GROUND MOTION ANALYSIS IN THE ANCHORAGE BASIN: 1-D APPROACH

Understanding the dynamic behaviour of soil in the Anchorage basin in southcentral Alaska is essential for seismic hazard assessment of this highly seismically active region. The analysis of site responses for 40 sites from weak-motion and strong-motion data with amplitudes less than 0.1 g showed a strong influence of subsurface geological conditions on the characteristics of ground motion. Particularly, the sites in the central part of the city, including the downtown area, showed prominent resonance peaks around 1 Hz with amplification values up to about 4. The numerical analysis, based on one-dimensional multi-layer soil models shows that site response characteristics, and especially aforementioned peaks, are largely related to the thick, soft layer of Quaternary deposits, particularly cohesive fades of Bootlegger Cove Formation. The computed transfer functions for soil profiles of six representative sites are in accord with the site responses in the frequency range from 1 to 5 Hz. There is no significant change in amplification values below 2 Hz corresponding to large-amplitude (up to 0.38 g) ground motions; however, above 2-3 Hz the amplification values are greatly reduced in this case.