A swarm of thousands of tiny earthquakes deep under western Washington, Oregon and British Columbia started again back in October 2020.
These small quakes are happening 40 to 60 miles deep, where the lower slab of the oceanic Juan de Fuca plate moves toward, and eventually is shoved beneath, the continent (North American plate) along the Cascadia Subduction Zone.
What are silent earthquakes?
These small tremors have been called “silent earthquakes,” which have a deep tremor and slow slip. Collectively, they move enough to slightly change the surface of the earth near them. The tiny epicenters pop as that plate slides. They will last for weeks until the event is over.
While most earthquakes are fairly random in time, the tremor runs on a schedule, occurring about every 14 months. The last slow slip event occurred in September 2019.
These silent quakes release the equivalent energy of a magnitude 6 earthquake.
Scientists didn’t even know these slowly evolving quakes existed 20 years ago.
“Maybe 2002, 2003 – that we started realizing that these tremor events take place deep under western Washington,” said University of Washington seismologist and professor emeritus, Steve Malone.
Malone continues to track these events and look at what they could potentially tell us about the giant mega earthquakes that Washington is expected to experience again.
Silent earthquakes occur at subduction zones
These types of silent quakes are now found around the world, mostly where crustal plates forming ocean floors are pushed under continental crust.
The scientific term for this crustal collision is called subduction. However, if the lower end of the plate moves quietly and unnoticed, the descending ocean plate is not as pliable as it gets closer to the surface. Usually, it is locked, only releasing when enough energy has built up that friction with the continental plate can’t hold it back.
How often this lock breaks in a giant quake depends on where you are. The last time it happened under Washington and much of the west coast was in January of 1700, in what’s called the Cascadia Subduction Zone. The subduction zone runs from Cape Mendocino in northern California, past Oregon, Washington, and off Vancouver Island, B.C.
Subduction earthquakes are violent and can range into the magnitude 9 range, including the great Tohoku earthquake in March of 2011 in northeastern Japan, triggering a tsunami, the failure of a nuclear power plant, and the loss of more than 15,000 lives.
But could silent quakes or deep tremors indicate that a big earthquake is coming, or at least creating a window when it’s more likely?
One theory is that it could add to the pulling force on the locked plate above it, one day triggering a massive quake.
However, there is no clear correlation around the world between deep tremor followed by a massive quake in the subduction zone.
Schmalzle et al., 2014 revealed the locations where the plate is locked (where friction prevents slip between the two plates) along the Cascadia Subduction Zone.
Large earthquakes occur in these locked sections when the lock is abruptly broken. Slow slip events however generally occurs in a region offset from the locked section of the plate interface.
This means that at present, these events are less likely to trigger large earthquakes than if they were located right at the edge of the locked zone.
However, it is possible that over time the episodic tremor and slip events will migrate closer to the locked zone over time. If this were to occur, it may indicate that the next big earthquake is on the horizon.
It is also possible that slow-slip events will become larger or more frequent when a large earthquake is imminent.
For now, the good news is that the main region of episodic tremor and slip in Cascadia is in an area with no locking. But the a megaquake along the Cascadia Subduction Zone is overdue and could erupt any time. So be preprared and have an earthquake plan, because Cascadia is not for fun!
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