Microtremor measurement to evaluate site characteristics by horizontal to vertical spectral ratio technique in Sikkim, Northeast Himalayas, India

The measurement of site amplification of microtremors is an important tool for evaluating seismic hazard at local and regional level. The H/V spectral ratio (HVSR) method is applied to estimate the amplification of ground motion at resonance frequency. The fundamental resonance frequency is estimated at 8 different sites in study region and shows the maximum vulnerability of the site to the earthquakes. This frequency ranges between a minimum of 2.8 Hz to a maximum of 9.3 Hz. The thickness of the sediments below the surface of recording sites is calculated using the empirical relationship that comes out to be in a particular range of 9.5 m?33.1 m above the bed rock. The sedimentary thickness and the resonance frequency clearly matched with geological setting of the study area. The observation shows a slow reduction in the thickness of the sediments and increase in the resonance frequency from South-western part to North-eastern part of the study region. The results show that most of the sites are highly vulnerable to seismic risks with high amplification of seismic waves in the area. Thus, current study provides a deep insight in to the seismic hazard of the Sikkim region with a clear emphasis on the site amplification and the sedimentary cover for the region.

Automated summary

The study utilized the HVSR method to estimate the resonance frequency of ground motion at different sites, revealing a correlation between sedimentary thickness and resonance frequency with the geological setting of the study area. The results indicate high vulnerability of most sites to seismic risks with significant amplification of seismic waves, providing insight into the seismic hazard of the Sikkim region.