One Quick Question: What's with all the earthquakes lately?
Western Washington has been hit with a series of "small" earthquakes in recent months (anything that jolts you out of a sound sleep can't be TOO small, can it?), leading to a plethora of media coverage on "what-if" scenarios surrounding our next big seismic event. We reached out to Geology's Emily Roland, one of two WWU scientists (along with Colin Amos) working on the NSF-funded Cascadia Region Earthquake Science Center, with a simple question: What's with all the earthquakes?
ER: "The earthquake some of us felt last week (my family included!) was a great reminder that we live on the edge of a very active, if sometimes quiet, plate boundary that’s capable of generating small or large earthquakes at any moment. The small earthquake in the morning of March 3 was a Magnitude 4.5 event that occurred at 17 km (~10 miles) depth, within the crust of North America, near the boundary between the North America and Juan de Fuca tectonic plates. But it did not occur right on the boundary fault - more likely on on a small thrust fault in the crust above the plate boundary.
We call these “crustal” earthquakes, and although they happen often and usually are not very big, in the PNW, we think they can grow to be as large as magnitude 7. The Magnitude 4.8 north of Vancouver, B.C. on Feb. 21 was also likely a crustal earthquake. Crustal earthquakes can be especially dangerous because they tend to be shallow in depth. Shaking associated with the 5 a.m. March 5 event may have felt in some places sharp or abrupt, and some people might even have reported hearing the earthquake like a freight train arrival - this is typical for shallow earthquakes, which tend to generate intense ground shaking.
When we hear about “The Big One” that our region could experience soon, it’s usually in anticipation of an earthquake that does not occur within the crust, but along the plate boundary itself. The plate boundary between the Juan de Fuca and North American Plates dips under the San Juan Islands and Bellingham, Seattle, Portland and Vancouver. It’s deeper here - more like 50 km (~30 miles) than the event we experienced this morning, but extends to the surface near the outer coast of Oregon, Washington and BC, and surfaces at the seafloor offshore. A large earthquake along the plate boundary, sometimes called a “megathrust” fault, could be as large as a magnitude 9, which releases more than a million times the energy of the event we felt near Orcas last week.
Interestingly, the Nisqually Earthquake from 24 years ago represents a third type of earthquake, also associated with the active plate boundary, but like last night’s event, not right on the boundary. The Nisqually earthquake (a magnitude 6.8, 57 km (35 mi) deep, in 2001), occurred deep below the plate boundary, within the subducting Juan de Fuca plate. This type of earthquake is probably actually the most common in the PNW in terms of how often they occur, but we often don’t feel them, because they are so deep and sometimes small in magnitude. I was in high school in Vancouver, Washington when the Nisqually earthquake occurred, and remember feeling it as long, slow rolling -- which is common for earthquake ground shaking that comes from a very deep event.
We now have an earthquake early warning system active in the Pacific Northwest, which produced an alert a few seconds after the earthquake started last night, but before the ground shaking reached Bellingham! If you’re interested in receiving these kinds of alerts and learning more about the system - you can check it out here: https://www.shakealert.org/
It’s run by the US Geological Survey, in collaboration with several universities and research groups. Thanks Federal Workers!"
Find out more about Roland and Amos' work with the CRESCENT lab here.