SSE Activity

Slow-Slip Events (SSE) / Episodic Tremor & Slip (ETS) / Slip / Tremor = interchangeable terms

“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.” Tohoku University & University of California, Berkeley4

“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.” National Institute of Geophysics and Volcanology & University Grenoble Alpes5

“Slow slip directly triggers seismic slip – we can see that” Oregon State University & University of South Florida6

Northern Slow-Slip events (Wrangellia Zone)

“Every slow slip event adds stress to the adjacent locked megathrust zone bringing it closer to failure.” Stanford University7

“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.” EOS: Getting to the Bottom of Slow-Motion Earthquake13

“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.” Berkeley Seismology Laboratory8

Central Slow-Slip Events (Siletzia Zone)

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. PENN State9

“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.” Université Côte d’Azur10

“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.” Science Magazine11

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

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 2/08/2022 ( 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. They write, “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:

One final thought. Sometimes, the north and south locations seem to be synced. Check out the images since mid-2018. )The final image is current as of 5/22/22 and is not considered complete).





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