Evaluating the seismic hazard in Anchorage, Alaska

Anchorage is situated in one of the most seismically active regions in the U.S. The Alaskan subduction zone, which underlies the city, is the source of the 1964 moment magnitude (M) 9.2 Great Alaskan earthquake. Intraslab and crustal earthquakes could also generate future strong ground shaking in the city. A sitespecific probabilistic seismic hazard analysis (PSHA) of the Port of Anchorage was performed to estimate future levels of ground motions. The Alaskan subduction zone, both the megathrust and Wadati-Benioff zone, crustal faults, and crustal background seismicity were included in the PSHA. Several Quaternaryactive and potentially Quaternary-active structures within the Cook Inlet were included as seismic sources. The new Next Generation of Attenuation (NGA) relationships for crustal earthquakes and recent attenuation models for subduction zones were selected for use in the PSHA. Based on these input, we calculated sitespecific probabilistic hazard for a firm rock site condition. The 2,475-year return period PGA at the Port is 0.58 g. The intraslab zone dominates the PGA hazard at all return periods. The intraslab zone and the 1964 segment control the longperiod (> 1.0 sec) hazard. The Castle Mountain fault, the closest significant crustal fault to the site, is not a major contributor to the probabilistic hazard in Anchorage.