Seismic Wave Amplification in the Central Indo-Gangetic Plains, India, Estimated from the Ratio of Soft to Hard Site Source Spectrum

Standard spectral ratio (SSR) technique has limited applicability in the estimation of spectral amplification in the vast Indo-Gangetic plains (IGP). Here we take recourse to an alternative approach using the recordings of three largest aftershocks of the 2015 Gorkha earthquake (Mw 7.9). We separately compute geometric mean source spectrum of an event from the recordings at hard sites in India and from the target IGP site. The ratio of the source spectrum from the target IGP site to the geometric mean source spectrum from the hard sites (denoted here, for brevity, as RSS) provides the desired spectral amplification. At four soft IGP sites where a comparison is possible, the spectral amplifications from RSS and SSR methods show a reasonable resemblance but also significant differences, owing to the difference in the definition of the reference spectrum. RSS method may be preferable if the input motion can only be prescribed at a generic hard site. We document amplification at 28 IGP sites using the RSS technique. The fundamental frequency, f0, of the sites increase from 0.12 Hz near the foothills of Himalayas to 2.0 Hz at the southern edge of the basin and the amplification reaches about 10. At several sites, f0 is difficult to select and the amplification of ~5 is broadband in the range 0.12–0.7 Hz. Application of SSR technique to teleseismic S-wave data recorded in the IGP reveals that this approach may be useful in the estimation of amplification at low frequencies (f < 0.5 Hz).