The 1964 Great Alaska Earthquake occurred on Good Friday and rocked the state with strong
ground shaking that lasted four and a half minutes.
At magnitude 9.2, it was the second largest quake ever recorded by seismometers.
Its memory haunts those who lived through the shaking and experienced the tsunami that
destroyed entire communities.
This earthquake also rocked the scientific community by confirming a major principle
of plate tectonics with the observation that the oceanic Pacific Plate dives beneath the
continental North American Plate at the Aleutian trench.
Alaska has over 24,000 measured earthquakes every year, more than the rest of the US combined.
Shown here are only those above magnitude 6.
On average, at least one strong magnitude seven occurs every year.
Between 1899 and 1986 eight earthquakes of magnitude eight or greater impacted different
areas of the Aleutian subduction zone.
Red circles mark the epicenter above the site where fault rupture began, whereas yellow
shows the fault-rupture area.
How can we understand the relative energy released by a great earthquake of magnitude
9 compared to smaller yet still damaging earthquakes?
And what is the rupture zone?
The energy released by earthquakes increases by a factor of 30 for each unit increase in
magnitude.
As an anology, if breaking a single strand of pasta represents a magnitude five earthquake,
then we need to break 30 strands to represent a six, 900 strands for a seven, and 27,000
noodles to represent a magnitude eight.
To represent the energy released in a magnitude nine earthquake, like the recent earthquake
in Japan, we would need to break an immense bundle of 810,000 noodles!
And the Alaska 1964 earthquake was even larger at magnitude 9.2.
To address the great-earthquake rupture process, let’s first examine the plate tectonics
of the North Pacific region.
The distribution of earthquake epicenters, shown earlier, roughly defines plate boundaries.
The Aleutian trench marks a 2-thousand-One-Hundred-mile-long subduction boundary between the Pacific and
North American plates, similar to the Nazca Plate diving beneath the South America Plate
at the Peru-Chile Trench.
Along southeast Alaska and western British Columbia, the Pacific Plate grinds against
the North American Plate in strike-slip motion as it does along the San Andreas Fault in
California.
The rate of subduction varies from 2.2 to 3 inches per year.
We will animate the 1964 earthquake in four consecutive perspectives to understand what
happened: Most people think of an earthquake as a sudden
release of energy at the epicenter.
In reality, an earthquake is caused by sudden displacement of rock across a fault at a given
depth within the earth at the hypocenter.
For great earthquakes, the displacement at the hypocenter is just the beginning.
The initial fault break in 1964 triggered a runaway rupture that displaced an area more
than 500 miles long, The entire fault rupture elapsed over four and a half minutes releasing
immense seismic energy equivalent to a cascade of 1,000 magnitude seven earthquakes.
Lets zoom in now to a cross section to view the plate interaction.
The dense oceanic plate dives beneath the more-buoyant continental plate at a rate of
about 2.5 inches per year.
Because tectonic plates are elastic like a spring, converging plates build energy at
the leading edge of the continent.
Locked by friction, the upper plate is forced back and shortened, raising the land surface.
GPS, not available in 1964, is now able to measure that strain.
In 1964, the force of the converging plates over came friction, and caused the leading
edge to lurch seaward, uplifting the seafloor, forming a mound of seawater that spread out
as a tsunami.
As coastal areas near the trench were uplifted, areas inland dropped.
During the 1964 earthquake, the broad area at the leading edge of the overlying plate
rose up as high as nine meters as the landward region subsided up to 2 meters, thus raising
the shoreline barnacles well above sea level and dropping the inland shoreline below sea
level killing trees, leaving its legacy in ghost forests.
The ground shook relentlessly as the rupture progressed along the fault surface.
Prolonged shaking served to liquefy underground water-saturated sediment.
Liquefaction triggered the landslides that caused in major damage in Anchorage.
Deformation of the seafloor, especially uplift along the Patton Bay fault, produced a tsunami
that spread across the Pacific Ocean.
Astonishingly only nine people died from the earthquake itself, but 130 were killed by
the subsequent tsunami that wreaked havoc across the northern Pacific region.
In the early 1960’s, Plate Tectonic Theory was in its infancy, and subduction was poorly
understood.
Measurements of land and seafloor deformation taken by U.S.G.S.
geologist George Plafker & co-workers soon after the earthquake, led them to propose
that underthrusting of the Pacific Plate beneath Alaska caused the earthquake.
This discovery helped establish subduction of oceanic plates beneath continental plates,
and megathrust earthquakes as fundamental processes of plate tectonics.
Since 1964, when there were only two seismometers in Alaska, monitoring earthquakes and ground
deformation to assess hazards and to mitigate future risk has greatly increased.
Research that characterizes earthquakes and fault lines serves to reduce the possibility
of casualties by better land-use planning and constructing earthquake resistant buildings,
allowing us to live more safely in earthquake zones.