Seismic hazard of Thailand and bi-directional response spectra

Abstract

When an earthquake occurs, a ground motion propagates from a seismic source where the epicenter locates on bedrock to structures located on soil layers which can modify characteristics of the motion. Therefore, a lot of factors need to be considered such as the probabilistic seismic hazard analysis, the effect of soil amplification for the wave propagation and the effects of bi-directional excitations on structures. The probabilistic seismic hazard map of Thailand and neighboring areas is developed. Earthquakes recorded from 1912 to 2009 by the Thai Meteorological Department and US Geological Survey are used in the analysis. The attenuation models which give good correlations with actual measured accelerations are used in predicting peak horizontal accelerations in Thailand. Maps of peak horizontal and spectral accelerations on bedrock with 2% and 10% probabilities of exceedance in 50 years are developed. For 10% probability of exceedance in 50 years, the maximum peak horizontal accelerations are about 0.25g in northern Thailand, 0.15g in western Thailand, and 0.03g in Bangkok. The spectral accelerations at the period of 0.2s are about 0.6g in northern Thailand, 0.3g in western Thailand, and 0.06g in Bangkok. And, the spectral accelerations at the period of 1.0s are about 0.15g in northern Thailand, 0.08g in western Thailand, and 0.03g in Bangkok. For 2% probability of exceedance in 50 years, the peak horizontal accelerations and the spectral accelerations are about 1.6 to 2.0 times the peak horizontal accelerations and spectral accelerations with 10% probability of exceedance in 50 years. For the effect of soil amplification study, seismic downhole tests were conducted at 6 sites in the northern Thailand and Bangkok to develop the relationship for predicting shear wave velocity in the areas. Soil response analysis was done for 33 sites in Chiangmai, Chiangrai, Kanchanaburi, and Bangkok to obtain the amplification factors. The amplification factors of peak ground acceleration are as large as 2.0 at locations where Vs30 is less than 200 m/s. This study also clarifies the effects of bi-directional excitations on structures and proposes the response spectra called “bi-directional pseudo-acceleration response spectra.” A simplified analytical model of a two-degree-of-freedom system was employed. 86 ground motion records were used in the analysis. The axis with the largest Arias intensity is referred to as the major axis and that perpendicular to the major axis is referred to as the minor axis. For design purposes, the mean plus a standard deviation of acceleration ratio response spectrum is proposed as 1.18. The proposed method is more conservative than 16%, SRSS and CQC3-rules for the direction with a shorter period.