If there is one thing that is true
with our knowledge of geology is that the actual explanatory science is out
there sitting in a text of some sort drawn from hard evidence from the
field. Mt St Helens was the classic example
of this. After all no end of volcanoes have
actually collapsed and disappeared catastrophically in even recent memory such
as in Indonesia.
Plate tectonics was struggling in
1963 and had been since first proposed some forty years earlier. In fact it was rejected outright by the
majority of geologists in exchange for some pretty bizarre explanations.
In Alaska seismic stations were able
to usefully refine the data and that made it quite clear what had actually
happened. This conformed completely to
the plate tectonic paradigm. Soon after,
the magnetic fields were mapped on the Atlantic sea bed and that put an end to
anyone’s doubts.
How the 1964 Alaska Earthquake Shook Up Science
By Becky Oskin, Senior
Writer | March 27, 2014 08:33am ET
There were great horrors, but what many
children remember is missing their supper.
The earthquake struck
at 5:36 p.m. Alaska Standard Time on Good Friday. When the first shaking hit,
many parents were in the kitchen, fixing dinner. For more than 4 minutes, the
earth buckled and lurched all across southern Alaska. Few people returned home
to their meals that night. In Anchorage, the ground cracked open and giant
fissures swallowed children whole, killing them in front of their siblings.
Landslides launched tsunamis that
swept away coastal villages before the shaking even ended. In Seward, spilled
oil slicked the water and caught fire. When the earthquake-triggered tsunami
hit minutes later, the wave was blazing. "It was an eerie thing to see — a
huge tide of fire washing ashore," survivor Gene Kirkpatrick told National
Geographic magazine in 1964.
In 50 years, no earthquake since has
matched the power of the March 27, 1964, Great Alaska earthquake. Now ranked a
magnitude 9.2, the second-largest ever recorded, the earthquake radically
transformed the young state. Important coastal ports, roads and rail lines were
destroyed. The liquefied ground in Anchorage led to the country's strictest
seismic building codes (now outpaced by California). President Lyndon Johnson
ordered a comprehensive scientific study of the earthquake.
The geologic
discoveries transformed how we understand the Earth.
"In 1964, earth
scientists were swept away by the plate tectonic revolution, which changed
everything we know about how the earth works," said Ross Stein, a U.S.
Geological Survey geophysicist. "That insight was triggered by the Great
Alaska earthquake 50 years ago."
In the 1960s,
geologists thought straight up-and-down (vertical) faults bounded the edge of
continents, similar to the San Andreas Fault that
slices through California. In 1965, Frank Press, who would become science
adviser to four presidents and head of Caltech's Seismological Laboratory, said
a vertical fault extending from 9 to 125 miles (15 to 200 kilometers) deep
caused the Great Alaska earthquake. His model was published May 15, 1965, in
the Journal of Geophysical Research. One month later, USGS geologist George
Plafker proved him wrong.
As a USGS geologist,
Plafker had studied Alaska's geology each summer since 1953. But he was in
Seattle when the 1964 earthquake struck. After Plafker heard the Space Needle
had swayed as the seismic waves raced past, he called his boss in Menlo Park,
Calif., recommending an immediate response. Any earthquake big enough to shake
the Space Needle from Alaska must be of interest to the USGS, he said.
"I
suggested we get up there fast before everything was bulldozed flat by the
engineers," Plafker said.
Plafker's work on the
1964 earthquake solved a key piece of the plate tectonic puzzle: How oceanic
plates recycle themselves at collision belts called subduction zones. At
a subduction zone,
one plate curves beneath another plate and sinks into the mantle, the hotter
layer beneath the crust.
"Before the 1964
earthquake, we did not have a unifying theory of how the earth works,"
said Peter Hauessler, a USGS research geologist. "The 1964 earthquake was
the first time people understood that there were places called subduction zones
that produce these really enormous earthquakes."
Plate tectonics is
now a widely accepted model that explains everything from why earthquakes
happen to how mountains grow. The model says that Earth's surface is divided
into stiff slabs of crust called plates. The oceanic plates are born and grow
at mid-ocean ridges, the long underwater volcanic chains that wind around the
Earth like seams on a baseball. Evidence for this growth was first published in
1963 — progressively older magnetic stripes on the seafloor record spreading
away from the volcanic ridges.
But in 1964, geologists
believed the Pacific Plate was rotating counter-clockwise. In that scenario, no
new crust was created at underwater volcanic ridges, nor was old crust shoved
under continents at subduction zones. (The counter-clockwise rotation was a
concept created to explain the hundreds of miles of offset recently discovered
along the San Andreas Fault.) However, this model didn't explain a strange
observation: Where some plates meet, earthquakes deepen, defining a
gently-dipping plane.
The careful geologic
mapping led by Plafker in the summer of 1964 would be key to solving the
mystery of oceanic plates sliding around Earth's surface, Stein said.
"George
discovered they were shoved underneath the continents. He solved this
incredible puzzle that triggered an understanding of what happens to the
Pacific Plate as it subducts."
Grinding plates
Beneath southern
Alaska, the Pacific Plate dives underneath the North American plate,
grinding northwest at a rate of 2.3 inches (5.8 centimeters) per year.
Friction between the two plates makes them lock together. Even though they're
locked, the plates keep moving, compressing the crust like springs. Where the
plates lock, they buckle and warp, similar to a piece of carpet wrinkling at
one end. Because of this compression, some areas of the Alaska coastline
warped downward before the earthquake and others bulged upward.
