1. Episodic Tremor and Slip (ETS)
By: Alex Bastyr & Maggie Whelan

2. Outline What is an ETS event? How do you identify an ETS?
What are tremors and what causes them?
The Cascadia Subduction Zone
How can we relate d ETS to earthquakes and megathrusts?
Why do we research ETS?

3. What is an ETS event? Episodic tremor and slip (ETS)
It is a seismological phenomenon observed in some subduction zones
Identified in Canada, Mexico, Japan and the USA
Characterized by non-earthquake seismic tremors and slow slip events
Persists over a period of days to months
Shows apparent cyclicity
What is an ETS?
ETS stands for Episodic Tremor and Slip as previously mentioned
This seismological phenomenon is observed in some subduction zones including zones off of Canada, Mexico, Japan and the USA
The events are characterized by non-earthquake seismic tremors and slow slip faulting
They occur on a time span of days to months and have an apparent cyclicity (as the name would suggest)

4. Identifying an ETS ETS = Slow Earthquake + Tremors Slow earthquakes
Imperceptible to humans
Slower form of strain release, relative to earthquakes
Faulting happens over a time scale spanning weeks to months
Trends in these events are apparent reversals of crustal motion, although the fault motion remains consistent with the direction of subduction
In subduction zones, occur below the active seismogenic zone
How do we identify an ETS?
An ETS event as previously noted consist of a Slow Slip Earthquake + non-earthquake seismic tremors
What are slow earthquakes?
For one, they are imperceptible to humans (so we can only tell via seismic data if one is occuring)
Relative to earthquakes they are a slower form of strain release, with faulting happening over a time scale of weeks to months even
Trends in slow earthquakes are apparent reversals of crustal motion
Slow earthquakes can occur on any fault surface, but in those related to ETS, occur below the active seismogenic zone (expand on why this is important later)

5. Slow vs. Regular Earthquake
Just to differentiate between slow and regular earthquake using this plot
We can see from the top seismic array that earthquakes operate on the scale of seconds to minutes and are typically accompanied by the rapid release of strain over large areas of the plate interface
earthquakes generate large, sharp, shock waves that subside very quickly -- which contrast greatly from the prolonged, low frequency waves from a slow earthquake
The vertical component is not to scale

6. Identifying an ETS
Slow earthquake with unique non-earthquake seismic signatures associated with tremors are classified as ETS
Tremors occur at a frequency content between 1 to 5 Hz
Signal consists of pulses of energy
Tremor can be gradual or jumping
So we have a slow earthquake and
If a slip event displays tremor signals it would be classified as ETS but not all slip events have to have tremors (if that makes sense)
The other indicator of ETS is the noticeable absence of tremors when no slip events are occuring
Seismic signals of these associated tremors occur at a frequency content between 1 to 5 Hz
This is much smaller than that of small earthquakes --> which have frequencies >10 Hz (so we can eliminate that hypothesis)
The signal we receive at the surface can be presented as pulses of energy that can last a few minutes to several days but we can narrow down ETS tremors to generally days
The tremor activity will migrate with deep slip events in a gradual or jumping fashion along the strike of the subduction zone at speeds from 5 to 15km per day and can be found up to 300km from source (so this is perpendicular to the subducting motion)

7. Tremors
Appear on seismic records as prolonged, intermittent ground vibrations, similar to those caused by windstorms  
Appear strongest on horizontal seismographs and propagate at shear wave velocities
Must look at more than one signal in order to identify the signal as an ETS event
On a seismic record these non-earthquake seismic tremors appear as prolonged, intermittent ground vibrations → similar to those caused by windstorms
Appear strongest on horizontal seismographs and propagate at shear wave velocities
To identify a tremor one needs to look at multiple seismometers from other locations in order to tease out the signal - looking at one it could be mistaken as just noise
Here we have 9 separate seismic signals corresponding to 9 geographically separated stations
Looking at one signal you can see that it would be easy to confuse with noise from say traffic or a windstrom
But looking at 9 a pronounced pattern is observed

