Interchangeable Terms

What are Slow-Slips?

According to the Pacific Northwest Seismic Network (PNSN)’s one-page explanation, “Episodic Tremor and Slip (ETS) is the name given to 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 sliding, one plate over the other, of a few centimeters over a period of several weeks, accompanied by energetic seismic noise, called tremor.”


As with other subduction zones around the world, Cascadia ETS events used to be semi-predictable. Broken into three primary regions—Northern (Wrangellia Zone), Central (Siletzia Zone), and Southern (Klamath Zone)—the timing of ETS events could more or less be predicted.

2007 Miami University and UC Berkeley research lists occurrences every 14 ± 2, 19 ± 4, and 10 ± 2 months, respectively.

In a September 2019 King 5 interview, referring to an event in the northern region, Pacific Northwest Seismic Network Director, Harold Tobin, said, “Every 13, 14 months for 20 years, we’ve been seeing these regular, almost like clockwork events.”

In the video, Professor Tobin goes on to explain that recent ETS events had behaved a bit unusual with respect to timing and intensity.

It’s been nearly three years since that interview. The “abnormal” behavior has continued. Predicting when a region will likely get an ETS is no longer possible. Check out this Quote from the PNSN Tremor Log: ETS Event of Fall 2021.

Aug 18, 2022 – Indeed the last batch of tremor reported above by Prof Creager stopped within a day of his post (Apr 7, 2022) but his observation of the northern Cascadia tremor section (Puget Sound – Vancouver Island) taking on a different pattern over the last five years seems to be continuing.  In early August a nine day burst of tremor took place right in the eastern Straights of Juan de Fuca.  While this burst of tremor didn’t last long enough to be considered a full ETS (10+ days) it did have both a slightly up-dip progression before moving slightly south to north.  It likely could be considered an “inter-ETS” event but what these things are called is starting to get quite arbitrary.  In any case the very regular 14 month cycle between large ETS events in this region seems to have changed over the past several years. Thus trying to anticipate when the next large event will occur is beyond me. However, the total tremor (and assumed slow slip) still fills in the whole zone if averaged over several years.”

Keep in mind, science has only known about the existence of ETS for roughly 30 years—not long enough to understand how ETS behave throughout the life cycle of subduction zone earthquakes. Take away: there’s no way to know if this change means anything with regard to risk. Still, a measurable geologic change is interesting to ponder.

The Link Between ETS and Subduction Zone Earthquakes

There are several research studies on the relationship between ETS and subduction zone earthquakes.

“Slow-slip events in Cascadia occur every one to two years, but geologists wonder if one of them will be the one that will trigger the next megathrust earthquake.”9

“A recent major study from their seismologists correlated every large earthquake to a slow-slip event that was happening at the time — including the 2011 Tohoku earthquake.”4

“Researchers have shown that the magnitude 7.3 quake that struck Papanoa on 18 April 2014 was caused by a slow slip event that had begun two months earlier.”5

“Slow slip directly triggers seismic slip – we can see that”6

“Every slow slip event adds stress to the adjacent locked megathrust zone bringing it closer to failure.”7

“Many people think faults with slow-slip events are in a transitional frictional state,” he said. “They are very close to failing (which would cause a typical earthquake), but something is holding back that failure.”13

“Both laboratory experiments and dynamic simulations suggest that earthquakes can be preceded by a precursory phase of slow slip… We show that during coalescence phases lasting for 2 to 5 days, the rate of energy (moment) release significantly increases. This observation supports the view proposed by theoretical and experimental studies that the coalescence of slow slip fronts is a possible mechanism for initiating earthquakes.”10

“The find reveals how the lethal offshore fault slipped slowly just before it ripped loose. That slow slip now appears to have loaded the fault to the breaking point, triggering the devastating quake last March.”11

“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. It is therefore important to monitor episodic tremor and slip in Cascadia over time.”8

Northern (Wrangellia Zone) Slow-Slip events: 2010-2022

Central (Siletzia Zone) Slow-Slip Events: 2010-2022

Southern (Klamath Zone) Slow-Slip events: 2010-2022

Tremor Behavior

Since 2018, tremor episodes in the northern and southern regions have often overlapped in time.

The images and dates from the three regions above come from data on the PNSN’s tremor map as of 8/24/2022 ( they reflect the PNSN’s recent software update). While the northern dates match what can be verified by the Pacific Northwest Seismic Network’s Tremor Logs, the PNSN does not list verified dates for southern or central region events.

However, UW Emeritus Professor Ken Creager posted the following graph of Cascadia tremor locations since 2017. The events are plotted by date and latitude as of 4/6/2022. The post states:

“Individual tremor epicenters are shown as black dots, and tremors that are clustered in space and time are marked by red circles.  Those with more than 50 hours of tremor show the hours as white numbers.” 

The large red circles with white numbers can be used as an approximate guide to check the dates listed above.

Monitoring (updated daily at 6:00pm) of these slips can be viewed at:




3) Luo, Y., & Liu, Z. (2019). Slow‐slip recurrent pattern changes: Perturbation responding and possible scenarios of precursor toward a megathrust earthquake. Geochemistry, Geophysics, Geosystems, 20, 852–871.

4) Uchida, Naoki & Iinuma, Takeshi & Nadeau, Robert & Burgmann, Roland & Hino, Ryota. (2016). Periodic slow slip triggers megathrust zone earthquakes in northeastern Japan. Science. 351. 488-492. 10.1126/science.aad3108.

5) Radiguet, Mathilde & Perfettini, Hugo & Cotte, Nathalie & Gualandi, Adriano & B, Valette & Kostoglodov, Vladimir & T, Lhomme & A, Walpersdorf & Cabral-Cano, Enrique & M., Campillo. (2016). Triggering of the 2014 Mw7.3 Papanoa earthquake by a slow slip event in Guerrero, Mexico. Nature Geoscience. 9. 10.1038/NGEO2817.

6) Oregon State University

7) Bruhat L., and P. Segall (2016), Coupling on the northern Cascadia subduction zone from geodetic measurements and physics-based models, J. Geophys. Res. Solid Earth, 121, doi:10.1002/2016JB013267.

8) Bartlow, Noel (2020), Faults slip slowly in Cascadia. Temblor,

9) Penn State. “Deep, slow-slip action may direct largest earthquakes and their tsunamis.” ScienceDaily. ScienceDaily, 21 December 2020. <>.

10) Université Côte d’Azur, IRD, CNRS, Observatoire de la Côte d’Azur, Géoazur, 250 rue Albert Einstein, 06560 Valbonne, France. 2 Institut de Physique du Globe de Paris, Université de Paris, CNRS, 75238 Paris, France. ✉email:
NATURE COMMUNICATIONS | (2020)11:2159 | |

11) Kerr, Richard. (2012). A Tantalizing View of What Set Off Japan’s Killer Quake. Science (New York, N.Y.). 335. 272. 10.1126/science.335.6066.272.

12) Pacific Northwest Seismic Network Facebook Page

13) Chakravorty, A. (2020), Getting to the bottom of slow-motion earthquakes , Eos, 101, Published on 24 April 2020.

14) doi:10.1126/science.aba2203