Site Effect Assessment in Ulaanbaatar, Mongolia through Inversion Analysis of Microtremor H/V Spectral Ratios

摘要

Due to the population growth and urban sprawl in Ulaanbaatar city (UB), Mongolia, hazard and risk analysis for future earthquakes have become an important issue for disaster mitigation planning. Evaluation of a site effect is one of the essential parts of the earthquake hazard estimation in this area. The site effect can be evaluated by site amplifications calculated from shear-wave velocity ( V S ) models including from bedrock to surface layers. However, it is difficult to assess the pattern of the site effects in UB because shallow mostly up to 15 m and a small number of investigated V S models are available in previous studies. In this study, the V S models are estimated using microtremor data at 50 sites and inversion analysis is applied to the observed data in order to evaluate site amplifications in UB. In particular, the joint inversion technique based on a diffuse field approach is applied to estimate the V S structures at three sites using the observed horizontal-to-vertical (H/V) spectral ratios and surface wave phase velocities obtained by Odonbaatar (2011). The rest of the sites are estimated by the single inversion technique using the observed microtremor H/V spectral ratios considering the results of the joint inversions. The seismic microzoning in UB is performed based on the site amplifications computed from the inverted V S models to characterize the pattern of seismic hazard. The result shows the largest site amplification zone is computed along the Tuul river in the southeastern part of UB.