Episodic tremor and slip (ETS)
Encyclopedia
Episodic tremor and slip (ETS) is a phenomenon observed in seismology
describing a particular type of tremor pattern observed in regions of convergent
plate
boundaries. These are characterised by non-earthquake
-like tremors, accompanied by aseismic slip in the same region of the local megathrust. For some areas around the ETS, there is an apparent slipping back or reversal of direction of the normal tectonic plate movement, although the fault motion remains consistent with subduction
.
ETS events are imperceptible to human beings and do not cause damage.
The term ETS was coined by the Geological Survey of Canada around 2003 to describe observations of GPS measurements in the Vancouver Island
area.
In the Cascadia subduction zone
, where the subducting Juan de Fuca plate
is underthrusting to the east relative to the North American plate
, a GPS point on the surface of the upper plate above the 'locked' plate boundary will slowly move east as it is dragged eastward by the subduction process. But during an ETS event, that same GPS point slips to the west. The "tremor" and "slip" along the fault at depth coincide in time with a movement at the surface to the west. This process happens repeatedly, hence "episodic tremor and slip."
ETS events in Cascadia were observed to be periodic, with an interval of 14 months, and analysis of measurements led to the successful prediction ETS events in 2003, 2004, 2005, and 2007. These events are marked by about two weeks of 1–10Hz trembling that are only detectable by sensitive seismometers, accompanied by aseismic slip on the megathrust that is equivalent to an M7 earthquake. The tremor and slip occurs downdip from the locked portion of the Cascadia megathrust that broke in the M9 1700 Cascadia earthquake, and which is expected to re-break in the future. The ETS phenomenon, as currently interpreted, suggests increasing stress on the locked portion of the megathrust fault, making a large earthquake on the megathrust more likely.
The first kind of ETS tremors are similar to those observed in the forearc region of southern Japan
, and have also been spotted in Alaska, Costa Rica, and Mexico.
The second triggered variety has now been seen under Vancouver Island, under Japan, on the San Andreas in California, and under Taiwan.
Week- to year-long episodes of slow slip not accompanied by tremor has been observed in New Zealand. One theory holds that ETS tremor is more common is the process of subduction
of younger oceanic crust
, which may be hotter and wetter, rather than older oceanic crust.
Seismology
Seismology is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other planet-like bodies. The field also includes studies of earthquake effects, such as tsunamis as well as diverse seismic sources such as volcanic, tectonic, oceanic,...
describing a particular type of tremor pattern observed in regions of convergent
Convergent boundary
In plate tectonics, a convergent boundary, also known as a destructive plate boundary , is an actively deforming region where two tectonic plates or fragments of lithosphere move toward one another and collide...
plate
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
boundaries. These are characterised by non-earthquake
Earthquake
An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...
-like tremors, accompanied by aseismic slip in the same region of the local megathrust. For some areas around the ETS, there is an apparent slipping back or reversal of direction of the normal tectonic plate movement, although the fault motion remains consistent with subduction
Subduction
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...
.
ETS events are imperceptible to human beings and do not cause damage.
Discovery
Vancouver Island
Vancouver Island is a large island in British Columbia, Canada. It is one of several North American locations named after George Vancouver, the British Royal Navy officer who explored the Pacific Northwest coast of North America between 1791 and 1794...
area.
In the Cascadia subduction zone
Cascadia subduction zone
The Cascadia subduction zone is a subduction zone, a type of convergent plate boundary that stretches from northern Vancouver Island to northern California. It is a very long sloping fault that separates the Juan de Fuca and North America plates.New ocean floor is being created offshore of...
, where the subducting Juan de Fuca plate
Juan de Fuca Plate
The Juan de Fuca Plate, named after the explorer of the same name, is a tectonic plate, generated from the Juan de Fuca Ridge, and subducting under the northerly portion of the western side of the North American Plate at the Cascadia subduction zone...
is underthrusting to the east relative to the North American plate
North American Plate
The North American Plate is a tectonic plate covering most of North America, Greenland, Cuba, Bahamas, and parts of Siberia, Japan and Iceland. It extends eastward to the Mid-Atlantic Ridge and westward to the Chersky Range in eastern Siberia. The plate includes both continental and oceanic crust...
, a GPS point on the surface of the upper plate above the 'locked' plate boundary will slowly move east as it is dragged eastward by the subduction process. But during an ETS event, that same GPS point slips to the west. The "tremor" and "slip" along the fault at depth coincide in time with a movement at the surface to the west. This process happens repeatedly, hence "episodic tremor and slip."
ETS events in Cascadia were observed to be periodic, with an interval of 14 months, and analysis of measurements led to the successful prediction ETS events in 2003, 2004, 2005, and 2007. These events are marked by about two weeks of 1–10Hz trembling that are only detectable by sensitive seismometers, accompanied by aseismic slip on the megathrust that is equivalent to an M7 earthquake. The tremor and slip occurs downdip from the locked portion of the Cascadia megathrust that broke in the M9 1700 Cascadia earthquake, and which is expected to re-break in the future. The ETS phenomenon, as currently interpreted, suggests increasing stress on the locked portion of the megathrust fault, making a large earthquake on the megathrust more likely.
Types
The ETS tremor has now been identified in two flavors:- many hours of tremor with geodetic deformation (as described above) identified by GPS, strainmeters, and tiltmeters, and
- 5–10 second bursts at the time of passage of waves from distant earthquakes.
The first kind of ETS tremors are similar to those observed in the forearc region of southern Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
, and have also been spotted in Alaska, Costa Rica, and Mexico.
The second triggered variety has now been seen under Vancouver Island, under Japan, on the San Andreas in California, and under Taiwan.
Week- to year-long episodes of slow slip not accompanied by tremor has been observed in New Zealand. One theory holds that ETS tremor is more common is the process of subduction
Subduction
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...
of younger oceanic crust
Oceanic crust
Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium...
, which may be hotter and wetter, rather than older oceanic crust.