Environmental noise measurements in Lalitpur area (Kathmandu) after the M7.8 Gorkha 2015 earthquake

In the framework of a scientific collaboration with the National Society for Earthquake Technology, Nepal, after the M7.8 Gorkha earthquake on 25 April 2015, the National Institute of Oceanography and Experimental Geophysics, Italy, organised a geophysical survey at Kathmandu, in order to have a better understanding of the seismic response of the valley in which the city is located. The main goal was to improve the knowledge of the spectral amplification of the ground motion, due to the fluvio-lacustrine sediments that constitute the Kathmandu basin, by combining the noise ratio I IN technique and velocity dispersion curve analysis. Ambient noise recordings were performed mostly in the Lalitpur area as a single station, and also by an array using ReMi measurement. This study mainly investigates the southern part of the city that was not covered by previous studies. In order to assess the resonant conditions of the soil foundation, microtremor measurements were also performed at Swayambhunath (Monkey Temple) hill and a site close to Boudhanath Stupa, the two prominent Buddhist temples-cummonasteries that had suffered significant damage during the Gorkha earthquake. The results show that the frequencies at the bottom of the basin are apparently lower than previous surveys and consequently also its depth would be greater.