- Published on Tuesday, 18 March 2014
- Written by Tim Wright
It was 1964, and College freshman Dorothy Moore was ironing in her bedroom. She was home in Valdez during Spring Break. "It was Good Friday," says Moore. About 5:30pm, her mother called the family into the kitchen to eat. "We were sitting down to a spaghetti dinner when the house began shaking."
"We'd had earthquakes before so you knew what it was," says Moore. "And I remembered I'd been ironing in the bedroom. I'd left the iron on. I thought, 'oh dear! The iron is going to fall off the ironing board. It's gonna catch the house on fire and I'd better go get it.' As I am trying to maneuver through the house, I find myself falling onto the couch." Undeterred, Moore manages to reach her bedroom and shut off the iron. "I had to hang on to the walls to get back to the kitchen area. At that time, I decided that maybe we should go outside."
"Our house was probably six to eight blocks away from the dock area where the ships were. I can remember looking down toward the ship and hearing the ship hoot its horn the way it would if it were leaving. And I'm thinking, 'that ship isn't supposed to leave yet. They haven't finished unloading yet.'"
Moore remembers that the streets were still covered with a sheen of winter ice. "I can remember seeing the ice crack and open and a little bit of liquid push up." Moore didn't know it then, but she was experiencing a quake destined for the record books. Lasting four and a half minutes, this wasn't just a big quake. With a magnitude of 9.2, it was the second largest earthquake ever recorded and it remains the largest earthquake to ever strike North America.
To give the public a sense of the power of the 1964 earthquake, the United States Geological Survey (USGS) turned to a little spaghetti. Pull a single piece of that brittle pasta from the box and hold it in your hand. Snap it in half, and you have just simulated a Magnitude 5 earthquake. To simulate a magnitude 6 quake, you'll need a handful of spaghetti because each magnitude is 30 times more powerful than the one before it...meaning now you'll need to break 30 pieces of spaghetti at once. An M7 quake simulation requires superhuman strength because you will have to break 900 pieces! If you do the math, at M9, you'll need to break a whopping 810,000 pieces of that brittle pasta!!! And the 1964 earthquake was bigger yet.
The epicenter for what would become known as the Good Friday Earthquake lay only 40 miles west of Valdez. When it began, it was like a massive underground zipper broke free. The quake spread southwest, creating a rupture zone 124 to 155 miles wide from Valdez to the south end of Kodiak Island. When it was over, approximately 130 people had died and much of southeast Alaska was damaged. Interestingly, most of the victims weren't crushed by crumbling buildings. They drowned.
Today, geologists have a good understanding of what happened. But in 1964, it was a different story as the mechanisms that created the quake hadn't been identified. Today, we know that Alaska's earthquakes are largely driven by the interaction of tectonic plates. Specifically, the Pacific plate is being forced underneath Alaska and the North American plate at the Aleutian Megathrust, which arcs underwater from the Gulf of Alaska across the foot of the Aleutians to Kamchatka.
The Pacific plate, being more dense and heavier than Alaska's North American plate, dives beneath Alaska at a shallow 7 degree angle at a rate of about 2 inches a year. But it's not a smooth continual movement as the two plates rub against each other. In localized regions, they lock, or stick together, causing the land above to compress and build enormous pressures. "It's entirely acting like a spring," says USGS Research Geologist Peter Haeussler.
Footage from the 1964 earthquake courtesy KTUU
As the Pacific plate adds compression, it drags the Alaskan shoreline downward AND it pushes the shoreline inland. While the shoreline dips, further inland the land actually rises as a result of this same compression. When the "spring" energy is enough to overcome the friction of the locked plates, an earthquake results and the land moves.
In the Good Friday Earthquake, areas along Alaska's shore sprang seaward and lifted to create new tidal flats. Further inland, the land that had been forced to rise by the compression, dropped —allowing seawater to flood areas that had been safely above sea level. Out in the Gulf of Alaska, the leading edge of the North American plate shot forward causing large areas of the seafloor to rise rapidly. The sudden rise displaced massive volumes of seawater, creating a tsunami that struck coastal communities in Alaska, Canada, the Pacific Northwest, Hawaii and elsewhere. But in some places, this tectonic driven tsunami wasn't the first to strike nor was it the most deadly. One town, of many in coastal Alaska, knows this story all too well.
