Second in a series
Originally published March 23, 2014
The Earth’s crust consists of massive slabs of rock called plates. The two largest, the Pacific Plate and North American Plate, make up more than a third of the world’s surface. They meet in a border that stretches from Baja California to Japan, and arches across the Gulf of Alaska. Two hundred miles southeast of Anchorage, the Pacific Ocean floor grinds into the North America continent at a rate of 2.3 inches a year, a nearly irresistible force meeting an almost immovable object. The two plates are locked at their line of and the lighter North American formation bulges upward.
At 5:36 p.m. on March 27, 1964, pressure overcame friction. Pacific Plate dove under the North American Plate, curling downward like a conveyor belt. Centuries of accumulated compression released violently and the coast of Alaska sprang forward as much as 64 feet.
The Great Alaska Earthquake released more energy than all other North American quakes since. Its magnitude, 9.2, made it the second most powerful earthquake ever recorded.
As shockwaves spread to the mainland of Southcentral Alaska, birch trees whipped back and forth, their tops touching the ground. Bridges gave way, roads split in half, rails twisted, power lines snapped. Buildings swayed and collapsed or slid into rubble as the weakened ground beneath them turned into mush.
Under the ocean, massive landslides displaced incalculable tons of water, sending waves as high as 200 feet into the coast. More waves generated by the rupture itself followed, racing to shores thousands of miles away. Water in wells on the other side of the globe oscillated up and down. In the words of Peter Haeussler, U.S. Geological Survey research geologist, the whole world jiggled “like a giant water balloon.”
When it was over, 139 were dead. Smaller settlements — Chenega, Whittier — suffered the greatest loss of life in proportion to their population. In the state‘s largest city, Anchorage, much of the business district lay in ruins, its airport was out of commission, fine homes in its most prestigious subdivision had been swept seaward in a morass of mud. Three of the most important commercial centers in the region — Kodiak, Seward and Valdez — faced catastrophic destruction. Communities that escaped the worst of the initial damage — Seldovia, Portage, Girdwood — were permanently altered by a new geography.
Something else also shifted that day. Science.
Rise and fall
“The 1964 event changed the way we thought about earthquakes,” said Mike West, state seismologist with the Alaska Earthquake Information Center at the University of Alaska Fairbanks. “It literally helped prove plate tectonics.”
The theory of plate tectonics, the constant movement of the Earth‘s crust, and its connection to earthquakes, was relatively new and not widely accepted by scientists in 1964.
“The existing theories were confusing,” said Haeussler. “There wasn‘t anything that unified the way that we were thinking about the earth.”
Most thought that earthquakes happened on vertical faults. But the scope of the 1964 quake could not be explained by the prevailing theory. An alternative model, that quakes could be caused by faults with shallow angles, more horizontal than vertical, gained traction when USGS geologist George Plafker arrived shortly after the disaster.
“He discovered that it was caused by the giant fault that lies underneath all of this southern Alaska margin,” said Haeussler. “Now plate tectonics is as crucial to understanding geology as natural selection is to biology.”
The information produced from studying the 1964 quake also helped scientists understand the connection between earthquakes and the huge waves and ocean surges known as tsunamis. “People had known they were associated for a long time, but they didn‘t know how,” Haeussler said. “But what we learned about the movement of the sea bottom in 1964 is incorporated into models used today.”
Plafker‘s hypothesis was supported by observations he made in Prince William Sound said USGS research geologist Rob Witter.
“He saw these really clear lines of barnacles,” the white-shelled mollusks that attach to rocks below mean high tide. “In some places they were jacked way above the water line. In other places he found that forests were drowned by rising tide levels.”
There had been an uplift on the “seaward” side, as much as 38 feet where the North American Plate shot over the top of the Pacific Plate in southern Prince William Sound. But in Cook Inlet, farther from the fault, where pressure was released, the land subsided 6 feet.
Forests growing next to the ocean at Girdwood, Portage and on the Homer Spit, began dying as their roots were smothered by silt brought in by saltwater tides.
Girdwood would be moved uphill to escape the high water. Portage was basically abandoned. Across Kachemak Bay from Homer, the boardwalk streets of Seldovia went underwater; the town would be rebuilt, but without its unique pre-quake character.
Plafker‘s work also led geologists to reassess previously known, but misunderstood, “wave-cut terrace” formations. Plafker noted a new one had emerged since the quake and that similar formations above it probably meant that similar uplifts had occurred in the past. Carbon dating and core samples determined that 1964-type quakes take place in our area every 500 years or so.
Built on Jell-O
Analysis of the quake provided another revelation, West said. “We all have this idea in our head of an epicenter. We‘d thought of earthquakes as things that happened in one particular place. But this one didn‘t have a point on a map. The rupture began under Prince William Sound, then continued almost like a zipper all the way past Kodiak.”
