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Tsunami Alert Is Downgraded Along Alaska's Coast After Powerful Quake

A map from the U.S. Tsunami Warning System shows the offshore location of a powerful earthquake that struck overnight, along with the revised tsunami advisory area.
U.S. Tsunami Warning System
A map from the U.S. Tsunami Warning System shows the offshore location of a powerful earthquake that struck overnight, along with the revised tsunami advisory area.

A powerful magnitude 7.9 earthquake struck off the coast of Alaska late Monday night, initially prompting a tsunami warning for a large section of the state's coast and parts of Canada. As more data came in, the U.S. Tsunami Warning System downgraded the threat to an advisory for Alaska's Chignik Bay.

Several smaller aftershocks were also felt after the quake, whose epicenter was located about 6 miles below the surface and 175 miles southeast of Kodiak, according to the U.S. Geological Survey.

Warnings from the National Weather Service were pushed out to cellphones in Alaska, saying: "Emergency Alert. Tsunami danger on the coast. Go to high ground or move inland."

But as the effects of the quake became clear, those alerts were soon canceled.

"The warning system reported several waves around the state of under a foot," member station KTOO reports.

In its initial response, the National Tsunami Warning Center at Palmer, Alaska, issued a tsunami warnings or watches for long swaths of the west coast of Canada and the U.S., from Alaska and British Columbia to parts of California. Public radio station KMXT in Kodiak issued this advisory over its airwaves: "This is a tsunami warning. This is not a drill. Please get out to higher ground."

In Sitka, Alaska, east of the quake's epicenter, schools that had been opened as shelters during the tsunami warning were later given the all-clear.

Reporter Emily Kwong of Raven Radio said via Twitter:

"Chimes going off in Sitka, Alaska, as booming emergency voice sounds the all clear: 'Repeat. The tsunami warning has been canceled. It is safe to return to coastal areas.' City says school will happen today. Kids I've spoken with have mixed feelings about that."

There were no immediate reports of damage or injuries related to the earthquake.

Copyright 2018 NPR. To see more, visit http://www.npr.org/.

Tsunami Alert Is Downgraded Along Alaska's Coast After Powerful Quake
Tsunami Alert Is Downgraded Along Alaska's Coast After Powerful Quake GUEST: Pat Abbott, geology professor emeritus, San Diego State University

>>> They say it caused no injuries or damage, but a large earthquake off Alaska's coast this morning, certainly caused some concern. The magnitude 7.9 earthquake struck off the coast of Alaska and tsunami warnings and advisories were issued as far south as here in San Diego. As it turns out, even though the earthquake was strong, it did not generate any significant wave activity. To find out why, we turn to Pat Abbott, geology professor emeritus at San Diego State University. Welcome back. >> Good to be here. >> A lot of people were spooked in the middle of the night by the tsunami warnings, why were authorities so concerned? >> This is an area where we have had the second biggest earthquake we've ever measured in our history, 9.2, 1960 for the Good Friday earthquake, but the killer tsunami that raged though the whole Pacific ocean basin. The computer's first before seismologists got to it said -- you think about the ocean floor being pulled to the interior there. The vertical motion is very efficient at putting energy into the water. To put off that warning right away for Alaskans would be critical. For us, farther down the lower 48, even though it was a tsunami warning in the northern part and tsunami watch for us, I hope they didn't wait people because it would take about five hours to travel to San Diego for example, but when we look at the patterns by the seismic waves, the surprise it turns out to be it strikes the horizontal movement, the things we are use to with the San Andreas. Here is the interesting thing. This is not the big, thick ocean plate subducting underneath Alaska, but if you bend those miles and miles of thickness of rock, they aren't meant to bend, they are brittle. So it is a fracture within the plate and that particular fracture propagated in a horizontal sense. In the sense it put the horizontal movement, it won't do anything much to disturb ocean water. Nothing like vertical motion. >> Because it was a strike slip that means it wouldn't generate a cinnamon? >> It wouldn't generate as big of one. How much energy gets out of the earth movement, how much is put into the water. The greater the vertical displacement up or down, that is the maximum for loading energy, you can visualize it with a pot of water if you slide your hand on the bottom, that won't disturb it like punching your fist into it. >> Let me go back to the subduction quake everyone is concerned about, because they have quakes, the potential in that Alaska area, that are more severe than the ones we have a potential for here, is that right? >> Most definitely. Of the 50 United States, the three states that have the really big earthquakes, not California, Alaska had a 9.4 in 1964, and in the year 1700, very elegant proof of how we know this, 1700, Washington and Oregon were hit with a 9.0, basically the exact same thing we saw in Japan in 2011. >> If there had been a subduction quake in that area, would we be at risk of a tsunami down here? >> As a career professor, one of the things I had the hardest time getting anybody to understand is the difference between us tsunami and tidal wave. They act like totally different things, our life experience does not prepare us for what a tsunami is. We go to the beach, you see a wave, it pulls back and the next one comes in. Your whole life experience teaches you something totally wrong. The tsunami, if you remember the video footage from 2011 from the Japanese news helicopter and use of the big black roiling mass of water and debris, that rolled inland for 6 miles. For a tsunami, it's not the height of the wave, it is the fact that they don't pull back, they keep coming and coming. Nothing can stand up to those. >> Would we get something like that if there was this big subduction kind of earthquake in Alaska? >> We would definitely get the tsunami, but mother nature has protected us. When we look offshore, we see Coronado Island, San Clemente Islands, those are the top of ridges and parallel between those bridges are down drop basins. It is like mother nature built this giant complex of subsea walls that takes most of the energy out of those kinds of tsunamis coming in. Go back to the 1964 Alaska earthquake. Yes, San Diego suffered damage, but not the big wall coming in. Most of the damage occurs when the impulse of energy surges in, it was mainly the boats shelter Island, where boats are tied next to each other and they are banged apart so there is millions of dollars of damage in that way. In terms of loss of life, we haven't had any tsunami loss of life here. It is not inconceivable if you were standing knee-deep in the water, it would take certain -- certain circumstances. Remember in 2010 we had an 8.8 in Chile. The earthquake catalog goes back to the beginning of the 20th century. For 110-115 years, that is the sick largest earthquake we have experience, the 2011 Japan was tied for the fourth largest. Then the 2004 Ocean one, the third-largest we have seen. Within our own lifetime, even for younger people, 2011, 2004, you have experienced some of the largest earthquakes we have experienced in over a century, and the largest tsunami as well. >> The possibility is there. I have been speaking with Pat Abbott, geology professor emeritus at San Diego State University. Thank you so much. >> My pleasure. .

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