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1 2022

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Determination of characteristics of the microearthquakes occurring in Lesser and Higher Himalaya India

Goswami Neetu, Gupta Subhash Chandra

Acta Geophysica

2022

Vol. 70, no 1

1--13

The present study aims to estimate the frequency-dependent attenuation relations and source parameters of local earthquakes that occurred in the Garhwal Lesser and the Higher Himalaya around Uttarkashi town, India. The local earthquake data of 234 events recorded from a 12-station digital telemetered local seismological network in the region during 2008 to 2017 have been used for this purpose. The first data set of 126 events occurred around Uttarkashi in the Lesser Himalaya, the second data set of 73 events located northwest of Uttarkashi in the Higher Himalaya and the third data set of 35 events located about 35 km east of Uttarkashi in the vicinity of 1991 Uttarkashi earthquake (Ms~6.8) in the Higher Himalaya. The source parameters such as: seismic moment (Mo), source radius (r) and stress drop (∆σ), have been estimated from the displacement spectrum of the SH component of S-wave by applying Brune’s circular source model (Brune in J Geophys Res 75: 4997 5009, 1970). The attenuation characteristics are examined by estimating the quality factor (Qc) from the decay of coda waves of vertical component record using the Single Backscattering model (Aki and Chouet in J Geophys Res 80: 23 3322 3342, 1975). The Qc estimated at different values of frequency (1.5 to 24 Hz) have shown that Qc a function of frequency. Here, we obtain the following frequency-dependent attenuation relationships: Qc=110 f 0.99 (using first and second data sets) and Qc=142 f 0.87 (using third data set). The source parameters of 234 events show that more than 96% of events (227 events) have a low stress drop (less than 10 bars), five events have a stress drop between 10 and 100 bars and two events have exceptionally high stress drop of 271 and 532 bars. This shows that the region produces, by and large, low stress drop events. The variation in stress drops with depth demonstrates no clear increase or decrease in stress drop with focal depth. The Mo-fc scaling relations have been obtained for events with magnitudes two and above, Mo=2.7× 1016 fc −3.98 (for data set 1), Mo=1.4× 1014 fc −2.74 (for data set 2), and Mo=1.1× 1015 fc −3.28 (for data set 3).