ESTIMATION OF SOURCE PARAMETERS OF LOCAL .4 EARTHQUAKES IN THE ENVIRONS OF KOLDAM SITE

Abstract

The Koldain site falls in the 1-limachal Lesser Himalaya in the vicinity of the Main Boundary Thrust. The Main Central Thrust is located about 80 km northeast of the dam site. The region around Koldam site is marked by the occurrence of moderate to largesized earthquake activity. In this study hypocenter parameters and source parameters of 70 local events (l.3M3.7) have been interpreted from the local earthquake data collected from the region employing a 5-station temporary network. Source parameters of events have been estimated adopting Brune model and using digital data from single station recordings. The seismic moments (M0), source radii and Brune stress drops (z\) vary from 9.83x 1017 dyne-cm to 4.77x 1021 dyne-cm, 132 m to 633 in, and 0.08 bars to 30 bars respectively. Other estimated parameters of local events include: rms stress drops, apparent stress, and/, rms stress drops vary from more than one bar to about 40 bars and were not found to be correlated with Brune stress drops. For majority of events, apparent stress and Brune stress drops are found to be correlated and follow the relation: Ta (bars) = 4.47A °89 The maximum stress drop of 30 bars is observercl at a depth of about 15 km. Apparent stress and Brune stress drops have almost same distribution with focal depths. Maximum envelope encompassing Brune stress drops show an increasing trend with focal depths upto focal depths of 20 km and then shows a decreasing trend and seems to be related to the strength of the crust. The estimates of radiated seismic energy vary from l.lxlo'' to 2.2x 10' and following the relation E (crgs) = 2x10 17 M0(dyne_cm)15. The source radii increased with seismic moments The values of J - < vary from 4.7 Hz to 15.6 I-Iz respectively. The corner frequency, f, and .tax follow almost same trend and decrease with seismic moments. Based on the local earthquake data, two scaling relations: M0 (N-rn/see3) = lxlO15 ç33 (between M0 and corner frequency) and frnax (Hz) = 314.28M&°°79 (between fmax and M0) are proposed for the region studied. These relations can be adopted to constrain the strong ground motion.