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Study evaluates impact of devastating Alaska earthquake on California shorelines

Ben Anderson

A new U.S. Geological Survey study conducted in cooperation with the California Geological Survey has determined that a massive earthquake off of Alaska’s Southwest coast could cripple California ports and potentially cause billions of dollars in damage and lost revenue to the Golden State.

The 12-chapter study is the result of extensive modeling and coordination with emergency management officials over what might happen if a magnitude-9.1 earthquake were to strike the southern coast of the Alaska Peninsula. That magnitude would be the equivalent of the devastating earthquake off the coast of Japan in 2011, and the study aimed to determine how best to prepare for a worst-case scenario in the event of a perfectly-positioned earthquake and tsunami pointed at the California coastline.

“The 2011 Tohoku earthquake was not anticipated, despite Japan having the best seismic and geodetic networks in the world and the best historical record in the world over the past 1,500 years,” the study said. “What was lacking was adequate paleogeologic data on prehistoric earthquakes and tsunamis, a data gap that also presently applies to the Alaska Peninsula and the Aleutian Islands.”

The scenario imagined the quake taking place “between Kodiak Island and the Shumagin Islands” in what’s known as the Semidi subduction sector. This was the most southwesterly tectonic region affected by the magnitude-9.2 earthquake that struck in Prince William Sound in 1964.

The Semidi sector was selected due to its potential for a “tsunamigenic” earthquake -- that is, one capable of generating a tsunami. Two historical, though potentially apocryphal, accounts of tsunamigenic quakes in the area were included in selecting the site: one in 1788, just a few years after long-term Russian settlement began in Alaska, and another “extremely doubtful” event in 1847 or 1848, which potentially resulted in a tsunami wave in far-away Tahiti. The most relevant document in the lengthy study related to the selection of the hypothetical earthquake site can be found here.

In the modeling, a massive tsunami wave would emanate from the origin of the earthquake, potentially raising wave height along the entire U.S. West Coast -- and the coastlines along Southcentral and Southeast Alaska -- all the way to the west coast of South America and even Antarctica.

The first of the increased wave activity would hit California in about four hours, first impacting the fishing community of Crescent City, which also suffered damage as a result of the farther-away 2011 quake in Japan. San Diego would see activity in six hours. The report estimates that strong currents would close ports at Los Angeles and Long Beach, resulting in more than $1 billion in lost revenue.

“In this scenario approximately 750,000 people would need to be evacuated, with 90,000 of those being tourists and visitors,” a statement from the USGS on the new study said. “Additionally, one-third of the boats in California's marinas could be damaged or completely sunk, resulting in $700 million in losses. It was concluded that neither of California's nuclear power plants would likely be damaged by this particular event.”

And though the study focuses on the impacts in California, Alaska certainly wouldn’t be exempt from tsunami waves on its shores.

“In the model, the Shumagin Islands, Alaska Peninsula, and Kodiak Island are hit with particularly large wave heights from the local tsunami,” the tsunami modeling chapter of the study said. Waves several meters high would be expected to reach shore in those areas, the model predicts.

The study's release comes during a period of heightened seismicity in Alaska's Aleutian Islands -- last week, a magnitude-7.0 rocked the remote and sparsely-populated region, and a magnitude-6.5 was measured on Tuesday. Neither event triggered a tsunami warning.

The study is far too comprehensive to summarize in full here. If you’d like to check it out for yourself, visit the report’s main page, here.

Contact Ben Anderson at ben(at)alaskadispatch.com