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Episodic tremor and slow slip events
A map of coastal Washington state and British Columbia shows the sweep of an episodic tremor and slow slip event, or ETS, from February to April 2017. The colors denote the time of the event as shown on the color-coded time bar at the bottom. The gray circles on the color bar indicate the number of tremor events per day. (UNAVCO Graphic / Kathleen Hodgkinson)

WASHINGTON, D.C. — Is it the tick of Earth’s heartbeat, or a ticking time bomb? Either way, instruments that monitor a 14-month pattern in seismic activity could serve as an super-early warning system for the “Really Big One,” the massive earthquake that’s expected to hit the Pacific Northwest sometime in the next few centuries.

The seismic ticks are known as episodic tremor and slow slip events, and they’ve been known about for more than a decade.

Such events are linked to the titanic clash between the Juan de Fuca tectonic plate and the North American plate, in a region known as the Cascadia subduction zone. The two plates grind into each other at a rate of an inch or two per year, about 25 miles below the surface.

Usually, it’s a slow grind, but every so often, there’s a sharp spike in the rate of movement. Along the Washington state coast, the spike comes roughly every 14 months. (The most recent spike occurred last May.) In California, the cycle takes 10 months. In Oregon, it’s more like 24 months.

Based on historical and geological records, seismologists have determined that the Cascadia fault can produce catastrophic earthquakes, on the order of magnitude 9.0 or more. In 2015, worries about the potential effects of a big Cascadia quake led to an eye-opening article in The New Yorker about the Really Big One.

Anne Trehu, a geophysicist at Oregon State University, isn’t saying the Really Big One is coming anytime soon. But during a presentation at this week’s annual meeting of the American Association for the Advancement of Science, she said a steadily expanding network of seismometers and strainmeter could give us advance notice.

The seismic detection network in the Pacific Northwest and California allows seismologists to map the pulls exerted by the episodic  in three dimensions, day by day.

“When there’s a little pull, it increases the risk, the stress increases, and the probability for a great earthquake increases,” Trehu said. “But it increases from one very small number to what’s still a very small number.”

Trehu said the key thing to watch for is a quickening in the pattern of episodic tremors.

“Potentially changes in the pattern, changes in that periodicity, could be indicative of something interesting,” she said. “But those are going to take longer monitoring times.”

For a time, it looked as if the West Coast’s earthquake early warning system was in trouble, but Congress kept the effort alive and added $10 million to install more seismic monitoring stations.

Efforts are already underway to extend the seismic monitoring network offshore: Those efforts include the Ocean Observatories Initiative and the Ocean Bottom Seismograph Instrument Center, operated by the Woods Hole Oceanographic Institution.

So there’s hope that when the Really Big One approaches, the West Coast’s network of seismic sensors will let us know it’s coming.

“It won’t lead to predicting to the minute or even to the hour,” Trehu said. “But there are potentially things one could do to improve the forecast.”

Update for 1:20 p.m. PT Feb. 18: Trehu sent along an email clarifying and expanding upon some of the points I laid out in the report above, and I’ve made a couple of tweaks as a result. Here are the bullet items from her email:

1) The Cascadia Initiative was a program to deploy ocean bottom seismometers across the entire Juan de Fuca plate for a limited time period (2011-2015). It is now over, although analysis of the data continues. The project was led by UO with team members from OSU, UW, WHOI and UCSC. For more information see: https://cascadia.uoregon.edu. Data were not available in real time.

2) More relevant to continued monitoring offshore is the Ocean Observing Initiative, a cabled observatory that currently includes 2 broadband and 3 short-period seismometers as well as many other instruments to monitor conditions along the continental margin. These instruments are currently recording data that are available in near real time. See:  https://oceanobservatories.org/cabled-array-seismometer-data/

3) More seismometers and geodetic instruments providing real-time data are needed. Japan has such systems. It is doable but expensive.

4) New insights about previously unobserved phenomena that can precede large subduction zone earthquakes are being obtained from large earthquakes that occur in well-monitored regions around the globe.  Examples include the 2011 earthquake off Japan and the 2014 earthquake off northern Chile.

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