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Swarm of more than 50 earthquakes off Oregon's Coast an indicator of previous activity

Since Tuesday, more than 50 small-to-moderate earthquakes struck off the Oregon coast.

The swarm of earthquakes in the Blanco Fracture Zone off the Oregon coast is part of a larger active system of faults, underwater volcanoes and actively spreading ridges that raise the risks of a large earthquake along the coast of the Pacific Northwest.

A fault is a crack between two tectonic plates, or pieces of the earth's crust, that move against each other, according to the National Park Service. When pressure builds along a fault and is suddenly released, earthquakes can occur. 

Scientists expect up to a magnitude 9 earthquake along a 600-mile-long fault that starts about 50 miles out from the West Coast. That fault is known as the Cascadia Subduction Zone

Since Tuesday, more than 50 small-to-moderate earthquakes struck off the Oregon coast. The quakes were roughly 200-250 miles off the coast of Newport. More than a dozen were magnitude 5 or higher. 

“These earthquakes don’t create a significant strain … to the subduction zone fault,” said Washington state Seismologist Harold Tobin. Tobin also leads the Pacific Northwest Seismic Network at the University of Washington. “So the slip we see in every one of these earthquakes is the fault catching up to the rest of the plate.”  

In other words, the earthquakes are a sign that the fault has already moved.

The Juan de Fuca Ridge runs along the western edge of a piece of seafloor; it's known as the Juan de Fuca Plate. That seafloor is part of the Earth’s crustal system of moving plates. In this case, the Juan de Fuca plate is stuck to the North American plate from Cape Mendocino, California past Oregon, Washington and part of British Columbia, Canada. Once pressure builds up enough, that plate will continue slipping below the North American plate, resulting in a massive earthquake.

The Juan de Fuca ridge is volcanically active. As more magma comes up from below it pushes the Juan de Fuca plate east several inches a year.  

For now, the smaller quakes are likely to continue. Researchers at the Pacific Northwest Seismic Network say the quakes will most likely be smaller than magnitude 5.8. Just how long they'll last is uncertain.