8. Causes of Tremors Emergent nature makes pinpointing origin hard
Hypocenters estimated to be at depths of 35 to 40km
Likely the result of the fluid movement in the subduction zone
At high temperatures and pressure aqueous fluid mixed with silicate melts exists as a supercritical fluid (this may reduce the friction and change the fracture criterion of the rock)
Tremors don’t occur within all slow earthquakes, so what causes them?
Hard to precisely locate the source  due to their emergent nature (lack of distinct P and S waves)
But their hypocenters (where they originate) were estimated to be near or just above the subduction interface (30 to 40 km deep)
Cause of tremor is unclear but most likely the result of fluid movement in the subduction zone
At the high temperatures and pressures associated with the inferred depth of the hypocenter aqueous fluid mixed with silicate melts exists as a supercritical fluid that could change the fracture criterion of the rock – causing a tremor

9. Origin of Fluids Abundance of fluids from subducting plate
At depths of 35-40km, the metamorphic dehydration from basalt to eclogite occurs
The released pressurized fluids travel along the subducting slab and lubricate the plate interface
How the fluid enables slow slip and tremors remains unclear
There are two possible sources for fluids in the subduction zone.
Would be seawater being subducted along with the subducting oceanic plate
2. Comes with a bit of an explanation - The events were talking about are happening at a depth of 35-40km, which happens to correspond with the depth (and subsequent pressure & temperature conditions) at which metamorphic dehydration from basalt to ecologite occurs
This reaction is when Serpentinite minerals dehydrate at temperatures in excess of 400 degrees celsius.  The released fluids travel along the subducting slab and lubricate the plate interface
This is helpful but how exactly the movement of these fluids enables the generation slow slip and tremors remains unclear

10. Cascadia Subduction Zone
CSZ runs from Northern California to Northern Vancouver island
Long term elastic deformation occurs along the Northern Cascadia Margin due to the locking of converging plate (seismic gap)
Zone of potential magnitude 8 or 9 megathrust or great earthquake
Megathrust earthquakes can involve 10 to 20m of fault movement
Now were going to focus in on the Cascadia Subduction Zone
Located off the Northwest Coast of North America, running from northern California all along the west coast of NA to Northern Vancouver Island
Long term elastic deformation occurs along the Northern Cascadia margin due to the locking of converging plate.  
The Juan de Facu plate subducts underneath the North American plate. The locking of plates has resulted in a noticeable seismic gap
Seismic gap means that this zone has the potential of generating a megathrust or great earthquake of magnitude 8 to 9 on the richter scale
A megathrust earthquake of this scale could involve 10 to 20 m of fault movement + generate a tsunami

11. Map illustrating patterns in episodic tremor and slip (ETS) along the entire Cascadia subduction zone
Dashed line = marks 40 km depth contour of the subduction interface  (estimated source area for tremors)
Locations of continuous global positioning system stations (squares) and broadband seismometers (triangles) that exhibit ETS
Different colours for different recurrence intervals
Recurrence intervals establish three zones: Klamath Zone, Siletzla Zone and the Wrangellia Zone
ETS occurs along the entire subduction zone → localized geology are not a controlling factor of the occurrence of ETS (supported by observations in other subduction zones)

12. Cascadia Subduction Zone
Monitoring of some of the GPS sites along the fault has revealed transient motion opposite to the long term linear trend  
At the Victoria GPS site:
Motion characterized by a sloped saw tooth function
Eastward movements for 13 to 16 months a a periodic motion reversal lasting 1-3 weeks
Some GPS sites along the fault have revealed transient motion opposite to the long term linear trend
At the victoria GPS site the motion is characterized by a sloped saw tooth function
Incredibly regular cycle drifting eastwards and slipping
So blue circles show day to day changes in the east component of the GPS site in millimeters - calculated this with respect to a GPS site assumed to be fixed on the North American Plate
Red line shows the mean eastward motion trend between slip events - lasting on the order of 13 to 16 months
There is a periodic motion reversal (the dips on the graph) lasting 1-3 weeks -- these are the ETS events

