Dynamic Characteristics of Subsoil Deposit of Tripura by Nakamura Method

Determination of local site effect by calculating fundamental period and amplification of motion using Nakamura (Railway Tech Res Inst Q Rep 30:1–9, 1989) proposed H/V spectral ratio approach was recognized to be a simplified, reliable and well-accepted method. Both strong motion recordings and ambient noise recorded from micro-tremor can be used to construct the H/V spectral ratio. It is a well-known fact that the affect of body wave in an earthquake signal is relatively higher compared to Rayleigh wave and H/V spectral ratio approach is a useful method to differentiate the influence of both the waves. Tripura is one of the Northeastern states of India surrounded three sides by Bangladesh and rest by Assam which was categorized as the highest seismically vulnerable zone (BIS 2016). In fact, the geological history of Agartala, the capital city of Tripura, and other river valleys of Tripura are relatively young of Holocene age group. Significant depth of sedimentation with soft alluvial and fluvial type at top layers is also reported. Hence, the possibility of local site effect may lead to significant effects. From the above viewpoint, present study is an attempt to investigate dynamic characteristics and amplification ratio after analyzing strong motion records following Nakamura (Railway Tech Res Inst Q Rep 30:1–9, 1989) proposed H/V spectral ratio approach. The acceleration history record of January 3, 2017, earthquake epicentered at North part of Tripura is used to construct the H/V spectral ratio. Results indicate that peak amplitude of H/V spectra is recorded at around 1.65–1.75 Hz for both the sites, which comprises SH waves of higher energy content and further troughs are noted at higher frequencies confirming moderate effect of Rayleigh waves. This limited study highlights amplification potential in order of 1.65–1.75 Hz indicating influence of local site effect at various locations of Tripura and need of microzonation map of important towns.