News

Don’t worry….

At 12:39 a.m. on the night of the great Alaska tsunami that wasn’t, the Anchorage Office of Emergency Management sent out an email of a “tsunami warning until 01:35 a.m.”

It was fake news, but most of the residents of Alaska’s largest urban area would be unaware for at least the next 26 minutes.

Not until 1:01 a.m. would come another e-mail notifying them that “there is no tsunami warning for the Anchorage area and vicinity. We are *outside*  the danger zone. Please do not call 911 unless you have an emergency. Thank you, APD Dispatch.”

The second message came from an Anchorage Police Department starting to field calls from a panicky public.

And there were people all over Alaska legitimately worried about tsunami dangers in the wee hours of Tuesday  after a 7.9 magnitude quake rattled the Gulf of Alaska some 175 miles southeast of Kodiak.

From the start, Anchorage residents had little to fear, but many of them didn’t know that. Meanwhile, a computerized, government emergency alert system designed to cover a broad area did exactly what it was designed to do. It spread the tsunami warning widely.

Technology is our friend only when it works.

Unfortunately, in this case, no one schooled the computer in how tsunamis are likely to behave in Cook Inlet. And a lot of people didn’t know much more than the computer even though the National Weather Service had tried to tell them two years earlier after another tsunami scare.

The federal agency produced a YouTube video warning people not to panic about a tsunami reaching Anchorage:

“…The National Tsunami Warning Center, the National Oceanic and Atmospheric Administration, and the United States Geological Survey (agree) Anchorage’s threat of a tsunami is ‘extremely low’ based on the natural geographical boundaries and the shallowness of Cook Inlet,” the video said.

“Tsunamis build in deep oceans, but they drag in shallow areas and lose energy to the friction with the bottom. While lower Cook Inlet communities such as Nanwalek and Homer face real threats from tsunamis, communities farther north do not.”

Unfortunately, it appears the video was viewed only 372 times in two years.

Pay attention

The Tuesday morning email warning Anchorage of a tsunami was in and of itself strange. It posted a warning to run until 1:35 a.m. and then repeated a message from the National Tsunami Warning Center in Palmer that made no mention of Anchorage.

The email listed “estimated tsunami start times” for Kodiak, 1:45 a.m.; Seward and Elfin Cove in Southeast Alaska at 1:55 a.m. and on through various communities to Homer at 2:55 a.m.  Homer is near the mouth of Cook Inlet, about 120 miles south of Anchorage.

A tsunami bound for Anchorage would have to pass Homer on its way up the Inlet. Tsunamis can travel at up to 500 mph in the open ocean where water is 6,000 feet or deeper, but their speeds rapidly slow as they enter shallow water and friction starts to strip power out of the wave.

But no matter how fast the wave was traveling, there was no way a tsunami scheduled to hit Homer at 1:45 an. could reach Anchorage 10 minutes earlier.

That alone should have tipped most people off to the fact something was wrong with the email, but then nobody had much reason to worry in Anchorage anyway.blurb

History

The biggest earthquake to strike North America in modern times hit Alaska on Good Friday 1964. It measured 9.2 on the Richter scale and struck in the Gulf of Alaska about 75 miles west of Valdez. It was the second largest earthquake ever recorded anywhere on the planet.

At 9.2, it is considered to be almost 20 times bigger than a 7.9.

“The earthquake was felt throughout most of mainland Alaska, as far west as Dutch Harbor in the Aleutian Islands some 480 miles away, and at Seattle, Washington, more than 1,200 miles to the southeast of the fault rupture, where the Space Needle swayed perceptibly,” scientists with the U.S. Geological Survey later reported. “The earthquake caused rivers, lakes, and other waterways to slosh as far away as the coasts of Texas and Louisiana. Water-level recorders in 47 states—the entire Nation except for Connecticut, Delaware, and Rhode Island— registered the earthquake. It was so large that it caused the entire Earth to ring like a bell: vibrations that were among the first of their kind ever recorded by modern instruments.

“Tsunamis produced by the earthquake resulted in deaths and damage as far away as Oregon and California. Altogether the earthquake and subsequent tsunamis caused 129 fatalities and an estimated $2.3 billion in property losses (in 2013 dollars).”

Anchorage was not among the communities impacted by a tsunami. Valdez and Seward were smashed, and Anchorage suffered massive damage as the land beneath the city  shifted. But there was no tidal wave in the state’s largest city.

“The tsunamis were not reported at all at Anchorage, probably because the waves were strongly attenuated by refraction effects and by friction as they traveled along the Cook Inlet,” the National Academy of Sciences reported in a summary of “The Great Alaska Earthquake of 1964.”

“Also, they would have encountered strong ebb currents from the outpouring astronomical tide which would have partly negated their advance. The question is an interesting one, however, and unfortunately the only data on the subject are contained in a report…that at Kenai the ice shifted position…three times along the edge of the basin.

