Sometimes when the earth moves, we need technology to notice it

This morning I learned that a "slow" earthquake expected to last several weeks is now in progress, under the Olympic Peninsula and en route via the Strait of Juan de Fuca to Vancouver Island. It started early Sunday morning in an area north of Olympia and west of Tacoma and is now on the move. I'd previously had no idea that such earthquakes even existed. (Good thing we can't actually feel them, yes?)

According to Vince Stricherz, writing for UWNews.org,

Episodic tremor-and-slip events have been associated with the entire Cascadia subduction fault zone, which runs along the Northwest coast, as well as a dozen other dangerous faultlines worldwide. The fault zone is created by the Juan de Fuca tectonic plate sliding beneath the section of the North American plate where western Washington is located.

It is believed the two plates are locked together by friction near the surface but that they slide past each other easily at greater depth, where heat has made the rock more pliable. Slow-slip events are likely to be occurring at a depth where the plates transition from being locked to being free-moving, Malone said.

"Models indicate these events are loading a little extra stress on the fault zone," he said.

Interest in slow-slip events has been intense in recent years because they alter stresses in the subduction zone, which ruptures in magnitude 9 megathrust earthquakes on the order of every 500 years. The last one occurred in 1700. (The earthquake that caused the devastating 2004 Indian Ocean tsunami had a magnitude of 9.1.)

With better understanding, changing slow-slip patterns might provide hints in advance of the next Cascadia megathrust quake, said John Vidale, a UW professor of Earth and space sciences and director of the seismic network.

"Maddeningly, we have no understanding of why the episodic slow slip lasts a month, rather than the few seconds of a normal earthquake or the continuous motion of flow deeper in the Earth, and we aim to figure it out," Vidale said.

As someone living in the Cascadia subduction zone, I couldn't be more fascinated.