Abstract

The horizontal components of S-wave data from 38 aftershocks (Mw 3.0–5.6) of the 2001 Bhuj earthquake (Mw 7.7) recorded at 18 strong-motion sites in the Kachchh seismic zone, Gujarat, India, were analyzed to estimate the site response, in the frequency range 0.2–10.0 Hz using the reference site generalized inversion technique. The site response values thus obtained at each site were averaged (logarithmically) over two frequency bands, 0.2–1.8 and 3.0–7.0 Hz, respectively, to study the spatial variation of site response in the Kachchh seismic zone. In the 0.2–1.8 Hz frequency band, the high site response values (1.0–1.7) were observed to be associated with the regions south of the Kachchh mainland fault that suffered extensive ground failure during the 2001 Mw 7.7 Bhuj earthquake. In the 3–7 Hz frequency band, the average site response estimates in the area of Jurassic and Quaternary formations vary from 0.22 to 1.53, whereas those of Tertiary formation lie between 0.60 and 1.30. The maximum site response values at both 0.2–1.8 Hz (∼1.7) and 3–7 Hz (∼1.53) are estimated at the Sikara site, which lies in a zone of geological contact between Jurrassic and Tertiary formations. The deamplification observed at the AMS station in the 0.2–10 Hz frequency band could be attributed to a very localized geological variation of the soil type or geometry of the basin near the site. It is apparent that in both 0.2–1.8 and 3–7 Hz frequency bands, the presence of geological contacts and sediment thicknesses controls the site responses in the Kachchh seismic zone. It is also inferred that larger site response (>1.2) values in the 0.2–1.0 Hz frequency band could be indicating the probable presence of soil class C (forumla) and D (180<Vs≤360 m/sec) in the Kachchh basin (according to 1997 National Earthquake Hazards Reduction Program [NEHRP] provisions).

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