George Plafker: "Everything was in chaos..."
Art Grantz: "I'd never seen anything that destructive 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.
Total property 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 electrical systems.
Narrator: The epicenter was in Prince William Sound,
74 miles southeast of Anchorage.
Yet effects were observed as far away as Texas and Louisiana.
What the 1964 Great Alaska Earthquake taught scientists 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 Anchorage International, was closed down because the
control tower had collapsed and killed the operator."
Arthur Grantz: "... we went out separately, mostly separately, to look at different things
so we could cover three times as much ground..."
Narrator: The scientists studied the effects from the
air, on land, and along shorelines.
They were astonished to find that the surface was disrupted over an area larger than California
-- 185,000 square 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 surface were suddenly several feet above.
Narrator: Mapping this uplift and down-drop became crucial
for understanding what happened.
But, with no faults visible at the surface to explain it....even with months of careful
observation and 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 the earth is broken up into roughly a dozen different
plates and that they move around with respect to 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-forming theory of plate tectonics.
He knew the theory had new crust forming at mid-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 being pushed underneath that part of southern Alaska
at a very low angle and there was slip on this, 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.
Periodic slip 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 colliding and 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 being compressed over years of the plates converging...it
releases like a spring - which is the earthquake...Seaward areas are uplifted while landward areas drop
down.
George Plafker identified this pattern common to megathrust quakes in subduction zones.
Peter Haeussler: "The 1964 earthquake was the first megathrust
subduction zone earthquake properly interpreted as such.
As a result of that, essentially, every other large subduction zone earthquake around 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 earthquake like this is how frequently do they occur?
Could you have one tomorrow or is it thousands of years?
Narrator: Plafker and his team drilled 50 feet into
the earth and collected core samples to find out.
They used carbon dating to identify when past megathrust earthquakes have occurred in south
central Alaska.
George Plafker: "... it's just an example of what has happened
in the past and the analog for that is what happened in the 1964 earthquake, namely abrupt
uplift of a broad area of mudflats that are intertidal 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 moment of sudden change...a past megathrust quake.
Dating the plant remains provides an age for that quake.
Narrator: The team discovered nine megathrust earthquakes
have occurred in south central Alaska over the 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 across
the Pacific, wreaking havoc in coastal Oregon, California, Hawaii and beyond.
Narrator: Locally, a number of extremely dangerous tsunamis
occurred in south central Alaska fjords like Whittier and Valdez.
Most deaths resulting from the 1964 quake came from these local tsunamis in fjords.
The scientists recognized that these were produced by underwater landslides that 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 were
made right here in Whittier was by underwater landslides.
... there is material at the edges of these fjords here and then it's shaken in the earthquake
and then it slides downward into 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 the
shoreline very soon after the beginning of shaking... and so here in Whittier the first
tsunami wave was really well observed, out in the middle of the fjord, but within three
minutes there were three waves that covered a large part of Whittier.
And it killed about 12 people.
There was a lumber mill located about where that hotel is in the background there - 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 of its population.
Narrator: Today, scientists use ocean-bottom sonar mapping
to identify submarine landslide deposits from the past.
Additional work like coring and dating these slides will help refine understanding of 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 big underwater landslide deposits at depth.
So, we know that these kinds of things happen over and over again.
Ya know, here we are at the margin of a fjord, we've got these big mountains there's glaciers
and streams eroding these things, they're putting sediment at the margins of the fjord
. We have the megathrust underneath us here at about 12 to 15 miles depth and these big
earthquakes it shakes like crazy releases these sediments into the deep parts of the
fjord and then generates tsunamis.
So, if you're living at the edge of a fjord, or recreating and an earthquake happens, a
really important thing to do is to travel to high ground right away.
You don't want to wait to hear a tsunami alarm or anything like that.
If you feel strong shaking that feels like a strong earthquake you 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.
Massive landslides 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 blocks that sort of slid sideways as a result of
that... and then some buildings collapsed into those areas, sometimes the edge of a
building was sticking off where it had failed underneath there.
There were a few people killed as a result of the damage to these buildings in the 64
earthquake.
Narrator: The widespread damage and loss of life from
this earthquake led to a determination to use science to save lives in the future.
Legacies from the 1964 Great Alaska Earthquake include:
-- The establishment of the USGS Earthquake Hazards Program.
-- NOAA's round-the-clock Tsunami Warning Centers.
-- New building codes and innovations in retrofitting older, 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 the largest population center in the state.
And the work that we do involves basically the fundamental characterizing of the earthquake
hazard and knowing which faults are active, which faults can produce earthquakes, understanding
how often those earthquakes occur...and then another part is understanding the local tsunami
hazard.
And getting an idea of how often they occur 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 at such risk for earthquakes
that the entire town was moved.
Narrator: In recent years, megathrust quakes in subduction
zones... accompanied by tsunamis have occurred in 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 to help reduce risks and save lives.
END