Estimation of Vs30 and site classification of Bhaktapur District using Microtremor Array Measurement

Abstract

Abstract The past earthquakes have depicted the strong need for the study of seismic behavior of sediments. Shear wave velocity directly relates to elastic shear modulus of the material and helps to incorporate the behavior of the soil in a design of structure which can be determined invasively as well as non-invasively. Multi-sensory Microtremor Array Measurement (MAM) is one of the non-intrusive methods of determining a subsurface soil characteristic by analyzing ambient vibration of the ground. In this method several geophones either in a linear or grid configuration are deployed on a firm ground surface for recording the ambient vibration. The ambient vibration data obtained by employing seismometer array of different sizes can then be jointly processed to obtain the single combined dispersion curve for a site. We determined averaged shear wave velocity (Vs30) of Bhaktapur district by two ways using non-invasive method of MAM using dispersion curve. The first method lies in obtaining Vs30 from dispersion curve directly and the second method extracts Vs30 from the inversion process using Genetic Algorithm. The first approach is direct and doesn’t require any prior knowledge of geology of site while the later one gives the velocity structure for the study area. The Vs30 thus obtained was tallied with previous works which illustrated a good correlation. The Vs30 at the south eastern part of the study area is higher than in other parts of the study area. Similarly, Vs30 at the western part of the study area is lower. The soil of the study area is found to be composed of both hard and soft soil but it is categorized as type D soil (i.e. very soft soil sites) in the new building code of Nepal. Also, the Vs30 from first method had high correlation so was used for the classification of soil of the study area based on NEHRP site classification system. As per this classification system, the study area lies in a site class C, D and E respectively.