1964 Alaska Earthquake
The Great Alaska Earthquake that struck the Anchorage area on Good
Friday, March 27, 1964 at 5:36 PM registered 8.6 on the Richter Scale,
although scientists now favor a different magnitude scale for very
large quakes that shows this quake as 9.2. This made it the largest
quake that has hit the United States in recorded history and one of the
largest known worldwide. Geologically, the effects were widespread and
dramatic. Large areas were lifted up or dropped by several feet,
landslides were extensive, ground failure led to large fissures in the
ground, landslides into bays caused huge seiche waves locally and a
tsunami caused damage thousands of miles away. Luckily, the casualties
were considerably lighter than might be expected for a disaster of this
magnitude. 115 deaths are attributed to the quake. This relatively low
number can be attributed to the sparse population of the area and the
fact that the quake occurred when most people were at home.
Cause of the Earthquake
The Great Alaskan Earthquake was the result of the movement of huge
plates of the earth's surface. This process of plate tectonics causes quakes when
neighboring plates interact. In this case the Pacific Plate containing
the Pacific Ocean is being pushed under the North American Plate. This
kind of subduction causes the largest and deepest earthquakes known. As
the Pacific Plate dives under the lighter continental crust it also
pushes up portions of the ocean crust which rise as mountain ranges.
Volcanos erupt as the descending ocean plate heats up in its descent
towards the earth's mantle. The rock melts and magma rises to the
surface in periodic eruptions.
Description
The earthquake started with a few seconds of small tremors. These
quickly built into intense shaking that knocked people down, threw
objects from shelves and caused buildings to collapse. Amazingly this
shaking lasted for a full 5 minutes. People reported that it seemed
like an eternity. For comparison, the Northridge and Loma Prieta quakes
in California each lasted less than 30 seconds. The time of shaking
generally increases with increased magnitude. The longer the ground
shakes, the more damage will occur as structures first weaken and then
collapse under the strain. The long period of shaking in this quake
doubtless caused much of the ground failure that was observed.
Downtown Anchorage was especially hard hit. Building facades crashed
into the street. In some places one side of the street dropped down
over 10 feet, leaving the facing buildings towering above. In places
ground waves of over 3 feet high were observed. People reported feeling
as if they were in ships at sea as the waves passed through. Fissures
opened up as blocks of earth dropped and tilted. Underground layers of
soil liquified, allowing the solider ground above to slide many feet,
sometimes in solid blocks. Cliffs collapsed in huge landslides. One
landslide occured under an expensive housing development overlooking
Cook Inlet. Other landslides into bays near Valdez and Seward sent 35
foot waves sloshing back and forth like water in a bathtub. In Seward
an oil tanker was wrenched loose from a pipeline, which erupted in
flames, spreading to the nearby oil tanks. Burning oil on the water
washed inland. Ships were battered against piers.
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