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
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
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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