Determination of Predominant Ground Frequency using the Micro Tremor Measurement in Kathmandu Valley of Nepal

Abstract

The Gorkha earthquake in Nepal on 25 April 2015 caused extensive damage to buildings and thousands of deaths and injuries felt all over the country. Many residential, commercial and historical buildings collapsed, temples have been ruined, roads destroyed and nearly 10000
people loss their life in this disaster. This quake was followed by more than 450 aftershocks. The capacity of the building and the ground property must be investigated before the construction procedure to minimize the loss of life and property due to earthquake disaster.
As the part of this project, micro tremor measurement was made in the study area. The property of micro tremor was used to analyze the predominant ground frequency of respective site. Micro tremor measurement of surrounding periphery of Thapathali in Kathmandu valley has been analyzed to determine the predominant frequency of the ground. The predominant frequency of the ground has been determined from H/V Spectrum and its value was found to be 0.38 Hz. The above result has been validated by using the 1D resonance frequency, fo which is the function of S-wave velocity (Vs.) and the thickness H of the soil deposit, given by (FO= VS/4H). The resonating frequency of the ground has been obtained as 0.18 Hz, which seems to be less reliable, it is because the input parameter for above expression were only approximated value. Shear wave velocity up to 30m depth was
only available during this study, but it was used for full depth of soil deposit. Similarly, thickness of soil deposit for location (Thapathali) was still unknown and it was approximated from contour map.