AP-Electric clocks on walls in Anchorage shut down at 5:36 p.m. on March 27, 1964. Time stopped at the start of the ’64 Great Alaska Earthquake, the second largest ever recorded at magnitude 9.2.
The ground shook for four to five minutes, roughly twice through “I Want to Hold Your Hand,” the Beatles No. 1 song that year. With the U.S. and the USSR facing off in the Cold War, some thought the Russians had attacked. The quake produced a tsunami that wreaked havoc along the West Coast. Overall damage was estimated at $2.3 billion in 2014 dollars. The quake and tsunamis killed 131 people.
Though now a distant memory, it helped make experts better prepared for future quakes. It played a role in the formation of the West Coast and Alaska Tsunami Warning Center and taught the U.S. Geological Survey what evidence to look for of previous great quakes, such as events in the Cascadia Subduction Zone off Oregon and Washington.
Geologists have a name for such strong earthquakes — megathrusts.
Plate tectonic theory says the Earth’s crust is made up of about a dozen major plates that rest on hot mantle. The thin, dense, Pacific Plate at Alaska’s southern edge moves at a shallow angle beneath the North American Plate at average rate of 2.3 inches (5.8 centimeters) per year and eventually is reabsorbed by mantle. Strain builds up until there’s a slip.
The Moment Magnitude scale rates quakes by the energy they release by calculating the area of fault rupture, the average amount of slip, and the force required to overcome the friction sticking the rocks together, according to the USGS. Every whole number higher on the scale represents 30 times the energy of the previous number.
More energy was released in the 1964 magnitude 9.2 quake than all other recorded Alaska quakes combined, said USGS research geologist Peter Haeussler. And drilling showed evidence of nine such megathrusts in south-central Alaska in 5,500 years. The average time span between them: 600 years.
The temblor caused two kinds of waves: local and trans-ocean tsunamis.
Eighty-five of the 106 Alaska tsunami deaths were caused by waves generated by underwater landslides. Rivers flowing into Alaska fjords build up huge deltas of unstable sediment on deep, underwater slopes, said USGS research geologist Rob Witter. Slides generate waves like a rock dropped in a pond.
During local tsunamis, water can at first retreat from shore in response to the underwater slide, then rush back. That’s what happened at Port Valdez. When the water swept back in, it smashed a freighter into the city dock. Longshoremen and children who had assembled to catch oranges or candy tossed down from the crew were killed. Thirty-two people died.
The great quake also rumpled the ocean floor like a rug, creating a trans-ocean tsunami. The deadly waves traveled down the West Coast. Four campers on a beach died at Newport, Ore. A dozen died in the northern California community of Crescent City.
