Investigation of Natural Frequency Reduction Mechanism of a Historic Masonry Building in Patan following the 2015 Gorkha Earthquake in Nepal

A Mw 7.8 earthquake struck the Gorkha district of Kathmandu, Nepal on April 25, 2015. In Patan, vibrational characteristics of a two-story masonry building near Patan Durbar Square had been measured prior the earthquake. During the damage survey of the building after the earthquake, several new cracks were found and the natural frequencies after the earthquake were found to be smaller than those before the earthquake. In this study, microtremor observation was conducted at the building site as well as at ground motion observation sites, and the ground motion at the building site during the Gorkha earthquake was estimated. An analytical model of the structure representing the pre-earthquake condition was established so that the natural frequencies of the first five modes match the pre-earthquake measurements. Then, the dynamic analysis of the building using the estimated ground motion as the input is conducted using the refined distinct element method. The reduction of the first natural frequency was explained using the numerical model which allows the damage at the locations where the actual damages were observed.