Estimation of current seismic hazard using Nakamura technique for the Northeast India

Some regions of Indian subcontinent are highly earthquake prone area, and some are not. However, among them north-east region of India is most geologically complex one, which are found to be highly earthquake hazard prone region. This region has experienced several disastrous earthquakes like Assam earthquake of 1950 for 8.1 and Shillong earthquake of 1897 for 8.7 magnitude in the past. Several methods are used for seismic hazard measurement. Among these methods the site response analysis is found to be most valuable part of the seismic hazard assessment. In this work we have used Nakamura technique (H/V site response spectral ratio) for estimating the site amplification of seismic ground motion. Fundamental frequencies estimated are in the range 0.6–10 Hz for the 15 broadband stations of India Meteorological Department distributed over the study region. ZIRO station and its surrounding area are found to have high liquefaction indices (kg) value of 250. The horizontal-to-vertical ratio curve variation of higher amplitude with lower frequency helped to find thick soil zones in Aizawl and Agartala. Similar analysis also gave flat response at some sites near Jorhat and Dibrugarh having thin soil. The H/V rotate analysis helped us to see the variation of fundamental frequency with azimuth. Based on the observations, frequency and amplitude counter map of the north-east region represented. Further considering the site amplification due to very high fundamental frequency for Shillong, Guwahati, Imphal and Itanagar city can be considered under high seismic hazard scanner in the north-east India region.