Estimation of the source parameters of the Himalaya earthquake of October 19, 1991, average effective shear wave attenuation parameter and local site effects from accelerograms

A simple and rapid methodology based on band-limited strong motion data to obtain estimates of double couple fault plane solution, average effective shear wave attenuation parameter (Q(seff)) and locate and analyze sub-events of the earthquake process has been presented. This method will be particularly efficient in the study of smaller earthquakes that may have been recorded locally at a small number of seismological or strong motion observatories. The method has been applied to investigate the moderate magnitude Uttarkashi earthquake of October 19, 1991, that occurred in the central seismic gap of Himalaya and was recorded locally on a strong motion array. The spectra of the high-energy packets observed in the 'SH waves' (as approximated by the transverse components of accelerograms) is modeled in terms of Brune omega square model. Thereby we estimate parameters of the earthquake comprising of the fault plane solution (strike: 315 degrees, dip: 19 degrees, rake: 55 degrees), seismic moment (1.7 x 10(26) dyne cm), stress drop (53 bar), fault slip (186 cm), source duration (8.2 s) and moment magnitude (6.7), which are found to be consistent with the corresponding values reported in published studies. The average value of Q(seff) in the region is estimated to be 731 264, suggesting a sufficiently cool thermal regime. Thus, brittle behavior of crustal rocks with capacity for supporting earthquakes is expected to be a dominant process in the region. The effect of site amplifications on peak ground acceleration (PGA) has been investigated using a comparison of observed PGA with a representative regression for the region. Furthermore, two distinct phases of energy releases, corresponding to two sub-events of the earthquake have about 6 s and 20 kin in space. The fault plane solution, seismic moment and stress drop of these asperities have also been obtained. (c) 2005 Elsevier B.V. All rights reserved.