Estimation of Subsurface S-Wave Velocity Structures in Yangon City, Myanmar Using Microtremors for Strong Motion Prediction

Abstract eng:
This study focuses on S-wave velocity structures in Yangon City, Myanmar, where the construction works have been performed without thorough consideration of present threat from strong shaking from large earthquakes. Especially, the most active and dangerous fault called the Sagaing fault is expected to cause large magnitude crustal earthquakes in the near future along the segment nearby Yangon City. Understanding the subsurface velocity structure is an important factor in mitigating seismic damages, because they can be used to estimate site amplification factors and simulate the expected strong ground motions due to crustal earthquakes nearby. In this study we investigated subsurface velocity structures based on Horizontalto-Vertical spectral ratios (HVRs) of microtremors as well as phase velocity estimates from the vertical component array observation inside Yangon City. Several array measurements with different radius were conducted at the site. From previous studies, the observed HVRs show consistent peaks as fundamental peak at lower frequencies ranging from 0.7 to 1.3 Hz, while another peak at higher frequencies range from 20 to 40 Hz, so we focus on frequencies below few Hz to consider the velocity structure that affects the fundamental peak. An S-wave velocity structure at the array measurement site was obtained so as to make both theoretical Rayleigh wave phase velocity dispersion curves and theoretical HVRs derived from diffuse field assumption, simultaneously agree with the observed ones. As a result, we found that in order to fit the Rayleigh wave dispersion curve obtained from the smaller and larger array measurement, we need a nine-layer model with the deepest layer at 1200 m depth with Vs=1800 m/s. A layer with Vs=800 m/s exists at around 100 m below the surface as the presumable engineering bedrock, while the uppermost softest layer with Vs=140 m/s have the thickness of 1.5 m in the area.

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