A Comparative Study of Response and Fourier Spectral Ratios on Classifying Sites

Abstract

It has been known for decades that predominant periods of soil columns have crucial roles in the evaluation of site effects of ground motion. For this reason, detecting reliable site predominant periods relies upon a diligent data process and frequency domain transform method. In engineering seismology literature, predominant period-based site classifications have been developed to mitigate the local site effects of earthquakes. The main purpose of this study is to investigate the differences between Fourier and response spectral ratios in order to estimate the clear predominant periods and seismic site classes using instrumental accelerogram data. Within this framework, Fourier and response spectra of 26 installed strong ground motion stations in southern Turkey operated by AFAD [T.C. Disaster and Emergency Management Presidency, Strong Ground Motion Database of Turkey (TR-NSMN)] were examined. A total of 527 acceleration records with magnitude ranging from 3 to 5 having shallow crustal depths (0–25 km) and near-field earthquakes (< 150 km) have been analyzed. Elastic response and Fourier spectral ratios of horizontal-to-vertical (H/V) spectra were evaluated for a fair comparison using the S-wave phase. Although there are no significant differences, it was observed in this study that the response spectral ratio’s predominant periods reveal more reliable results for engineering purposes, especially in many sites where Fourier spectra show superficial peaks at engineering bedrock. As a pioneer study for a part of Turkey’s seismic station network, it will be an important step to establish a relationship between response and Fourier spectra to detect the site classes of stations alternatively in a global extent manner in the future.