Lessons Learned: What caused the magnitude 7.1 earthquake in 2018
Right below our feet in Alaska, huge tectonic plates are crashing into each other, making this one of the most seismically active places in the world. This force is strong enough to create volcanoes and some of the tallest mountains in the world while also causing the Earth’s crust and the plates themselves to break, crack and rupture as the rock is constantly being bent, pushed and pulled.
When the Earth ruptured on Nov. 30, 2018, the rupture was deep inside the ocean plate — a very different type of quake compared to what we experienced here in 1964 or what we saw on the Denali Fault in 2002.
There are three different types of earthquakes that can happen in Alaska, but they can all be traced back to a root cause of the Pacific Plate being forced underneath the North American Plate, where Alaska sits.
In 1964 we experienced what happens when there's friction right at the boundary between these two plates.
"They're moving together at a rate of about 2 1/2 inches per year. And where they collide, friction causes them to stick together,” explained U.S. Geological Survey geologist Rob Witter.
When they stick together, it can force what is called a megathrust earthquake. These types of quakes are close to the Earth's surface and can cover a large area. In 1964 the area was more than 500 miles long and 150 miles wide.
The boundary between the plates isn't the only place we see tension.
As the Pacific Plate moves north, Alaska is being compressed from north to south. This forces southern Alaska west, which causes numerous fault systems.
This compression and sheering was the cause of the 2002 magnitude 7.9 Denali Fault earthquake, another shallow and powerful quake that ruptured more than 200 miles of the Earth's surface.
Unlike the shallow quakes of 1964 or 2002, the earthquake that forced the ground to shake across Southcentral in 2018 happened deep inside the Earth.
"The third type of fault we have is a fault in the subducting ocean plate here, that dives below Alaska, and the place where it broke was 34 miles below Anchorage right there — a magnitude 7.1 caused by extension of the subducting ocean slab," Witter said.
Witter explains that, as the Pacific Plate is forced under Alaska, it is forced to bend, which causes it to stretch, eventually causing faults where earthquakes happen.
On Nov. 30, 2018, Witter says, an area roughly 15 miles by 20 miles ruptured within the plate, causing shaking that was felt at the surface.
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