Episodic tremor and slip (ETS)

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. These are characterised by non-earthquake like tremors, accompanied by aseismic slip in the same region of the local megathrust. 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 [ [http://gsc.nrcan.gc.ca/geodyn/ets_e.php Natural Resources Canada web page about ETS] ] .

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 [ [http://adsabs.harvard.edu/abs/2004AGUSM.G23A..01D Abstract of a 2004 paper of the 2004 spring meeting of the American Geophysical Union, "Geodetic And Seismic Signatures of Episodic Tremor And Slip Beneath Vancouver Island, British Columbia"] ] , and have also been spotted in Alaska, Costa Rica, and Mexico.

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.

ee also

* Geodynamics
* Seismology

References

External links

* [http://www.nrcan-rncan.gc.ca/com/index-eng.php Natural Resources Canada]