13. ETS and Earthquakes
ETS events can be used as a real- time indicator for stress loading
Tremors sometime seem to be triggered by relatively large earthquakes nearby.  While some tremor activity finishes right after a nearby earthquakes.
It has been proposed that ETS events can be used as real-time indicators for stress loading
Tremors sometimes seem to be triggered by relatively large earthquakes nearby.  While some tremor activity finishes right after a nearby earthquake
The arrows indicate major earthquakes greater than magnitude of 4. The spikes are the tremor activity
We can see that tremor activity can be brought upon by an earthquake event such as the Geiyo (M6.7)
And that tremor activity can subside as an earthquake finishes -- Western Aichi (M4.1)

14. ETS and Megathrusts
Each ETS episode adds stress to the locked portion of the subduction zone
Since slow faulting occurs below seismogenic zone – it increase stress
If, slow faulting occurred above seismogenic zone – it would decrease stress
As stress increases and approaches a critical level → an ETS event may provide the additional stress needed to overcome the friction on a fault → triggering a great earthquake and the generation of its subsequent tsunami
ETS episodes are adding stress to the locked portion of the subduction zone because slow faulting related to ETS occurs below the active seismogenic zone.
If, the slow faulting was heppening above the seismogenic zone it would decrease stress.
So while stress increases and approaches a critical level due to subduction alone, we have this additional stress due to slow faulting from ETS below the seismogenic zone being added on top of that MEANING the event may provide the additional stress needed to overcome the friction on a fault → triggering a great earthquake and the generation of its subsequent tsunami  VERY DANGEROUS

15. Importance of ETS WHERE: WHEN:
ETS events define the eastern or landward limit of the zone that will rupture during the next great earthquake
Provides a more accurate estimate of how close the rupture could be to a major west coast city and of the potential shaking experienced
WHEN:
Provide a basis for improved earthquake forecasting
During times of ETS, it  raises the probability for a megathrust earthquake occurrence
The importance of ETS
They can tell us WHERE:
•ETS events define the eastern or landward limit of the zone that will rupture during the next great earthquake
•provides a more accurate estimate of how close the rupture could be to a major west coast city and of the potential shaking experienced
• and WHEN:
•provide a basis for improved earthquake forecasting
•during times of ETS, it  raises the probability for a megathrust earthquake occurrence

16. References
Alevizos, S., Poulet, T., & Veveakis, E. (2014). Thermo-poro-mechanics of chemically active creeping faults. 1: Theory and steady state considerations. Journal Of Geophysical Research: Solid Earth, 119(6),
Audet, P., Bostock, M., Christensen, N., & Peacock, S. (2009). Seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing. Nature, 457(7225),
Brudzinski, M., & Allen, R. (2007). Segmentation in episodic tremor and slip all along Cascadia. Geology, 35(10),
Dragert, H., Wang, K., & Rogers, G. (2004). Geodetic and seismic signatures of episodic tremor and slip in the northern Cascadia subduction zone. Earth, Planets And Space, 56(12),

17. References
Episodic Tremor and Slip. (2011). Seismescanada.rncan.gc.ca. Retrieved 3 April 2018, from pprp/pubs/GF-GI/GEOFACT_ETS_e.pdf
Melbourne, T. (2003). GEOPHYSICS: Enhanced: Slow But Not Quite Silent. Science, 300(5627),
Obara, K. (2002). Nonvolcanic Deep Tremor Associated with Subduction in Southwest Japan. Science, 296(5573),
Rogers, G., & Dragert, H. (2003). Episodic Tremor and Slip on the Cascadia Subduction Zone: The Chatter of Silent Slip. Science, 300(5627),