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.
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, 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.
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Last updated on Saturday May 10, 2008 at 01:47:34 PDT (GMT -0700)
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