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.[1]
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The term ETS was coined by the Geological Survey of Canada around 2003 to describe observations of GPS measurements in the Vancouver Island area.[2]
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 ETS tremor has now been identified in two flavors:
The first kind of ETS tremors are similar to those observed in the forearc region of southern Japan,[3] 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.