Episodic tremor and slip (ETS) is a phenonemon recently observed in seismology describing a particular type of tremor pattern observed in regions of convergent plate boundaries. Seismology (from Greek grc σεισμός seismos, "earthquake" and grc -λογία -logia) is the scientific study of Earthquakes In Plate tectonics, a convergent boundary – also known as a convergent plate boundary or a destructive plate boundary – is an actively deforming region Plate tectonics (from Greek τέκτων tektōn "builder" or "mason" describes the large scale motions of Earth 's Lithosphere These are characterised by non-earthquake like tremors, accompanied by aseismic slip in the same region of the local megathrust. An earthquake is the result of a sudden release of energy in the Earth 's crust that creates Seismic waves Earthquakes are recorded with a Seismometer For some GPS stations 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.
The term ETS was coined by the Geological Survey of Canada in the latter part of the 20th Century to describe observations of GPS measurements in the Vancouver Island area[1]. The Geological Survey of Canada (GSC (Commission Géologique du Canada (CGC is part of the Earth Sciences Sector of Natural Resources Canada. Basic concept of GPS operation A GPS receiver calculates its position by carefully timing the signals sent by the constellation of GPS Satellites high above the Earth Vancouver Island is a large Island in British Columbia, Canada, one of several North American regions named after George Vancouver, the British
Tremor now has been identified in two flavors - (1) many hours of tremor with geodetic deformation identified by GPS, strainmeters, and tiltmeters, and (2) 5-10 second bursts at the time of passage of waves from distant earthquakes.
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 first kind of ETS tremors are similar to those observed in the forearc region of southern Japan[2], and have also been spotted in Alaska, Costa Rica, and Mexico. For a topic outline on this subject see List of basic Japan topics.
The second triggered variety has now been seen under Vancouver Island, 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. In Geology, a subduction zone is an area on Earth where two tectonic plates meet and move towards one another with one sliding underneath the other Oceanic crust is the part of Earth's Lithosphere that surfaces in the Ocean basins