Lapse time and frequency-dependent attenuation characteristics of coda waves in the Northwestern Himalayas

Abstract

We analyzed the local earthquakes waveform recorded on a broadband seismic network in the northwestern Himalayan Region to compute lapse time and frequency dependence of coda Q (Qc). The observed Qc values increase with increasing lapse time at all frequency bands. The increase in Qc values with lapse time is attributed to an increase in Qc with depth. This implies that attenuation decreases with increasing depth. The approximate radius of medium contributing to coda generation varies from 55 to 130 km. By comparing the Qc values with those from other regions of the world, we find that they are similar to those obtained from tectonically active regions. The estimated Qc values show a frequency-dependent relationship, Qc = Q0fn, where Q0 is Qc at 1 Hz and n represents degree of frequency dependence. They represent the level of heterogeneity and tectonic activity in an area. Our results show that northwest Himalayas are highly heterogeneous and tectonically very active. Q0 increases from 113 ± 7 to 243 ± 10 and n decreases from 1.01 ± 0.05 to 0.85 ± 0.03 when lapse time increases from 30 to 70 s. As larger time window sees the effect of deeper part of the Earth, it is concluded that Q0 increases and n decreases with increasing depth; i.e., heterogeneity decreases with depth in the study area.