1964 Quake: The Great Alaska Earthquake

George Plafker: "Everything was in chaos.

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" Art Grantz: "I'd never seen anything thatdestructive that close up.

" Narrator:In 1964, Alaska was shaken by the largest U.

S.

earthquake ever recorded:Magnitude 9.

2.

Narrator:Shaking went on for over four minutes.

One hundred forty three people died.

Totalproperty loss in 2013 dollars is estimated at 2.

3 billion.

There were gaping fractures,massive landslides, and the destruction of water mains, gas, sewer, telephone and electricalsystems.

Narrator:The epicenter was in Prince William Sound, 74 miles southeast of Anchorage.

Yet effectswere observed as far away as Texas and Louisiana.

What the 1964 Great Alaska Earthquake taughtscientists was as profound and far-reaching.

Narrator:Initially, no one understood how or why the earthquake occurred.

Immediately, three U.

S.

Geological Survey scientists were sent to figure it out.

George Plafker: "The main airport, the AnchorageInternational, was closed down because the control tower had collapsed and killed theoperator.

" Arthur Grantz: ".

we went out separately,mostly separately, to look at different things sowe could cover three times as much ground.

" Narrator:The scientists studied the effects from the air, on land, and along shorelines.

They wereastonished to find that the surface was disrupted over an area larger than California — 185,000square miles.

Some areas dropped down as much as 8 feet,and others rose up by as much as 38 feet.

barnacles once two feet below the ocean surfacewere suddenly several feet above.

Narrator:Mapping this uplift and down-drop became crucial for understanding what happened.

But, withno faults visible at the surface to explain it.

Even with months of careful observationand field work the cause of the quake remained a mystery.

Peter Haeussler:"It was right at this time that this idea of plate tectonics, that the surface of theearth is broken up into roughly a dozen different plates and that they move around with respectto each other.

It occurred right at the time when this idea was being put forth.

" Narrator:One of the scientists, geologist George Plafker, considered the quake in terms of this newly-formingtheory of plate tectonics.

He knew the theory had new crust forming atmid-ocean ridges but there was no explanation for where this crust went.

George Plafker:"And so the most likely one, we could, came to mind was that the oceanic crust is beingpushed underneath that part of southern Alaska at a very low angle and there was slip onthis, on the interface between the oceanic crust and the overlying continental crust.

" Narrator:These two crusts are converging at the rate of an inch and a half each year.

Periodicslip between the crusts produces great quakes, which Plafker called Megathrust earthquakes.

Narrator:His realization changed our understanding of these great earthquakes.

Megathrust quakes are the largest known on planet Earth.

They occur in areas of collidingand descending crusts known today as subduction zones.

Narrator:The uplift and down drop of large areas from these quakes is a result of the crust beingcompressed over years of the plates converging.

It releases like a spring – which is the earthquake.

Seawardareas are uplifted while landward areas drop down.

George Plafker identified this patterncommon to megathrust quakes in subduction zones.

Peter Haeussler:"The 1964 earthquake was the first megathrust subduction zone earthquake properly interpretedas such.

As a result of that, essentially, every other large subduction zone earthquakearound the world sort of falls in the shadow of what we learned from the 1964 earthquake.

" Narrator:Next, the question became: "How often do these quakes happen?" George Plafker:"One of the obvious things that everybody wants to know when you have an earthquakelike this is how frequently do they occur? Could you have one tomorrow or is it thousandsof years? Narrator:Plafker and his team drilled 50 feet into the earth and collected core samples to findout.

They used carbon dating to identify when past megathrust earthquakes have occurredin south central Alaska.

George Plafker:".

it's just an example of what has happened in the past and the analog for that is whathappened in the 1964 earthquake, namely abrupt uplift of a broad area of mudflats that areintertidal and then sudden appearance of fresh water plants growing on that surface.

Narrator:In the cores, where the remains of land plants overly ocean sediments this marks a momentof sudden change.

A past megathrust quake.

Dating the plant remains provides an age forthat quake.

Narrator:The team discovered nine megathrust earthquakes have occurred in south central Alaska overthe past 5,500 years.

The average time span between these quakes is was 630 years.

Narrator:Another devastating effect of the 1964 Great Alaska Earthquake was a series of deadly tsunamis.

The largest, triggered by the shifting of plates when the quake began.

traveled acrossthe Pacific, wreaking havoc in coastal Oregon, California, Hawaii and beyond.