Witter prefers the analogy of a Velcro strap. “It wasn‘t a line. It was like a wide surface breakage. We‘re talking about something 580 miles long by 100 miles wide.”
The rupture field tore from Prince William Sound to Kodiak at more than 100 miles an hour, the land and seabed shuddering for the duration of the rip.
The main action lasted 4½ minutes with strong aftershocks rumbling for days and weeks after. Kristine Crossen, Department Chair of Geological Sciences at University of Alaska Anchorage said the continuous shaking had a multiplier effect.
“If you hit your hand on your desk once, not much happens,” she said. “But if you keep hitting it over and over again for four minutes, stuff‘s going to start moving around.”
The constant shaking particularly affected parts of Anchorage built on the clay of the Bootlegger Cove formation. The nature of the terrain that underlies much of the Anchorage bowl can be seen in the silt and mudflats that surround the city. If you step onto them (never advisable!) the mud may initially hold your foot in a shallow depression. But move your foot back and forth and it will begin to sink.
Saturated soil can be stiff and cohesive when it‘s still. But it behaves like a liquid when disturbed, a process called “liquefaction.”
Nearly every geologist interviewed for this series used the popular dessert Jell-O to describe upper Cook Inlet soils. “You make it and turn it out of its mold onto a plate and it‘s beautiful,” Crossen said. “Then you put it in your trunk and drive over a bumpy road and by the time you get to the church social, it‘s spread out all over the plate.”
The jiggling clay beneath the city led to landslides, not the familiar down-a-mountain-type avalanche but what geologists call “translatory landslides” in which the movement splays out rather than falling downward. In level parts of town surrounded by other level land, that wasn‘t so much of problem.
But along bluffs and slopes where there was nothing to retain the liquefied soil ground crumbled and broke apart.
“In Turnagain, houses moved into the water,” Crossen said. “Along Government Hill and Fourth Avenue the ground moved into Ship Creek Valley.”
Some of the deaths and damage in Anchorage was due to buildings shaking apart. But most happened when previously solid land was vibrated into a quicksand-like ooze.
“All Alaskans should be haunted by liquefaction,” West said. “It‘s a particular hazard in Alaska. Most of our large communities — Fairbanks, Anchorage, Mat-Su — are built on flat sediments. Our soils tend to be very wet. Or we‘re built on river valleys. Or permafrost. And it doesn‘t take a giant earthquake for trouble to happen.”
Not if, but when
Future quakes are a certainty, West said. “Four out of five earthquakes in the U.S. occur in Alaska. Between 1960 and 2010, more than 99 percent of the earthquake energy released in the U.S. occurred in Alaska, and 78 percent occurred during the 1964 earthquake.”
Even a small quake can bring down a city if it happens in close proximity.
“The New Zealand earthquake in 2011 was ‘just‘ 6.1, but it killed 150-some people,” West said. “That quake was so lethal because it struck right smack in town and had soil perfectly situated to exaggerate that motion,” he said, soil not that different from Anchorage.
For the past 50 years, the Pacific Plate has steadily continued to press against North America. At the moment the compression since 1964 amounts to 10 feet, and it keeps building every day.
“The exact earthquake of 1964 is not expected to occur again for a few hundred years,” West said. “But that‘s a really dangerous statement if it sits there on its own. People think, ‘500 years? We must be safe right now.‘ But that‘s not true. There are countless other earthquake possibilities and no one will be surprised if there‘s another 8-point quake pretty much anywhere along the Alaska-Aleutian arc tomorrow.”
Just where is a mystery, however.
Seismically speaking, “Alaska is not exactly mapped,” West said. “If you look at a fault map of California, every little trace and feature has been studied and documented. We don‘t have that. Some people point to known features — like the Castle Mountain Fault — as the focus of the next earthquake. But I‘m more scared by the unknown ones.
“History has proven that the most dangerous earthquake is usually the one we‘re not expecting.”
What‘s in a number?
8.6 or 9.2?
The Great Alaska Earthquake of 1964 was initially declared to have a magnitude of 8.6. That has been revised to 9.2 as new ways of measuring seismic action are developed and technology is improved. Cindi Preller, NOAA‘s Alaska Tsunami Program Manager, noted that there were only three seismographs in Alaska at the time, one each in Fairbanks, Sitka and Adak. “Today we say seismographs have to be within 100 miles to give an accurate reading,” said UAA geology professor Kristine Crossen. “In 1964, that wasn‘t the case. The 8.6 was just a good guess.
It is hard to describe the power of a 9.2 earthquake. One analogy using spaghetti sticks is used in the USGS video “The 1964 Great Alaskan Earthquake” (see the Online listings); if the energy of a 5.0 quake is represented by snapping a single stick, then the energy of 9.2 exceeds what’s needed to break 800,000 sticks. Preller supplied estimates that equated it to more than 4 million Hiroshima-size atom bombs or enough energy to power the entire world at current rates for the next 179 years.