Strong ground motions from two moderate size (Mw5.5) Kachchh intraplate earthquakes, Gujarat, India

During 2006–2016, a strong motion seismic network of twenty 3-component accelerogarphs was deployed by the National Geophysical Research Institute, Hyderabad, India, in the epicentral region of the 2001 Mw7.7 Bhuj earthquake. On 7 March and 6 April 2006, two earthquakes of Mw5.5 have occurred in the Kachchh seismic zone, which led to an excellent dataset from 16 to 18 strong-motion accelerograph sites. This dataset enabled us to estimate strong motion parameters for these earthquakes that allowed us to examine the suitability of the design response spectra of BIS (criteria for earthquake-resistant design of structures, part I—general provisions and buildings, Bureau of Indian Standards, 2002) in Kachchh, Gujarat. Our study reveals that the estimated normalized response spectra at strong-motion accelerograph sites, which are lying in tertiary formations or near a zone of geological contact between Jurassic and tertiary formations, exceeded the design response spectra at 0.07–0.2 s, correlating with complete collapse of low-rise buildings, water tanks and dams in the area during the 2001 Bhuj earthquake. While the normalized acceleration spectrum of hard sediment (rock site) is found to not exceeding the design spectrum, correlating with the lack of damage in the Mesozoic hill zone. The estimated normalized acceleration spectra at SMA sites lying on hard sediments of Jurassic/Mesozoic formations are found to be lower than the design spectra, which suggests that these stations are less hazardous. We also notice that spectral acceleration values at few sites lying on quaternary formations have exceeded the design spectra at 3–4 s, suggesting these sites hazardous for engineered reinforced structures like bridges. Our estimated mean pseudo velocity spectra show peaks at 0.2, 0.5, 1.2 and 6 s. The peak at 6 s probably can cause damages to the reinforced engineered structures while the peak at 1.2 s could be causing damages to the multistoried reinforced buildings, as observed during the occurrence of the 2001 Bhuj mainshock. Thus, our estimated normalized acceleration spectra show peaks exceeding the design spectra at shorter (0.07–0.2 s, 0.5 s, 1–1.2 s) as well as longer periods (3–4 s, 6 s), which could be attributed to local site effects. We also notice that the modeled normalized response spectra for the shallow March event shows a better correlation with the NGA West2 model for tectonically active regions of US while the modeled normalized spectra for the deeper April event suggest a better correlation with the NGA east model for the stable parts of USA. We propose that the design spectra for the Kachchh region, Gujarat, India, should be modified incorporating local site, path and source effects.