Investigation of site effects in kathmandu basin from array microtremor observations

The observed horizontal ground motions of the 2015 Gorkha earthquake, Nepal, on sedimentary sites were of long-period, high-velocity type and with long shaking duration comparing with those on rock sites nearby. The effect of soil sediments on ground motion characteristics was evident but some important information such as velocity structure of deep soil strata has not been investigated. In this study, the primary objectives are to explore site characteristics of thick basin sediments by large-size, array microtremor observations and to examine the seismic amplifications using site response analysis. The Centerless Circular Array (CCA) technique was employed for array microtremor observations in Kathmandu valley close to four seismic stations, one is on a rock site and the others are on soil sites. The depths of rock-like material ranging from 10 m to 400 m could be clearly identified from the shear wave velocity profiles. Sedimentary layers from surface to deep basement rock were modelled according to the derived velocity structures for equivalent linear and nonlinear site response analyses. The calculated ground motions in horizontal components at deep sedimentary sites from analysis using the records from a rock site as input motions were compared to the records from the mainshock event. The spectral amplification at long period and suppression at short period could be predicted satisfactorily by the analysis using the derived velocity models. Associate Professor, Dept. of Civil Engineering, Thammasat University, Pathumthani, Thailand (email: pnakhorn@engr.tu.ac.th) 2 Assistant Professor, Department of Geology, Tri-Chandra M. Campus, Tribhuvan University, Kathmandu, Nepal Graduate Student, Dept. of Civil Engineering, Thammasat University, Pathumthani, Thailand Professor, School of Engineering and Technology, Asian Institute of Technology, Pathumthani, Thailand Poovarodom N, Chamlagain D, Jirasakjamroonsri A, Warnitchai P. Investigation of Site effects in Kathmandu Basin from Array Microtremor Observations. Proceedings of the 11 National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Los Angeles, CA. 2018. Eleventh U.S. National Conference on Earthquake Engineering Integrating Science, Engineering & Policy June 25-29, 2018 Los Angeles, California Investigation of Site effects in Kathmandu Basin from Array Microtremor Observations N. Poovarodom, D. Chamlagain, A. Jirasakjamroonsri and P. Warnitchai