In 2018, the Cascadia Subduction Zone saw 36,377 Episodic Thrusts & Slips (ETS). These tremors are different from earthquakes, which are generally more sudden, and that of other, shallower faults, which can be generated from the pressure buildup of magma sitting under Cascadia’s many volcanoes along the Pacific Ring of Fire. Instead, Episodic Thrusts & Slips occur along the Cascadia subduction zone, and are generally slower, longer vibrations that occur as the two massive continental forces clash together.
A big thank you to the Pacific Northwest Seismic Network and John Cassidy (@earthquakeguy) who made this information and map available. For more information, you can see a full history of the regions seismic activity from previous years at: https://pnsn.org/tremor/overview
This map shows the boundary between the Juan de Fuca Plate system and the North American Plate which forms the Cascadia Subduction Zone. For those unfamiliar, Cascadia, almost more than any other sense, is defined through the geologic dynamicsm of this faultline, taking it’s borders from the watersheds that flow from the edge of the Continental Crust, uplifted by the Cascadia Subduction Zone (also known as the Cascadia fault) as the Juan de Fuca plate slips underneath the North American in a convergent plate boundary that stretches from northern Vancouver Island in Canada to Northern California.
This fault is responsible for deep earthquakes known as Cascadia megaquakes and the active volcanism of the Cascade mountains that has shaped our region for tens of thousands of years. A Cascadia Megaquake refers to large, deep thrust earthquakes along the Cascadia Subduction Zone. The last Cascadia Megaquake occurred January 26, 1700 with an estimated moment magnitude of 8.7–9.2 and stretched from mid-Vancouver Island, south along the Pacific Northwest coast as far as northern California stretching for more than 1,000 kilometers (620 miles), with an average slip of 20 meters (66 ft), and resulted in a massive tsunamai felt as far away as Japan.
Cascadia Megaquakes occur every 300-500 years. These earthquakes are known as megathrust or great earthquakes. They can involve 10 to 20 metres of fault movement, along the hundreds of kilometres that make up the fault line. After a great earthquake, the two plates become locked together again and the stick-slip cycle is repeated.
Much of Cascadian planning and co-operation still relies around increasing the resilience of our communities in the face of large earthquakes above a 9.0 magnitude that could impact the largest population centers in the region, such as Vancouver, Victoria, Seattle and Portland, leading to tsunamis or volcanic eruptions. In the past these have included such notable eruptions as Mount Mazama (Crater Lake) about 7,500 years ago, the Mount Meager massif (Bridge River Vent) about 2,350 years ago, and Mount St. Helens in 1980.
Episodic Tremor and Slip (ETS) is a process that occurs deep below the Earth’s surface along faults that form the boundaries of tectonic plates. It involves repeated episodes of slow fault slip of a few centimetres over a period of several weeks, accompanied by seismic tremors. Tremors appear on seismic records as prolonged, intermittent ground vibrations, similar to those caused by windstorms. They differ from earthquakes which generate large, sharp, shock waves that subside very quickly.
Erik Klemetti from Discover Blog talks about other types of earthquakes that can strike the Cascadia bioregion:
With all that being said, not all earthquakes in the Pacific Northwest are the same. The Cascadia Megaquake will be generated by the stress built up as the Juan de Fuca plate off the coast of Oregon, Washington, California and British Columbia slides underneath North America (see above). The plates can stick, creating stress that is sometimes released as giant earthquakes (and many smaller ones). The “snapping” back of the plate can create tsunamis, just as we saw in 2004 off Indonesia and 2011 off Japan.
However, those aren’t the only earthquakes that might be felt in the Pacific Northwest. The volcanoes of Cascadia, formed from the same subduction that is going on with the Juan de Fuca plate, can generate earthquakes as magma moves underneath them. Smaller faults that are far inland and shallower than the boundary between the two plates can also accumulate stress and form earthquakes that tend to be smaller (but potentially as damaging if they were to happen under a big city).