Abstract

A study of source, path, and site characteristics was conducted for the Uttarakhand Himalaya region using accelerogram data from 15 earthquakes (ML≥3.5). These earthquakes were recorded at the 16‐station accelerograph network operated by the Indian Institute of Technology, Roorkee, during 2005–2011. The average seismic moment (M0) of the studied earthquakes ranges between 1.20×1022 and 1.02×1024  dyn·cm, and the average moment magnitude (Mw) is between 4.0 and 5.3. The estimated corner frequency (fc) varies from 1.1 to 3.3 Hz, radius of rupture (rd) from 0.5 to 1.4 km, and stress drop (Δσ) from 6 to 172 bars, indicating continuous seismic energy release in the Uttarakhand region. The interdependence between estimated source parameters is shown by determining scaling laws for the studied region. The constant Q0 of the shear‐wave quality factor (QS=Q0fn) varies between 40 and 300 and exponent n varies between 0.85 and 1.5, providing an average relation of QS=174f1.27. However, least‐square fitting of the observed data set in the frequency range 0.1–20 Hz gives QS as 159f1.16. The value of QS demonstrates that the region is heterogeneous, seismically active, and attenuative. The high‐frequency spectral fall‐off factor (γ) varies from 1.3 to 2.1 and the upper crustal attenuation factor (κ) from 0.023 to 0.07 s at different sites, with an average of 0.044 s. The site response characteristics are estimated by horizontal‐to‐vertical spectral ratio (HVSR) and generalized inversion (GINV) techniques; results obtained from both the techniques show 1:1 correspondence. The site amplification factor varies between 2.3 and 9.4 using HVSR and between 2.6 and 10.9 using GINV among different stations. The predominant frequency ranges from 1.3 to 8.3 Hz with HVSR and from 1.3 to and 9.0 Hz with GINV.

Online Material: Tables of source, path, and site parameters with associated errors at different sites for the earthquakes recorded from December 2005 to March 2011. Figures represent accelerogram records, source spectra matching, and site amplification patterns.

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