Evaluation of horizontal to vertical spectral ratio and standard spectral ratio methods for mapping shear wave velocity across Anchorage, Alaska

The use of horizontal to vertical spectral ratios (HVSR) of earthquake ground motions has become a standard technique to characterize sites, especially those lacking subsurface measurements. Several studies have developed relationships between HVSR results and time-averaged shear-wave velocity in the upper 30 m (VS30). Other studies have utilized standard spectral ratios calculated from horizontal ground motion Fourier amplitude spectra to estimate VS30. Anchorage, Alaska (USA), has a network of strong-motion recording stations, many of which have no site-specific subsurface characterization. This study compares measured VS30 and HVSR results from 18 strong-motion stations to four regional models developed by others. A relationship between the 1 Hz band-averaged (0.5–2.5 Hz) spectral amplification results and VS30 is presented. VS30 estimates for the strong-motion stations are made, and a regional model is developed between HVSR and VS30, both in terms of fpeak (the frequency of the peak HVSR amplitude) and Apeak (the amplitude of the peak). In addition to the regional model, additional VS30 data from other sites in Anchorage, including 19 downhole VS30 measurements and 22 microtremor VS30 estimates from others, are used with the strong-motion station VS30 estimates to develop a VS30 contour map of Anchorage. The contouring represents the spatial distribution of the site classes of the local building code, which are based on VS30. This map may be incorporated into planning documents for future developments in the city.