Expert: Juneau tsunami unlikely

Posted: Sunday, March 07, 2010

Since Southeast Alaska sits on a major fault, most residents know it's not "if" a big shaker will happen here but a matter of when. Many of the city's buildings are made to withstand the kind of violent shaking that caused so many deaths in Haiti, but then there's also a tsunami to worry about.

Juneau Empire
Juneau Empire

Or is there?

Scientists predicted the magnitude 8.8 earthquake last Friday in Chile would create a large tsunami, issuing alerts around the Pacific basin and even in Alaska.

The size of the waves turned out to be less than half what was expected but still traveled tens of thousands of miles.

Tsunami forecasters need to get better at predicting tsunamis, said Pacific Tsunami Warning Center Science Officer Bill Knight, with the center's office in Palmer, but the basic premise of the program worked.

"We did put out the right threat level messages," Knight said.

A tsunami advisory was issued, which meant for people to get off beaches, docks and the water. The next highest level, a warning, would have caused a lot of unnecessary evacuations.

The highest wave in Alaska from the tsunami generated by Chile's earthquake, which was centered about 200 miles south of Santiago, occurred in Alaska in King Cove last Saturday. The maximum wave height there was about two feet.

Seward experienced a wave height of about one foot, according to recorded data at the center, but residents reported half that to media.

The center did not record any surge in Juneau.

"In general, Juneau is in a pretty good location as far as tsunamis go," Knight said.

During the Good Friday Earthquake in 1964 in Southcentral Alaska, Juneau recorded a 1.3-meter-high amplitude wave, or about 4 feet. The quake was the second-largest measured by seismograph and caused destruction along much of the region's coastline.

A magnitude 9.1 earthquake on March 9, 1957 in Alaska's central Aleutian Islands caused an 8-inch wave in Juneau.

Earthquakes are caused by shifts in tectonic plates, huge landmasses moving around the earth at about a few inches per year. Scientists call areas where plates collide faults.

Southeast sits on the Queen Charlotte-Fairweather fault, which is actually an extension of the San Andreas fault in California. It is made up of the Pacific plate, which is moving northwest toward the North American plate.

The large fault can produce huge earthquakes but because the plates collide horizontally with no vertical or uplifting movement, a large tsunami is not likely, said Alaska Earthquake Information Center Seismologist Natasha Ruppert.

The Queen Charlotte-Fairweather is a strike-slip fault, similar to the one in Haiti but unlike the one in Chile.

In Chile, and other parts of Alaska such as the Aleutian Islands, one land mass dives under the other, causing an uplift in the earth's crust.

Incidentally, this is what creates mountains, but the two plates crashing vertically together also displaces so much ocean it can cause large tsunamis like the one predicted last weekend.

Since the Queen Charlotte-Fairweather fault near Juneau is moving horizontally, even a big earthquake is not likely to cause a tsunami - at least in theory.

The fact is that several tsunamis could result from underwater landslides caused by the earth shaking.

That's what happened in Lituya Bay during one of the region's largest recorded earthquakes along the Fairweather Range in 1958. The resulting splash ripped trees out of the ground more than 1,700 feet above sea level when the water sloshed in the bay during one of the largest tsunamis in history.

Underwater landslides during the 1964 Good Friday Quake caused local tsunamis in Valdez, Resurrection Bay and elsewhere in the state.

And of course, a subduction earthquake caused one of the deadliest natural disasters in history, the Indian Ocean tsunami in 2004 that killed nearly 230,000.

Obviously last week's tsunami event shows that scientists haven't perfected their work. There's still some art to the science of prediction but for its location on the Pacific Ring of Fire, history shows Juneau has been protected from great tsunamis.

But Ruppert cautions against basing disaster probability on history.

She notes the 1958 quake along the Queen Charlotte-Fairweather fault measured 7.9 on the Richter scale.

"We know it has produced large earthquakes in the past," she said. "We know it will happen again, but it's impossible to tell when."

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