Seismic Microzonation Map of Chiang Mai Basin, Thailand

Abstract

The seismic site effect which is controlled by local geological conditions is a key parameter of seismic hazards analysis. This paper presents an observational study of microtremor data to investigate the dynamic characteristics of soil in the Chiang Mai basin (Chiang Mai and Lumphun province), Northern Thailand. The Chiang Mai basin was formed on terrace sediments and alluvium sediments. The horizontal vertical spectral ratio (HVSR) analyses of ambient noise data at 101 sites in the basin were analyzed for an average smoothed HVSR curve to estimate the amplification factor and fundamental resonance frequency of each observation point. We also evaluated the shear wave velocity using the HVSR inversion technique based on the diffuse field assumption. Tests undertaken include a comparison of the HVSR of a significant earthquake that was detected at stations in the basin and a nearby bedrock site. The results indicate that the resonance frequency ranges between 0.15 - 0.4 Hz at sites having large thicknesses of soft sediments. The lowest resonance frequencies occur in the center of the basin, whereas higher resonance frequencies were observed in the areas of the shallowest bedrock in the west and east of the basin. It can be inferred that the western margin of the Chiang Mai basin is a steep slope, while the eastern margin of the basin is a low-angle west-dipping basement. The amplification factor ranges between 3 - 5 times, in the middle of the basin. Most of the Chiang Mai basin area is classified as site D soil (stiff soil) relative to alluvium sediments, and a region of class C soil (very dense soil) conforms to the terrace sediments located on the eastern edge of the basin. The soil classification is based on shear wave velocity (Vs30) determined by HVSR inversion. HIGHLIGHTS HVSR analysis and inversion of HVSR from 101 survey points in Chiang Mai basin The Chiang Mai basin area is at high risk from seismic amplification that amplifies a ground motion between 3 - 5 times in the middle of the basin at a dominant frequency range from 0.15 - 0.4 Hz The lowest Vs30 values location is in the middle of the basin classified as site class E (soft soil), while most of the basin area is classified as site D soil (stiff soil) corresponding to alluvium sediments. The class C soil (very dense soil) area conforms to the terrace sediments located on the eastern edge of the basin GRAPHICAL ABSTRACT