Episodic tremor and slip (ETS) represents a newly discovered mode of fault behavior occurring just below the locked zone that generates great earthquakes. Initially discovered in subduction zones, this new slip mechanism can release energy equivalent to at least a magnitude 7 earthquake! While this is a tremendous energy release, no one ever feels these events because they occur as slow slip episodes lasting weeks or months. As the plates move, high-precision Global Positioning System (GPS) monuments record the magnitude and direction of motion while seismometers record the low amplitude seismic waves released. The importance of this discovery lies in its potential relationship to the part of faults that generate destructive earthquakes. Considering that ETS occurs immediately below to the locked zone of faults, it may be possible for energy released in slow slip episodes to concentrate stresses at the deep edge of the locked zone, incrementally bringing it closer to failure. Thus ETS episodes might be a trigger for great earthquakes or aid in monitoring the stress state of faults as they lead up to the big one.
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