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California Could Suffer From Alaska-borne Tsunami

Tsunami graph of a strong earthquake along the Semidi segment of the Alaska-Aleutian subdivision zone
A strong earthquake along the Semidi segment of the Alaska-Aleutian subduction zone would send powerful waves toward the California coast line. Courtesy of the U.S. Geological Survey Ports map

A scenario prepared by the U.S. Geological Survey estimates that the state could suffer nearly $10 billion in losses from a tsunami generated by a powerful earthquake off the coast of Alaska.

September 17, 2013—A potential strong earthquake along the Semidi segment of the Alaska-Aleutian subduction zone would send powerful waves toward the California coastline, forcing the evacuation of as many as 500,000 people and causing $3.5 billion in physical damages and another $6 billion in economic costs from business interruptions.

Those are some of the key findings from a scenario developed recently by the United States Geological Survey (USGS). The accompanying report, “The SAFRR (Science Application for Risk Reduction) Tsunami Scenario—Executive Summary and Introduction,” examines the effects of an earthquake measuring 9.1 on the Richter scale along the Semidi segment of the subduction zone.

Northern and central California would be hit hardest in the scenario. Waves could reach heights of 24 ft in the central part of the state and 23 ft in the north. South of Point Conception, residents are protected somewhat by the state’s curving coastline. The area would see peak waves of approximately 10 ft.

The ports of Los Angeles and Long Beach are especially vulnerable in the scenario, with an estimated $100 million in repair costs and a possible $4.3 billion in economic losses unless appropriate mitigation strategies are developed. The commercial fishing industry and the state’s coastal marinas would also be hard hit.

“We do have three to four hours between when you know it really is a big earthquake and the first wave arrives,” says Lucile Jones, Ph.D., a science advisor specializing in risk reduction for the USGS. “That’s a very short time frame within which to receive the information, process the information, make a decision to act, and actually pull off the evacuation.”

An evacuation would be further complicated by the prevalence of resorts and hotels along the California coast. The majority of people who need to evacuate could be tourists, making public education campaigns difficult. If there is good news, it is that the state’s steep, rocky coastline means that many evacuees would have to travel less than a mile to safety.

The USGS was already developing the geologic and project models for the scenario when a 9.0 temblor struck along a subduction zone in shallow waters off the coast of Tōhoku, Japan, in March 2011, Jones says. The seabed in the Tōhoku earthquake thrust up as much as 8 m along a vast swath of the subduction zone, creating a powerful tsunami with waves as tall as 128 ft. The death toll was 15,883, with 2,654 missing. 

 Aerial view of a tsunami

 The Port Los Angeles and the Port of Long Beach would be
especially hard hit, with possible economic consequences of
$4.3 billion.  Courtesy of the U.S. Geological Survey

“Tōhoku changed what we did,” Jones says. “We realized that we hadn’t [modeled] a big enough earthquake. We went back and discussed it and decided to use the Tōhoku earthquake as a model for this event, but moving it into Alaska.”

The research team found that the subduction zone in the Tōhoku quake and the Semidi segment of the Alaska-Aleutian subduction zone share many key similarities, Jones says. Sedimentary wedges above the two subduction zones indicate that they move at a similar rate. Also, the age of the geologic materials in the zones is similar. Furthermore, the Semidi segment is locked at the surface, while deeper measures indicate movement.

“It’s been a long time since the last earthquake,” Jones says. “There is slip accumulated on it and it’s not moving right now. A locked section is one that can produce a big earthquake. We have been seeing it slipping at depth, so we know that we are stressing it, but it’s not moving at the shallower part.”

The team determined that an earthquake similar to Tōhoku was plausible on the Semidi segment, and predicted the return rate of such an event to be between 100 years and 700 years.

“That’s what we aim at in these scenarios. This is that sweet spot between what you already know and what you don’t need to worry about,” Jones says. “If it was more common than that, you wouldn’t need science to tell you what to do. You would have it in your experience. And if it gets too much longer than that, it doesn’t make financial sense to get ready for something that’s really, really uncommon.”

The scenario is the third in a series developed by the USGS to help emergency preparedness efforts in the state. Previous scenarios looked at a strong earthquake and a massive, powerful rainstorm. Although the tsunami report has captured a great deal of public attention, it is the massive storm that is projected to cause the most damage. Jones says the state’s extensive flood control network so effectively manages 10- to 20-year storms that residents vastly underestimate the effects of a powerful storm with heavy rains and strong winds. The USGS’s scenario, called ARkStorm, projected losses of nearly $800 billion for a storm similar to one that struck the state in 1862.



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