Abstract
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 (VS) 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 VS models are available in previous studies. In this study, the VS 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 VS 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 VS 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.
Highlights
Mongolia has been one of the most seismically active intracontinental regions in the world with four large earthquakes with the magnitudes of around 8 along with their active faults in the western part of the country during the last century [1]
Most of the sites in Ulaanbaatar city (UB) are classified to class C or B in the VS30 categories of the National Earthquake Hazards Reduction Program (NEHRP) [23], indicating that the site conditions are basically stiff and almost no very soft soils are expected in UB
Computed usingzoning microtremor data and previous investigations forshear-wave evaluating site amplifications andisassessing seismic in Ulaanbaatar the previous investigations for evaluating site amplifications and assessing seismic zoning city (UB)
Summary
Mongolia has been one of the most seismically active intracontinental regions in the world with four large earthquakes with the magnitudes of around 8 along with their active faults in the western part of the country during the last century [1]. The microtremor H/V spectral ratio (MHVR) has been used to estimate VS profiles by assuming that the microtremors are mainly composed of surface waves [9,10]. Diffuse field assumption (DFA) has been proposed to understand MHVR by assuming a diffuse wavefield containing all types of elastic waves [11,12] In this theory, MHVR can be numerically computed as the imaginary parts of Green’s functions, and the theoretical H/V ratio depends on underground layer thickness and elastic properties of soil. We observe the microtremors at multiple sites in UB areas and apply the inversion technique to the MHVR in order to reveal the patterns of the VS profiles considering the observations in the previous studies such as Odonbaatar [3] and JICA’s report [5]. The site amplification characteristics obtained from the estimated VS profiles are assessed by zoning the UB area
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