Site effects and their relationship with geological structure in Tainan, Taiwan

Tainan area is one of the areas have the highest risk of earthquake disasters in Taiwan. This research focuses on the microtremor characteristics and site effect of the Tainan Plain, the tableland, lowland and the foothills regions. We analyze the predominant frequency, amplification and fragility index of the seismic stations. Horizontal-to-vertical spectral ratio (HVSR) is used for the evaluations of site effect, and the data used are the micro-tremor received by the 173 single stations placed in the Tainan area from March to June 2021, and the duration of each station is 1-3 months. We used the stratigraphy borehole data as the initial model, f-k and HV-INV methods to obtain the velocity model in this area, and combined this velocity model with H/V to estimate the interface thickness. This is then used to compare different geological structures showing different site effect. The results at this stage point out that the stratum in the Tainan area contains two predominant frequencies, 0.2 Hz and 1~3 Hz respectively. Between them, the frequency of 0.2 Hz is dominant in the plain and the lowland areas. The stations above the tableland and the hills are dominated by 1~3 Hz, and their amplification is 2~3 times larger than that of the plain area. This illustrates that the site effect in the area is controlled by local geological conditions, and when seismic waves approach tableland and hills that the amplifications become stronger due to the site effect. However, the maximum value of PGA and the destroyed houses observed in the 2016 Mw 6.4 Meinong earthquake were found in the lowland area. This may be due to the multiple reflection signals observed in this area. We will sum up the fragility index for the entire interested frequency bands hoping to understand the overall geological strength of Tainan. In addition, we will compare the HVSR differences between the daily microtremor and the earthquake strong ground motion. If there is a certain relationship between them, we can use daily microtremor to estimate amplification and predominant frequency when earthquakes strike, and use this to evaluate the severity of disasters that may occur in different geological environments.