Amplification of Seismic Waves in the Central Indo-Gangetic Basin, India

Abstract

Abstract Although the Indo-Gangetic basin is adjacent to rupture areas of largeHimalayan earthquakes (M ≥6), a quantitative study of the amplification of seismicwaves in the region is still lacking. To obtain a first estimate of the amplification, fortwo years we operated an array of 10 broadband seismographs that crossed the centralIndo-Gangetic basin in a north–south direction. Using earthquake recordings ofshallow earthquakes at soft sites and hard reference sites, we computed standard spec-tralratios( SSR s). SSR satsitesneartheHimalayanfoothills,wherethesedimentthick-ness is ∼4 km, reveal a broadband amplification with a fundamental frequency of0.13 Hz. The amplification at this frequency varies between 20 and 60. The funda-mental frequency increases to the south as the thickness of the sediments decreases,becoming ∼0:8 Hz at the southernmost site. The amplification at the fundamentalfrequencies exceeds 10 at all eight soft sites. Calculations based on reasonable earth-quake source and attenuation models and application of random vibration theorysuggest that peak ground acceleration and peak ground velocity at soft sites nearthe foothills, located 100 km from the epicenter, would be amplified by a factorof2–4and6–12,respectively.All ourresultsassumelinear behaviorofthesediments.Although this assumption would not be valid during intense motions resultingfrom large earthquakes, our results, nevertheless, provide basic building block forincorporating nonlinear behavior.Online Material: Supplementary figures of amplification, spectral ratio, andacceleration spectra.Introduction