Local geological conditions and seismic site response in Fairbanks, Alaska

The town of Fairbanks is mostly situated on alluvial river deposits, but some portions of the city are located in other geological units, such as thick unconsolidated loess. The different geological conditions influence the seismic site responses at different locations. We studied strong motion data recorded at ANSS (Advanced National Seismic System) stations in Fairbanks for an eighteen-month period from January 2004 to June 2005. We selected forty-nine local earthquakes in the magnitude range from 1.1 to 5.3 and compared the amplitudes and spectral characteristics of accelerations at different locations. The horizontal peak ground accelerations of these records range from 0.1 to 4.3 cm/s/s. Since data from the bedrock site were not available, the frequency-dependent site responses at seven sites were obtained from the spectral ratio of horizontal components of the individual records and the network averages. We have used a twelve-second segment of the records following the S-wave arrival. Though our data were relatively limited, we noticed some prominent low-frequency amplification trends in regions containing thicker river alluvium, as well as some low-frequency attenuation at a station located in silt-fan deposits. It is possible that the latter is related to the presence of permafrost in these deposits; however, further studies are needed to confirm such a relationship. This study was the first to examine local seismic site response in the Fairbanks area, and marks an important first step toward understanding the possible seismic hazards present in Fairbanks.