7± 0.09 at different stations. The stress drop is found to be 76.3 ± 11.5 bars, while source radius for the earthquake is estimated to be (850.0 ± 38.0) m. The value of f max for this earthquake is 9.1±1.7 Hz obtained from records at various stations of different site conditions. However, this single event data is not sufficient to conclude whether f max is a source or site related phenomenon.
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