Abstract

Although the Indo-Gangetic basin is adjacent to rupture areas of large Himalayan earthquakes (M≥6), a quantitative study of the amplification of seismic waves in the region is still lacking. To obtain a first estimate of the amplification, for two years we operated an array of 10 broadband seismographs that crossed the central Indo-Gangetic basin in a north–south direction. Using earthquake recordings of shallow earthquakes at soft sites and hard reference sites, we computed standard spectral ratios (SSRs). SSRs at sites near the Himalayan foothills, where the sediment thickness is ∼4 km, reveal a broadband amplification with a fundamental frequency of 0.13 Hz. The amplification at this frequency varies between 20 and 60. The fundamental frequency increases to the south as the thickness of the sediments decreases, becoming ∼0.8 Hz at the southernmost site. The amplification at the fundamental frequencies exceeds 10 at all eight soft sites. Calculations based on reasonable earthquake source and attenuation models and application of random vibration theory suggest that peak ground acceleration and peak ground velocity at soft sites near the foothills, located 100 km from the epicenter, would be amplified by a factor of 2–4 and 6–12, respectively. All our results assume linear behavior of the sediments. Although this assumption would not be valid during intense motions resulting from large earthquakes, our results, nevertheless, provide basic building block for incorporating nonlinear behavior.

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