A Comparative Study of Site Response in Western Washington Using Earthquake Data

Abstract

Broadband and strong motion seismic data from three moderate to large earthquakes were used to determine site response characteristics in the Seattle and Tacoma, Washington, area. The three earthquakes chosen for analysis were the 2012 Mw 6.1 Vancouver Island earthquake, the 2012 Mw 7.8 Queen Charlotte Island earthquake, and the 2014 6.6 Mw Vancouver Island earthquake. Resonant frequencies and relative amplification of ground motions were determined using Fourier spectral rations of velocity and acceleration records from three-component seismic stations within and adjacent to the Seattle and Tacoma basins. Recordings from the sites were selected based on their signal to noise ratios. Both the Standard Spectral Ratio (SSR) and the Horizontal-to-Vertical Spectral Ratio (HVSR) methods were used in the analysis and results from each were compared. Although 56% of the sites exhibited consistent results between the two methods, other sites varied considerably. Sites that had acceptable recordings from more than one of the earthquakes were compared. Using the results, several factors postulated to influence site response were examined for this study. These included depth to bedrock and age/type of geologic material. Although the scope of this study is limited, results of the analysis suggest that sites located on the Pleistocene continental glacial drift tend to have high amplification at 1-1.5 Hz by both HVSR and SSR results. Acceleration data from the Seattle Liquefaction Array (SLA) were also used to determine the site response in different depths. The spectra and the SSRs from this station indicate consistent frequency characteristics of the near-surface amplification among the earthquakes. The depth of the sediments to the bedrock along with the age and the type of the geologic units are investigated as the two major factors involved in the SSR and the HVSR results.