January 28, 2018

USA: A Massive 8.2 Magnitude Earthquake Rocked Alaska On Tuesday. A Tsunami Warning Was Triggered, Then Canceled. Scientist Explain Why B.C. And Alaska Avoided Massive Tsunami.

Fox News
written by Edmund DeMarche, Travis Fedschun
Tuesday January 23, 2018

A large 7.9 magnitude earthquake struck off Alaska's Kodiak Island early Tuesday, prompting a tsunami warning for a large swath of coastal Alaska and Canada's British Columbia that was later downgraded to an advisory as possible destructive waves failed to materialize.

The U.S. Geological Survey said the quake was recorded about at 12:31 a.m. local time about 155 miles off of Chiniak, Alaska. The Pacific Tsunami Warning Center initially said "widespread hazardous tsunami waves were possible," but later said waves failed to show up in coastal communitites.

Buoy 46410, located northeast of the quake's epicenter, recorded a “water displacement” of 32 feet, the National Weather Service said. The reported 32 foot wave, however, failed to materialize in coastal areas, which only saw between a one and three-foot rise.

Caltech Professor Thomas Heaton said at an afternoon briefing that "mainly fish felt this earthquake."

"Since it was a strike slip earthquake it was primarily horizontal motion of the ocean-floor and very little chance of tsunami from this type of earthquake," Heaton said. "Of course when the earthquake first happened semiologists had to take some time to analyze the seismograms to analyze it was a strike-slip earthquake."
The initial concern from the size of the quake caused officials in coastal areas to order residents to evacuate and then wait for an "all clear" before returning to low-lying areas. The town of Kodiak has several shelters above the 100-foot mark, and police encouraged people below that level to evacuate.

The Alaska Earthquake Information Center said the quake was felt widely in several communities on the Kenai Peninsula and throughout southern Alaska, but categorized the shaking intensity as light with no reports of damage.

"This is a very large earthquake, and Alaska is no stranger to earthquakes or tsunamis," Fox News' Senior Meteorologist Janice Dean said on "FOX & friends." "They've had four of them in the past."

The Kodiak Police Department also posted a video to Facebook imploring residents to leave their homes and to head for higher ground.

"This is not a drill," Kodiak Police Sergeant Derek Beaver said in the video. "This is an actual tsunami warning. Everyone please get at least 100 feet above sea level."

The PTWC originally included Hawaii and the U.S. West Coast in the tsunami watch, but later canceled it, saying "a tsunami was generated but does not pose a threat to these areas."
In San Francisco, emergency officials had warned residents who lived within three blocks of the Pacific Coast and five blocks of the San Francisco Bay should be prepared to evacuation if needed before giving the all clear.
In Kodiak, one resident told the Anchorage Daily News that hundreds of cars had driven up Pillar Mountain, the site of the town's utility-scale wind turbines, to escape the tsunami threat.

"Pretty much everyone in town went up Pillar Mountain," he said.

Keith Perkins got the phone alert and later heard sirens going off in his southeast Alaska hometown of Sitka. He said people on Facebook were talking about whether the threat was real and what they should do.

Given the magnitude of the earthquake, Perkins told the Associated Press he thought it best to head to the high school, a tsunami evacuation point, even though in the past he felt his home was at a "high-enough spot."

"I figured I'd probably just better play it safe," he said.

Larry LeDoux, superintendent of the Kodiak Island Borough School District, said schools were open as shelters and estimated there were about 500 people at the high school. He described the atmosphere inside as calm, with people waiting for updates.

Warnings from the National Weather Service sent to cellphones in Alaska warned: "Emergency Alert. Tsunami danger on the coast. Go to high ground or move inland."

Alaska Gov. Bill Walker said on his Twitter feed that he has been in contact with local officials and the state's adjutant general, and he urged residents to heed any warnings to move inland or to higher ground
CBC News, Canada
written by Emily Chung, Nicole Mortillaro
Wednesday January 24, 2018

Coastal communities in British Columbia and Alaska were evacuated to higher ground early Tuesday morning after tsunami warning sirens blared following a large earthquake off the coast of Alaska. But the warning was later cancelled without any reported tsunami damage. Why?

Tsunamis are giant waves — which can be more than 30 metres high — generated by large earthquakes, and can cause massive damage. The 2004 Boxing Day tsunami in the Indian Ocean killed more than 200,000 people. More recently, a massive earthquake and tsunami in Japan in 2011, caused major damage and a meltdown at the Fukushima nuclear power plant.

Tuesday's earthquake struck at 1:31 a.m. PT in the Pacific Ocean about 250 kilometres southeast of Chiniak, Alaska. The U.S. Geological Survey initially reported the quake's strength at 8.2 and later revised that to 7.9, with a depth of 25 kilometres below the surface. At least three aftershocks were reported.

Earth is comprised of moving slabs of rock called plates. Areas where they meet are called fault lines. When these move against each other we get earthquakes.

According to the International Tsunami Information Centre, the most destructive tsunamis are generated from large, shallow earthquakes — those with a magnitude higher than 7.5 and less than 70 kilometres below the surface — with an epicentre or fault line near or on the ocean floor.

Tuesday's earthquake met those criteria.

But the type of earthquake also matters, reports CBC seismologist Johanna Wagstaffe.

She said the quake struck in an area where there is a transition between two types of earthquakes:
  • Strike-slip, where two tectonic plates slide horizontally past each other.
  • Megathrust, where one plate slides under the other, also called subduction.
"This kind of earthquake was what is called a strike-slip earthquake," Wagstaffe said. "That means there isn't as much vertical displacement. So when you're thinking of these rocks on the ocean floor, it didn't punch up like some other earthquakes have."

Instead of the water being displaced upward and forward, this water was moved much less by the horizontal motion. A fortunate circumstance for anyone living along the coast.

n the case of the 2004 and 2011 earthquakes, there was more vertical motion, which had devastating consequences. Roughly 225,000 were killed in the 2004 earthquake; 20,000 people were killed in Japan in 2011.

Motion of the plates
The West Coast is an area with quite a few fault lines that can result in various motions, Wagstaffe explains.

"Where the Gulf of Alaska curves, you end up getting a splay of fault lines," she said. "There's a really interesting section where this earthquake occurred where you can get all different kinds of motion happening. If it had occurred 90 kilometres to the west then it would have been a subduction earthquake and we could have seen a massive tsunami."

On March 27, 1964, a 9.2 magnitude earthquake occurred off the coast of Alaska at a depth of 25 kilometres. Rather than being a strike-slip, it was a megathrust and produced a tsunami that rose 30 metres in some areas, killing more than 130 people.

"This is a reminder that we live in a seismically active area of the world.… It's also a reminder that we get different kinds. We're waiting for the 'Big One,' but we could also get a smaller earthquake, shallower closer to a major city. That would be devastating."

Wagstaffe said the West Coast can expect after shocks over the next few days or even weeks.

For now, seismologists will gather data collected from the earthquake to better understand the risks along the West Coast.

"We are quite vulnerable to a lot of different earthquake risks here."

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