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.”

The terms slow-slip events (SSE), episodic tremor & slip (ETS), and tremors are often used interchangeably.

Image: USGS

Relationships between slips and “Big Ones”

Slow slips can’t be felt by people, but they slowly, gently change the land in measurable ways seen through GPS monitoring. The change is small—just a few centimeters—but that movement is thought to transfer stress to the “stuck” part of the fault shown in red on the lower right image above. Check out this research by Oregon State University, Stanford, NIWA, Penn State, and Université Côte d’Azur.

“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

“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

“Both laboratory experiments and dynamic simulations suggest that earthquakes can be preceded by a precursory phase of slow slip.” 10

Subduction Zone Earthquakes Preceded by Slip

Fortunately, devastating subduction zone earthquakes don’t happen often (on human timescales), but that infrequency makes them difficult to study. However, there have been a few which have occurred since the discovery of these slow slip events. Check out this research from Tohoku University, the National Institute of Health, and the European Global Ocean Observing System.

“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

“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

“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

The Possible Implication of Slow Slip Timing

Before reading this section, it’s important to note that slow slip events were only discovered around the early 1990s. From a geologic standpoint, that’s an incredibly short amount of time. That said, some research suggests a change in how often slow slips occur may indicate that a fault is approaching failure. Check out this research from NASA’s Jet Propulsion Lab and Berkeley

“SSE is often recurrent, that is, repeats itself in space and time, and recurrent SSEs are mostly regular and predictable.” 16

“We find right before the megathrust earthquake (tens of years in our simulation case) the SSE recurrence intervals will rapidly decrease, accompanied by a rapid decrease of SSE slip (rate) if the magnitude of the megathrust earthquake is much larger than that of the SSEs, consistent with the observations.” 16

“We find that due to the interaction between the SSE and megathrust earthquake regions, SSE become more frequent before megathrust earthquake, in line with observations. Our results suggest such distinctive SSE pattern change might serve as a potential precursor for future large earthquakes.” 16

“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

Slow Slip Changes in Subduction Zone Quakes

With the short amount of time these slow slip events have been monitored, modeling is a primary tool used by scientists to learn about how they may trigger a major earthquake. There have, however, been real-world examples to support the modeling. A couple of examples aren’t proof of correlation, and there may be several other factors not yet understood. That said, check out this research from the Geospatial Information Authority of Japan, NASA, and the National Research Institute for Earth Science and Disaster Resilience (NIED).

“Capturing the intrinsic pattern change would require an extensive observation period, at least over three or more SSE cycles. However, if a sequence of SSE is observed with increasingly shortening of recurrence intervals (drop-off), then it will be a clear indication that the fault is possibly close to a megathrust earthquake, which seems to be the case of Boso SSEs before the 2011 Tohoku-Oki earthquake.” 16

“The recurrence intervals of reproduced short-term SSEs decrease during a long-term SSE, as observed in western Shikoku, in the Nankai region. The recurrence intervals of both types of SSEs become shorter in the later stages of interseismic periods… The recurrence intervals of long- and short-term SSEs shorten before a large earthquake compared to those after a large earthquake, similar to the 2-D model results.” 17

“The SSE significantly reduced its recurrence interval intrinsically prior to the [2011 Tohoku-Oki] earthquake… The change of the SSE recurrence intervals and slip was also observed in the Guerrero segment in the Mexican subduction zone before the 2014 Papanoa earthquake.” 16

“The recurrence interval has decreased from approximately 6.4 to 2.2 years from 1996 to 2014. One explanation of this shortening is the change in Coulomb failure stress due to the 2011 Mw9.0 Tohoku earthquake and its afterslip. Another interpretation is related to a scenario observed in several numerical simulation studies, in which the recurrence interval of slow slip becomes shorter as the time nears a large earthquake.” 15

Recent Cascadia Slow Slip Behavior Change

As with other subduction zones around the world, Cascadia Slow Slip Events (SSE)/Episodic Tremor & Slip (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.

Check out this research from Miami University, UC Berkeley, the Pacific Northwest Seismic Network (PNSN), and NASA.

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, PNSN Director Harold Tobin said, “Every 13, 14 months for 20 years, we’ve been seeing these regular, almost like clockwork events.”

Professor Tobin explained that recent ETS events had behaved a bit unusual with respect to timing and intensity.

It’s been over 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.”

As a follow-up to the research listed above from the Jet Propulsion Lab, 2021 research out of NASA states, “Slow earthquakes along the Cascadia subduction zone display a diverse behavior at different spatiotemporal scales and an intriguing increase of events frequency with depth. However, what causes such variability, especially the depth-dependent behavior is not well understood.” 18

Jan 12, 2023 – Recent ETS sequences have NOT followed a similar pattern that was common from around 2006 until 2017.  For many cycles the area involved was similar and they occurred at about 14-month intervals.  In the past two cases, the overall area was broken up into at least two segments that were active at different times—the most recent almost 5 months apart.”

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. Takeaway: there’s no way to know if this change means anything with regard to risk. Still, a measurable geologic change is interesting to ponder.

A Different Slow Slip Behavior Change to Note

Below are two comparison charts showing slip occurrences in the northern region (green), the central region (blue), and the southern region (purple). The chart on the left shows 2010-2016, while the chart on the right shows 2017 to the start of 2023. There appears to be a slip in process in the southern region as of 01/25/2023 and the activity is listed.

As you can see in these comparison images, tremor episodes since 2018 in the northern and southern regions have often overlapped in time and appear to be more or less synced. Prior to 2018, this wasn’t the case.

Northern dates match up with those listed on the PNSN Tremor Logs. There aren’t any compatible logs for the other regions, but as you will see farther down the page, there are certain charts that can be used to verify activity. Based on those charts, tremor migration patterns, and the number of days of activity, these are the best estimate I can provide. Images of the activity in each region, dating back to 2010 (as far back as I can view on the PNSN Tremor Map) are shown below.

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

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

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

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-ish) of these slips can be viewed at:

Could Smaller Earthquakes Relieve Stress?

This section can also be found on Surviving Cascadia’s Offshore Earthquakes page. Because earthquakes that occur during a slow slip can measure around a magnitude 2.0, the information is relevant here, as well. Many wonder if small earthquakes could relieve built-up stress along a fault. According to the Government of Canada, smaller earthquakes are unlikely to lower the risk level. Here is a print screen from their earthquake Q&A page.

Click the links below to jump to a new page!

Megathrust EarthquakesTsunamisHow Big Will it Be?
How Big is The Fault?CSZ Frequency10,000 Years
When Will it Happen?37% in 50 Years17% in 50 Years
Summer EarthquakesThe True RiskWorst Case Scenario
Expected Regional ImpactsWhy PrepareDrop, Cover, Hold?
Water PreparednessFood PreparednessSanitation… & Whales
Life Without ElectricitySlow-Slip Events (ETS)Offshore Earthquakes
ResourcesCERT SalemWest Salem CERT
Research PapersBook RecommendationsContact




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6) Oregon State University

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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