Alaska Earthquake and Tsunami, 1964

Garry Gamber asked:

The 1964 Anchorage, Alaska, earthquake and the resulting tsunami struck without warning on Good Friday, March 27.

It was a quiet spring day in Anchorage, a holiday. Temperatures were seasonably mild with a moderate amount of snow on the ground. Children had the day off from school, and customer traffic in the stores downtown was light. Many residents were preparing or enjoying dinner at home. At 5:36 p.m. a major earthquake began to shake the ground, and the earth beneath Southcentral Alaska moved in waves for the next four long minutes.

Parents and children slipped, stumbled and fell on shifting floors in a panicked effort to get outdoors to escape breaking windows. Two inch cracks appeared in the ground in many places. Roads wrinkled and split and Fourth Avenue in downtown Anchorage broke apart and collapsed 10 feet or more. The Government Hill Elementary School twisted, shifted and became unusable in a moment. The outside wall of the J.C. Penney building crashed to the street. In the Turnagain residential district the ground liquefied like quicksand, slid away, and swallowed up 75 or more homes.

The four minute earthquake released the energy roughly equivalent to 10 million times the force of an atomic bomb. The mass of the earth and ocean absorbed most of the force, but manmade structures in the area could not absorb the rest of the force without suffering massive damage. Total property damage was estimated at $500 million.

Anchorage was crippled as gas lines and water lines were severed abruptly. Residents resorted to melting snow for water while awaiting repairs. Four days later students returned to available schools as life in Anchorage began to recover.

The Earthquake

The center of the Alaska earthquake was located about 75 miles east of Anchorage and about 55 miles west of Valdez. It began 14 to 16 miles deep in the earth’s crust, a comparatively shallow depth, where the Pacific plate dives beneath the North American plate. The huge subduction zone is located at the north end of the Ring of Fire, a semicircle of volcanic and earthquake activity that defines the rim of the Pacific Ocean.

The earthquake fault, more precisely the thrust fault, which was the cause of the Good Friday earthquake stretched 750 miles from Alaska’s Aleutian Islands to Valdez. The Pacific plate that day moved an estimated 25 to 30 feet northward, diving beneath the North American plate. The grinding of the two massive tectonic plates caused the Alaska earthquake and measured 8.4 on the Richter scale. In later years the measurement of the Alaska earthquake was upgraded to 9.2 on the Mw, or moment magnitude, scale as the Richter scale was determined to be inaccurate at measuring very large earthquakes above 8.0. Within a day of the initial major earthquake 11 more tremors of 6.0 or greater shook an already nervous population. In fact, aftershocks continued for nearly a year.

The earthquake caused the ground to displace upward by as much as 25 feet on several Alaskan islands and by nearly 3 feet upward at the city of Valdez. In other areas the ground displaced downward as much as 9 feet, for example in the town of Portage.

The Alaska earthquake on Good Friday was the strongest earthquake ever recorded in North America. It was the second strongest ever recorded worldwide, surpassed in strength by the 9.5 Mw earthquake in Chile on May 22, 1960. The recent December 26, 2004, earthquake off the coast of the Indonesian island of Sumatra measured 9.0 Mw. The deadliest earthquake occurred in Shensi Province, China, in 1556 where over 830,000 residents perished.

The Tsunami

Tsunami is an adapted Japanese word meaning “port wave,” a reference to the fact that the wave’s danger and destructive power only become evident as it approaches the shore.

During the 1964 Alaska earthquake the North American plate released upward, displacing a huge volume of ocean water and causing a seismic wave, a tsunami, to travel outward. The wave traveled at an estimated 450 miles per hour in the deeper ocean in a long wave of almost imperceptible height.

As the tsunami wave passed over the continental shelf and approached shore its length shortened, its speed decreased and its height increased as the massive volume and weight of water prepared to release its incredible energy on anything in its path.

At the shallow Valdez Inlet the wave reached a maximum height of nearly 200 feet. Further on, at the old town of Valdez, a 30 foot wall of water struck and demolished all structures. Twenty eight Valdez residents died when the tsunami crashed ashore. Valdez was later rebuilt at a higher elevation and further from the waterfront.

In Seward, Alaska, the earthquake caused a portion of the bay to slide. The slide caused a local tsunami which devastated Seward’s port and downtown district, both of which were eventually rebuilt. Twelve residents perished in Seward.

The small town of Portage was leveled by its own local tsunami and never relocated or rebuilt. Another local tsunami struck the small port of Whittier killing 12 residents.

The Destruction

The original tsunami traveled about 8400 miles. It caused damage in the Hawaiian Islands and along the Oregon and California coasts. A 20 foot wave struck Crescent City, California, and killed 10 residents. The tsunami was responsible for the deaths of 16 people in Oregon and California.

The tsunami killed a total of 122 people in three states. By comparison, the earthquake resulted in 9 deaths.

It has been more than 40 years since the Alaska earthquake and tsunami. In the meantime construction materials and building practices have been enforced to produce structures more capable of surviving strong earthquakes. Also in the meantime, the population in Alaska’s vulnerable areas has increased tremendously.

Smaller earthquakes along Alaska’s subduction zone and other fault zones occur on a daily basis, presumably relieving the internal pressures that would otherwise produce another massive earthquake.

However, nobody knows with certainty when, where, or whether another huge and destructive earthquake will strike Alaska.

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