The old town of Valdez sits in a fjord at the far northern end of Prince William Sound, 75 miles from the open waters of the Alaska Gulf and surrounded by some of the most spectacular scenery Alaska can offer. Steep mountainsides plunge to the water's edge and descend hundreds of feet down into the cold, glacier fed waters. Valdez sat on the flattest terrain in the area, created by sediments from mountain erosion and glacial deposits that settled at the water's edge at the eastern end of the fjord.
The shaking that Dorothy Moore felt was the first aspect of the quake to make itself known. But according to State Seismologist Michael West, "the real story in Valdez, in the earthquake, is the tsunami."
"Valdez was hit almost immediately after the earthquake" by a tsunami, says West. "But it wasn't actually hit from the massive movement of the land way out at sea. It was caused by landslides immediately adjacent to Valdez. Underwater landslides."
"One of the things we've unappreciated in Alaska, and we haven't actually understood until recently, is that our tsunamis, the deadly portion of some of our tsunamis, is actually caused by these landslides. I think it's safe to say," says West, "that they've only been fully appreciated in the last decade or two. They were not understood at the time of the 1964 earthquake."
On the day of the quake, the steamship Chena, a 400' long vessel, was tied to the docks in Valdez unloading supplies. Onboard, a crewman was shooting an 8mm movie of the dock below where school kids with their mothers and dogs mingled with dockworkers. When the quake struck, the first shocks caused soft, underwater sediments along the steep shoreline to slip downward in underwater landslides. The shocks also caused the dock to separate from the shoreline, trapping the workers, kids, moms and dogs. The Chena was quickly rocked by a local tsunami, a wave created by a nearby submarine landslide. This wave, before it rolled inshore, lifted the Chena and dropped it on the dock where moments before people had been trying to escape. The Chena was momentarily stranded on the dock until a second wave refloated it. Seconds later, as the Chena floated in the debris along the shore, a chasm opened up beneath it.
"If you look at that video," says Haeussler referring to the crewman's 8mm film, "there's this place in the video where there's water pouring over the edge of something. And what that is, it's water [flowing back off the shore] pouring over the head scarp of the underwater landslide. Sorta like a miniature Niagara Falls."
"This underwater landslide was occurring right beneath the Chena. As the land went down, so the Chena went down. And so the water flows back in to fill the hole."
At the other end of the fjord, at the mouth of Shoup Bay, ancient blocks of submerged glacial moraine from the Shoup Glacier broke free. Massive 80-meter blocks of moraine tumbled into the bottom of the fjord. The slide at Shoup Bay was much smaller in volume "but these large blocks are far more efficient tsunami generators than the loose sediment near the town of Valdez," says Haeussler. "They were just really good at making tsunamis."
"The highest wave run-up anywhere in the '64 earthquake was just a little east of the Shoup Bay moraine," says Haeussler. "And I think 170 feet is how high it went up due to these blocks falling down." According to Haeussler, it would have taken the Shoup Bay waves about five to six minutes to hit Valdez.
By contrast, the tsunami generated out in the Gulf of Alaska needed about six hours to strike. "The way to think about it is not as a wave," says Haeussler. "It's probably more like a really fast tide change that goes much higher than normal. It would have been strong currents, but it wouldn't have been any sort of cresting wave." Coming at high tide, large areas of Valdez were inundated by the tectonic tsunami. Buildings that survived the shaking found themselves flooded.
Following the earthquake, it was decided to move the town of Valdez three miles to the west where it rests today atop a solid outcrop of granite. The town could still fall victim to another earthquake but it is far more likely to suffer another tsunami, one that isn't generated far out to sea. The real tsunami threat comes from a nearby landslide.
"There's a big difference between an earthquake and knowing that you've got a couple of hours to evacuate people and put in place a plan versus having an earthquake and waves that hit the shore one minute later," says State Seismologist Michael West. "Those are some you prepare for in very different ways. But if you feel shaking for 30 seconds, you'd better head for higher ground. The details you can figure out later."
To mark the 50th anniversary of the Great Alaska Earthquake, KTUU has been working on an hour-long special program called "Unstable Ground," looking back at the quake and how it impacted the state. It airs March 27 from 7 to 8 p.m. on Channel 2.