Determination of Site Response in Anchorage, Alaska, on the Basis of Spectral Ratio Methods

Abstract

This paper deals with the site response (SR) in the Anchorage basin in south‐central Alaska. The investigation is based on the analysis of seismograms of 114 earthquakes recorded by 22 weak‐motion stations and 46 earthquakes recorded by 19 strong‐motion stations in the study area. We have computed SR for 41 sites, using standard spectral ratio and horizontal‐to‐vertical spectral ratio methods in the frequency range from 0.5 to 11 Hz. Based on these results, we have calculated band‐average site response values in two frequency ranges: low frequency (from 0.5 to 2.5 Hz) and high frequency (from 3 to 7 Hz). There is a good correlation between SR values and surficial geology of the Anchorage area in the low‐frequency range. SR values increase by a factor of three from the foothills of the Chugach Mountains in the east to the west towards the deeper part of the basin. The highest site response values (SR>2.5) in the same frequency range are observed in the west‐central part of the city, which is underlain by cohesive facies of the Bootlegger Cove formation. The SR has a good correlation with the uppermost 30‐m time‐average shear‐wave velocity with a correlation coefficient of 0.82. Moreover, the low‐frequency SR values are close to the NEHRP site coefficients for 1 sec. However, high‐frequency SR values lack correlation with 30‐m average shear‐wave velocity and short‐period NEHRP site coefficients.