“It is not known whether any oil companies working in Cook Inlet possess information that would show any unusual tide state during and after the earthquake. Nevertheless, there was evidence of wave activity in Turnagain Arm shortly after the earthquake. The shock occurred about one hour before predicted high tide for Hope….

“Shortly after the quake, water swept in from the northwest like a 40-foot tide, running 200 yards inland and flooding homes and property.”

It was never determined, however, whether the Hope flooding was due to the tail end of a tsunami or the “tilt that Turnagain Arm received from the land subsidence,” according to the Sciences Academy report, or “opposed horizontal thrustings of land” at either end of Turnagain Arm.

The quake caused some lands along the Arm to drop up to 10 feet. 

Some of the damage the quake caused can still be seen in the ghost forests along the Seward Highway at Girdwood south of Anchorage, and in the Placer, Portage and Twentymile river drainages east of there.

When the land dropped below the tide line, the roots of spruce trees sucked up saltwater that killed them and in the process preserved them.

Fifty-four years later, the still standing dead spruce are a stark reminder of the power of nature.

Tsunami proof

In the wake of the Good Friday earthquake, Anchorage’s location at the head of Cook Inlet more than 100 miles from the Gulf of Alaska came to be considered largely tsunami proof.

“In theory, a tsunami is possible at any ocean side location,” the Anchorage Daily News’ Mike Duhham reported in 2014. “But it’s considered improbable in upper Cook Inlet.

“‘Generally speaking, tsunamis travel better through deep  water,” Kristine Crossen, head of the University of Alaska Anchorage Geology Department told him. ‘Cook Inlet is fairly shallow. It creates a lot of friction on the base of the wave.”’

“Anchorage is pretty much safe,” Bo Bahgn, an oceanographer at the Tsunami Center agreed Thursday, because it has two things going for it.

The first is simple elevation.

Most of Anchorage, Bahgn noted, “is higher than 30 feet above sea level,” and even the wave generated by the massive quake of ’64 topped out at about 30 feet.

Secondly, he said, the long shallow run up the Inlet into Anchorage makes it hard for a wave to maintain itself as earlier noted. As the wave hits shallow water or land, it slows down and eventually collapses on itself.

Tsunamis, Bahgn noted, are just another version of the bore tide, and you don’t see a bore tide in the Inlet. You only see them in Turnagain Arm where the water is confined to a narrow channel comparatively deeper than its banks.

The Inlet is not narrow enough, he said, and the bottom slope is not conducive to the propagation of a big wave.

But saying a tsunami in the Inlet is improbable is different from it being impossible, cautioned Elena Suleimani, a Tsunami modeller at the University of Alaska Fairbanks. 

Like everyone else, she doubted that a tsunami generated by an earthquake in the Gulf of Alaska could reach the state’s largest city, and if it did, she said, “I guess it would be insignificant.”

But she wasn’t so sure about a tsunami generated by an earthquake in Cook Inlet itself. That, she said, could cause a problem.

“Nothing,” she said, “is totally safe.”

Nature is unpredictable. There are wild cards. If the mother of all disasters were to befall Cook Inlet – a massive earth quake of the size of ’64 coming at a time when the tide is pouring into the Inlet and there is a big storm surge from a Gulf of Alaska storm – who  knows.

In that case, Suleimani said,  some sort of ocean generated wave might reach the city. Scientists are still trying to sort out the exact interactions of tsunamis and rivers, she said, and the Inlet is in many ways just a big, two-way river running in and out with the tides.

The earthquake that hit days ago came at a time when the Inlet was near bottoming out at low tide. Any wave moving up the Inlet would have spread out over a lot of tideland on either side of the channel, and the friction from moving through shallow water or across land quickly takes the steam out a tsunami.

Thus a tsunami wouldn’t have a had chance of moving far up the Inlet even if there had been a tsunami. But, of course, there never was a tsunami.

It never materialized because the movement of the seabed off Kodiak was a strike-slip earthquake, which is a geologist’s way of saying two pieces of the earth’s crust slid alongside each other instead of one piece sliding under the other.

As they repositioned themselves, the movement caused some considerable shaking, but because they remained in the same plane there was no up-down movement of the sea floor to cause a wave.

Alaskans as a result got lucky. There’s no guarantee on next time. But if you’re in Anchorage, and it happens again, don’t panic.

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1 reply »

  1. Seems like this all boils down to a common theme throughout US government. And that is they are very poor at communicating to the public. If the message sent out simply had a little additional information like: “Please ignore this message if you live in Anchorage, Girdwood, etc” … then there would be less confusion. I should be so simple. Just before sending out the message a screen pops up. It has the text of the tsumani warning on it. Below the text are cities that you want to include/not include. Just uncheck the boxes for the cities where the tsunami is not a threat. Then send out the message and the note for the cities that are unaffected is automatically generated. A jr. high computer programming student could figure this out. Alas, that’s why the US government hasn’t figured it out.

    Like

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