SSE Activity

2010-2021 ETS History in Cascadia

Northern Slow-Slip events (Wrangellia Zone)

Central Slow-Slip Events (Siletzia Zone)

Southern Slow-Slip events (Klamath Zone)

The images and dates from the three regions above come from data on the PNSN’s tremor map as of 5/05/2021 ( 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. I am not qualified to say what is or is not an ETS, so the events listed are best guesses based on migration patterns and duration of each event (minimum of two weeks). I am working to find where the dates for these regions can be confirmed by a professional in the field and will update this when and if I can locate a list.

Recent Tremor Behavior

Slow-Slip Events (SSE), also known as Episodic Tremor and Slips (ETS) normally occur in the Pacific Northwest like clockwork. However, their pattern in our northern region has changed.

Harold Tobin, Director of the PNSN
“Every 14 months for 20 years, we’ve been seeing regular, almost like clockwork events. This one is definitely looking a little bit different than those. That’s interesting. We don’t know if that’s a major change, or if the next one will cycle back to the normal pattern.”1 – Quote regarding the August 2019 slip.

Since that interview, the pattern has not cycled back to normal. (See the slideshow toward the bottom of the page for the Northern Region. The 4.5 month interval preceded the August 2019 event.)

PNSN Professor Emeritus Steve Malone, February 2021
“There is no question but that it seems what used to be a fairly standard 14 month ETS that covered similar areas is breaking into different patches that cover the same general area but scattered out in time.”2 -Quote following the last 2.5 month interval show above on the right.

Why do Slow-Slip events and their timing matter? Many in the science community believe these SSEs are key to better understanding the complex system of subduction zone tectonics.

NASA’s Jet Propulsion Laboratory
“Capturing the intrinsic pattern change would require an extensive observation period at least over several or more SSE cycles. However, if a sequence of SSE is observed with increased shortening of recurrence intervals, then it will be a clear indication that the fault is close to a megathrust earthquake, which seems to be the case of Boso SSEs before the 2011 Tohoku-Oki earthquake (Supplementary Figure S1, Hirose et al. 2012, Kato et al. 2014). Similar SSE shortening has also been observed before some other subduction earthquakes, for instance, the Guerrero SSEs before the 2014 Papanoa earthquake (Radiguet et al. 2016).”3

“We find that due to the interaction of the SSE region and the megathrust earthquake region, SSE become more frequent before megathrust earthquake, in line with observations.”3

Words of Caution from the Pacific Northwest Seismic Network
“It’s important to keep in mind that “ordinary” still isn’t a very well understood term when it comes to tremor. We’ve been studying tremor for less than two decades, so what may seem irregular right now might be normal over the 300+ years since the last CSZ earthquake.”12

“We have been monitoring this sort of activity for less than two decades; a short period over the 300+ years since the last major earthquake. We really don’t know what variations to expect over decade periods of time.”2

Tremor Research

Tohoku University & University of California, Berkeley
“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

National Institute of Geophysics and Volcanology & University Grenoble Alpes
“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

Oregon State University & University of South Florida
“Slow slip directly triggers seismic slip – we can see that”6

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

EOS: Getting to the Bottom of Slow-Motion Earthquakes
“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

Berkeley Seismology Laboratory
“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

PENN State
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

Université Côte d’Azur
“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

Science Magazine
“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

Science Magazine
“At the Cascadia subduction zone off the Pacific Northwest, for example, GPS stations on land suggest that enough strain has accumulated to drive a magnitude-9 earthquake when the fault finally ruptures.”14

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