As the operation time of tunnels grows, the potential damages due to earthquake have become an issue that cannot be neglected. This research implemented accelerators in a highway tunnel passing through a slope in southeast Taiwan. After a seismic event with 5.0 Richter magnitude from 12.7 km beneath, 12.8 km to the case tunnel, the coseismic acceleration at five different locations in the tunnel returns satisfactory outcome. The Fourier spectra after Discrete Fast Fourier Transform reveal a 4–6 Hz predominant frequency of the tunnel lining. The maximum coseismic acceleration is 37.38 gal. All five accelerators on vault and two sidewalls show similar predominant frequencies in three dimension, proving that the predominant frequency of a site governs the predominant frequency of a tunnel. However, the secondary frequencies of accelerators on hill side sidewall of 11.4 and 11.7 Hz in vertical direction are apparently higher than the other two accelerators on creek-side sidewall of 8.6 and 6.4 Hz. The results describe the influences of both site characteristics and topography on the seismic response of a tunnel. The rare in-situ monitored data sets a starting post to quantify and verify contemporary methodologies on seismic-related problems of tunnels.

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