Understanding the 7.0 earthquake, aftershocks, and how it compares to 1964
Aftershocks have become a part of daily life in Southcentral since the magnitude 7.0 earthquake. Each one brings us back to the moment we felt the world rumble under our feet, stirring up feelings of fear or prompting a desperate search for a safe place to ride out the next wave of energy emitted from the constantly moving ground underneath Alaska.
While these tremors are normal, dealing with them is not. U.S. Geological Survey research geologist Rob Witter is working to get a better understanding of exactly what happened on Nov. 30 and how residents can have peace of mind moving forward.
After a big earthquake, the ground is left unsettled. From the tectonic plates to the soil beneath our feet, it all went through an incredible change after the Earth ruptured during the magnitude 7.0. After any big shift, things need to settle and that’s what aftershocks are — settling. Each shake felt here on the surface is the ground, deep beneath our feet, settling back in to an equilibrium.
“These aftershocks are going to continue for a week, where we could perceive them. And then, on into months, but they’ll become much rarer. And the further away we get from the event the less frequently we’ll experience these aftershocks. But it would not be unusual if we had another, larger, like magnitude 5 aftershock in the coming weeks,” Witter said.
Even the seemingly larger aftershocks are just that, aftershocks. A bigger rumble doesn’t necessarily have an ominous meaning and more quakes months down the road doesn’t mean we are starting all over again. In the grand scheme of things, these tectonic plates are moving slow — about 2 to 2.5 inches per year — and they’ve been around since Earth took shape. So a few months, even a year, of shaking is a blip in the timeline of these massive plates.
Understanding the November 30th Quake
Not all earthquakes are created equally. There are a host of factors that go into the makeup of each one, from the size and shape to the movement of the plates and how they interact with each other. The earthquake Alaska experienced on that Friday morning was along the subduction zone where the Pacific Plate is slowly sliding under the North America Plate at a rate of about 2 inches per year, which is the reason Alaska is considered the most seismically active region in the United States. Normally, these quakes are small enough that we only notice a fraction of the thousands of earthquakes under us each year. But on November 30th, as the Pacific Plate was forced deeper into the Earth, the weight of the dense tectonic plate caused it to tear under its own weight.
"It happened inside that down-going ocean plate and it happened because it’s so dense and heavy it’s getting pulled apart deep within the earth. And this is about 30 miles below our feet," Witter said. "We liken it to the fractures and the type of deformation you might see if you took a Snickers bar and bent it because the ocean plate is diving beneath Alaska, it bends and deforms and cracks. The earthquake that happened on Friday is what we call a normal fault… it’s caused by the bending and cracking of the subducting ocean plate.”
The bending and cracking he described was the shaking felt across Southcentral. While the tear itself lasted just 11 seconds, the shaking continued for about an additional 30 seconds depending on where you were. As the ground shook deep beneath us, it sent the surface into a fit of commotion. Places with softer soil beneath them were able to move more and therefore shook longer and sustained relatively more damage.
Swamps or bogs around town sustained the most damage, while parts of town with a harder topsoil didn’t notice the effects of the quake as much.
Location location location
Just like the old real estate adage, earthquake impacts are all about location.
Distance from the epicenter and depth of the hypocenter play as much of a role as the magnitude itself.
Think of the magnitude 7.1 quake that struck near the Iliamna volcano back in January of 2016. The magnitude of that earthquake was larger, the impacts of it in Anchorage were almost negligible, while parts of the Kenai Peninsula took a larger hit. The reason for the variance in damage was because it was more than 160 miles from Anchorage and almost 77 miles deep. The 2018 Anchorage earthquake was closer in more ways than one, just 9 miles north of Anchorage and only 27 miles deep.
This close proximity was also the reason it came on so quickly. The energy of the shock didn’t have to travel very far to get to us. There wasn’t much of a buildup nor the freight train sound often described before feeling an earthquake. The close proximity of this tear in the Pacific plate is the reason many have compared it to the devastating magnitude 9.2 earthquake that struck Alaska in 1964.
1964 vs. 2018
Though both the 1964 and 2018 earthquakes shook Anchorage, they were very different. For starters, a magnitude 9.2 earthquake is about 160 times bigger than a magnitude 7.0 earthquake and releases almost 2,000 times the amount of energy.
“The area that ruptured in 1964, causing that great earthquake was hundreds of miles long and 200 miles wide. Versus the size of the earthquake that happened in the ocean slab, was only about 12 to 20 miles in its area, so it’s a much smaller event [in 2018] and the amount of energy that it released was much smaller as well,” Witter said.
The Richter scale is logarithmic, meaning as the magnitude approaches 10, each increment higher on the scale releases exponentially more energy than the last. When you start breaking down what exactly happened below the ground, it paints a picture of exactly how different these two quakes really were.
The 2018 earthquake was the Pacific plate tearing under its own weight and only affected a rather small area. The tear itself measured about 20 miles across. Being so small, the actual earthquake or tearing of the plate only lasted 11 seconds.
The 1964 earthquake was caused by the Pacific plate surging north underneath the Alaska plate, more like pressing your two hands together and pushing up on one until it lurches forward; an interaction of two plates instead of the tearing of just one.
It was also bigger, a lot bigger. The 1964 quake stretched hundreds of miles, from the eastern side of Prince William Sound to the southern edge of Kodiak — almost 400 miles. But it wasn’t just the area that was bigger. The quake itself lasted for more than four minutes.
The 1964 earthquake changed Alaska, but it’s also the reason the magnitude 7.0 earthquake wasn’t as destructive as it could have been.
Building for the Future
Earthquakes similar in magnitude to the one on November 30th have caused mass devastation around the world. From the 1989 Oakland earthquake to the temblor that leveled Haiti in 2010, it's apparent that the destruction of a magnitude 7.0 earthquake can be devastating.
"Because of the 1964 earthquake, Anchorage has adopted a very progressive building code. I think we’re seeing the benefits of that smart building philosophy because we use the international building code in the municipality. I think that’s why, relatively speaking, there was very little damage to most homes and commercial facilities," Witter said. "Generally speaking, a magnitude 7.0 earthquake like this in any other urban environment could cause substantially greater damage than what we experienced here in the Anchorage Mat-Su area.”
Building for the future is what helped Alaskans through the 2018 Anchorage earthquake. As we move forward, Witter says we need to remember that we live in the most seismically active region of the United States. Preparation saved many lives and got us through that Friday morning and we need to have plans in place, and know what to do, when another aftershock hits or on the outside chance of another earthquake.
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