During the 1964
earthquake, giant sections of coastline rose or fell as each plate relaxed and
released the centuries of compression. The rupture was like unpeeling a piece
of Velcro, with a segment of the subduction zone 580 miles long (930 km) by 100
miles (160 km) long shuddering apart at more than 100 miles an hour (160 km/h).
Pfalker and his
colleagues surveyed the uplift and sinking after the 1964 earthquake. Areas
around Montague Island rose 13 to 30 feet (4 to 9 meters) and Portage dropped 8
feet (2 m). Overall, the Pacific Plate slid under North America by about 30
feet (9 m). Like bathtub rings, the boosted-up islands showed the vertical
changes. Masses of dead barnacles and starfish proved the land had just been
underwater.
Plafker concluded the
pattern could only have been caused by a hidden fault, releasing tension about
9 miles (15 km) below the surface. They never found a significant surface break
from a vertical fault, just minor cracks from secondary faults. The results
were published in the journal Science on June 25, 1965.
"If you do the
things right, you can reveal some of nature's secrets," Plafker said.
Aftershocks also
confirmed the findings. Following the massive megathrust in March, small
earthquakes jangled the sinking plate all along its length. Detecting these
quakes with seismometers showed
the Pacific Plate bent beneath the North American Plate. (Before March 1964,
Alaska had only two of the earthquake-sensing instruments — one in Fairbanks
and one in Sitka. A bigger network was installed after the quake.)
And with hindsight,
researchers can inspect the seismic record of the 1964 earthquake and see the
pattern of a subduction zone earthquake hidden in the needle scratches. The
pattern suggests one block thrusting over another, not the up-and-down motion
of a vertical fault.
Future hazards
After the coastline
sank, trees began dying as saltwater and silt invaded their roots, creating
ghost forests still visible today. Decades later, these Alaska ghost forests
were the clue to figuring out that the Cascadia subduction
zone offshore of Washington also had a magnitude-9 megathrust earthquake in
1700.
"The 1964
earthquake gave birth to modern megathrust earthquake detection," Haussler
said. "The patterns have now been recognized in many other regions."
The raised islands and
tree graveyards along Alaska's coast suggest that megathrust earthquakes similar
to the 1964 temblor happen sometime between every 330 and 900 years. But
geologists are more concerned about the hazards Alaskans face from more
frequent, smaller quakes along the Aleutian subduction zone,
between magnitude 7 and magnitude 8.
State seismologist
Michael West thinks Alaskans have grown too lax about earthquake hazards.
"After the 1964
earthquake there was a visceral understanding of the hazards we faced, and I
think we've lost a little bit of that edge," he said.
In Anchorage, wet,
silty soils liquefied and a massive landslide destroyed 75 homes in 1964. Now
known as Earthquake Park, the Turnagain Heights landslide is where children and
homes were swallowed in the fissured ground. Some of the city's most expensive
houses slid into the ocean atop liquefied soils. Yet people were allowed to
rebuild along the bluff.
Saturated soil can be
stiff when it's still, holding up houses and buildings. But when it shakes, the
soil jiggles like gelatin and behaves like a liquid. Two-thirds of Alaska's
population lives on top of these mixes.
Since the 1964
earthquake, geologists have learned that the speed of earthquake shaking plays
an important role in destruction due to liquefaction.
The shaking in 1964 was long and slow, instead of the fast, high-frequency
shaking similar to Christchurch, New Zealand, which killed 185 people with a
magnitude-6.1 quake in 2011. Christchurch and Alaska share similar mixes of
unconsolidated sediments, West said.
Terrible waves
The earthquake also
proved the link between subduction zone earthquakes and tsunamis. The movement
of the seafloor during the earthquake shoves the sea, giving it a big slap that
translates into a massive tidal wave.
For an earthquake and
tsunami larger than any in the past decade, the death toll was remarkably low,
just 131 people. Throughout the southeast, the worst damage wasn't from ground
shaking, but from soil failure, tsunamis and landslides. The state had
few residents, and they lived in low-rise wood-frame buildings, the most
resistant to shaking. [11 Facts About The 1964 Alaska
Earthquake]
Of the 119 deaths
attributable to ocean waves, about one-third were due to the open-ocean
tsunami: four at Newport Beach, Ore.; 12 at Crescent City, Calif.; and about 21
in Alaska. The most terrible damage was from tsunamis triggered
by underwater landslides, as thick piles of sediment slumped and slid during
the earthquake. In some cases, these waves hit before the earthquake ended,
sweeping away entire villages. Eighty-two people were killed by these
"local waves."
"The victims in
Seward, Chenega, Valdez and Whittier barely had a chance. The tsunami washed
over them in a matter of seconds," West said.
In Seward, the tsunami
inundation zone, where water destroyed the town and docks, was turned into a
park and public campground. But new development has crept into the flood zone
in recent years, prompting debate over safety and tsunami hazards.
In the past 50 years,
Alaskans have endured scores of powerful earthquakes that would have devastated
other states, such as a magnitude 7.9 earthquake in 2002 and a 7.5 shaker in
2012.
"If you’re not
careful, the take-home message is that these big earthquakes don't hurt anyone
in Alaska," West said. "That’s tremendously naive."
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