Narrator:Locally, a number of extremely dangerous tsunamis occurred in south central Alaska fjords likeWhittier and Valdez.

Most deaths resulting from the 1964 quake came from these localtsunamis in fjords.

The scientists recognized that these were produced by underwater landslidesthat occurred as the quake began.

Peter Haeussler:In the 1964 earthquake, of the people who died, most people were killed by tsunamis.

And there's kind of two ways you can make tsunamis, but the way that the tsunamis weremade right here in Whittier was by underwater landslides.

there is material at the edgesof these fjords here and then it's shaken in the earthquake and then it slides downwardinto the deep part of the fjord, that generates tsunami waves which then hit the shoreline.

And the thing that's really notable about those kinds of tsunamis is that they hit theshoreline very soon after the beginning of shaking.

and so here in Whittier the firsttsunami wave was really well observed, out in the middle of the fjord, but within threeminutes there were three waves that covered a large part of Whittier.

And it killed about12 people.

There was a lumber mill located about where that hotel is in the backgroundthere – there were 13 deaths in Whittier and 12 of them were over there.

" Narrator:Chenega, a small native village in Prince William Sound, lost 23 people — a third ofits population.

Narrator:Today, scientists use ocean-bottom sonar mapping to identify submarine landslide deposits fromthe past.

Additional work like coring and dating these slides will help refine understandingof the tsunami hazard and how often these quakes occur.

Peter Haeussler:.

And at Valdez in particular, it looks like there may be like six to ten of these bigunderwater landslide deposits at depth.

So, we know that these kinds of things happenover and over again.

Ya know, here we are at the margin of a fjord, we've got thesebig mountains there's glaciers and streams eroding these things, they're putting sedimentat the margins of the fjord.

We have the megathrust underneath us here at about 12to 15 miles depth and these big earthquakes it shakes like crazy releases these sedimentsinto the deep parts of the fjord and then generates tsunamis.

So, if you're living atthe edge of a fjord, or recreating and an earthquake happens, a really important thingto do is to travel to high ground right away.

You don't want to wait to hear a tsunami alarmor anything like that.

If you feel strong shaking that feels like a strong earthquakeyou need to head uphill right away.

Don't wait until the earthquake is over.

" Narrator:Some of the most stunning destruction from the 1964 quake came from sub-aerial landslides.

Extreme shaking led to significant ground failure and liquefaction in Anchorage.

Massivelandslides struck the downtown area, Government Hill, and in the Turnagain-By-The Sea subdivision.

Peter Haeussler:.

what happened is through the ground shaking in the 64 earthquake there were these blocksthat sort of slid sideways as a result of that.

and then some buildings collapsedinto those areas, sometimes the edge of a building was sticking off where it had failedunderneath there.

There were a few people killed as a result of the damage to thesebuildings in the 64 earthquake.

Narrator:The widespread damage and loss of life from this earthquake led to a determination touse science to save lives in the future.

Legacies from the 1964 Great Alaska Earthquakeinclude: — The establishment of the USGS EarthquakeHazards Program.

— NOAA's round-the-clock Tsunami WarningCenters.

— New building codes and innovations in retrofittingolder, vulnerable structures.

As part of the Advanced National Seismic System,the USGS now routinely monitors all earthquakes that occur in the U.

S.

Peter Haeussler:"South central Alaska here is the infrastructure center of the state and it's also by far thelargest population center in the state.

And the work that we do involves basically thefundamental characterizing of the earthquake hazard and knowing which faults are active,which faults can produce earthquakes, understanding how often those earthquakes occur.

And thenanother part is understanding the local tsunami hazard.

And getting an idea of how often theyoccur and doing tsunami modeling to understand where people could be hit by these tsunamis.

" Narrator:All together, these programs can help predict strong ground motions from future earthquakes,and minimize risks.

For example, scientists learned that Valdez was so unstable and atsuch risk for earthquakes that the entire town was moved.

Narrator:In recent years, megathrust quakes in subduction zones.

accompanied by tsunamis have occurredin Indonesia, Japan and Chile.

Narrator:The 1964 Great Alaska Earthquake changed our understanding of earthquakes and tsunamis.

and had a profound and lasting impact on how scientific knowledge can be used tohelp reduce risks and save lives.

END.

Source: Youtube

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