SITE AMPLIFICATIONS OF CHIANG MAI BASIN, THAILAND FROM 1D AND 2D SITE RESPONSE ANALYSIS

Abstract

Chiang Mai, the largest city in northern Thailand and the second largest city in the country after Bangkok. The city is located on a flood plain composed of unconsolidated sediments. The city is occasionally threatened by tremors originating from nearby faults in Thailand and neighboring countries. Although ground motion amplifications have been observed, limited research studies have been conducted to comprehensively understand the seismic mitigation measures required for the area, particularly regarding site effects. This study aims to investigate seismic site effects in Chiang Mai basin and analyze site response in central part of Chiang Mai basin using differences 1-dimensional (1D) soil model. An approximately 140 sites distributed across the center of the basin, covering an area of approximately 1,500 sq.km, were investigated by microtremor observations and 50 sites for site response analysis. The first part, microtremor observations using the Centerless Circular Array (CCA) method was conducted to estimate phase velocity dispersion curves and derive shear wave velocities (Vs) profile through inversion analysis. The findings from this part provide information of average Vs from the surface to 30-m depth (Vs30) and estimated Quaternary sediment thickness. The second part, site response analysis using 1D soil model was involved to examine site amplification. Ground motions obtained from Probabilistic Seismic Hazard Assessment were input as rock outcrop acceleration, and their propagation through the 1D soil models was analyzed using an equivalent linear analysis. From the strong motion database, motions with the same mechanism of occurrence were selected and scaled to match response spectra with the conditional mean spectrum (CMS) at periods of 0.2, 0.5, 1.0, 1.5, 2.0, and 3.0 seconds, corresponding to a return period of 2475 years. The average spectral accelerations were then used to evaluate the Maximum Credible Earthquake (MCE) design spectrum for each site. Finally, this study presents a comparison between site amplification from 1D and 2D soil models and provides recommendations for site response analysis in irregular basins. This study is expected to significantly contribute to enhancing our understanding of seismic site effects in the Chiang Mai basin. Furthermore, it would provide invaluable insights that can guide the implementation of appropriate measures to effectively mitigate earthquake hazards in the region.