Evaluating Tsunami Hazard for Anchorage and Upper Cook Inlet, Alaska

The Alaska Earthquake Center, in cooperation with the Alaska Division of Geological & Geophysical Surveys, develops tsunami hazard maps for coastal communities in Alaska. This state-level program is a component of the National Tsunami Hazard Mitigation Program. Historic tsunami events, including the most recent tsunamis in Alaska, have led to a common misconception that this region is completely immune to tsunami impacts. Multiple tsunami warnings in the past 5 years have generated confusion, incorrect public information, and unnecessary evacuations. Because there has been no thorough study of tsunami potential in Upper Cook Inlet, a coordinated effort is now underway to evaluate tsunami hazards in the area.

The only tsunami effects in Upper Cook Inlet from the 1964 Great Alaska earthquake included a localized wave in Turnagain Arm that flooded the small community of Hope. The 1964 rupture did not produce a substantial amount of slip in the presently creeping Kenai segment of the plate interface, between the Prince William Sound and Kodiak Island asperities. This slip pattern resulted in little tsunami energy propagating into Cook Inlet in 1964. However, there is geologic evidence that the Kenai segment can rupture independently of the presently locked Prince William Sound segment.

Our modeling results indicate that some tsunami hazard does exist in low-lying areas of Upper Cook Inlet. These results are based on modeling of geologically realistic earthquakes using nationally benchmarked methodologies and incorporate knowledge from recent earthquakes around the world, including the 2011 Tohoku earthquake in Japan. Conclusions are further confirmed by geologic evidence from the coast of Southcentral Alaska suggesting that earthquakes larger and more impactful than the M9.2 1964 Great Alaska Earthquake have occurred in the past thousand years. Finally, our results show that coseismic slip at certain depths on the Kenai segment of the plate interface can generate substantial waves in Upper Cook Inlet assuming only M8.7 hypothetical scenarios.

Our tsunami mapping team is in communication with local emergency management officials and stakeholders throughout the project. The report release will include a coordinated public information campaign.