Seismic Microzonation and Soil-Structure Resonance Condition of Suryabinayak Municipality using Ambient Vibration Measurements

Abstract

Bhaktapur, lying in the eastern part of the Kathmandu basin suffered major damages during the 2015 Gorkha Earthquake. The major reason for this might be the abrupt change in topography and soil conditions even at short distances, ground motion amplification due to fluvio-lacustrine deposits, and ground-structure resonance. This study mainly focuses on the study of frequency-based seismic microzonation and soil-structure resonance possibility in an urbanized area of Suryabinayak Municipality. This study is divided into two parts. The first part deals with ambient vibration measurement in a 200 x 200 m grid for assessing the fundamental frequency of the basin deposits. Ambient vibration measurements were done at 115 station points. The fundamental frequency of the basin sediments of the area ranges from 0.77 Hz to 9.09 Hz. The higher fundamental frequency is found in the southern part of the study area than that in the northern part. The sediment depths of the study area were estimated which agrees well with available borehole and geological data. The results reveal northern part of the study area comprises thick sediments than the southern side. The second part includes microtremor measurements on different typologies of buildings. The predominant frequency of building in both longitudinal and transverse directions was determined using Floor Spectral Ratio (FSR) method. The dominant period of ground was categorized as A, B, C, and D which ranges from 0.11-0.32 s, 0.32-0.51s, 0.51-0.73s, and 0.73-1.29s respectively. The range of predominant periods for 2, 3, 4, and 5-story buildings ranges from 0.1 – 0.15 s, 0.19 - 0.26 s,0.29-0.36 s, and 0.29-0.39 s respectively. The predominant period of building was then compared with the free field period obtained. The result shows resonance between ground and structure may be the major reason for extensive damage in the study area. The experimentally obtained fundamental period of building was compared with codal provisions. The results depict a lower fundamental period from experimental provisions than that from codal provisions. This study's results will help in seismic hazard mitigation and the construction of earthquake-resilient buildings in the study area.