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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