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In 2001, CWU researchers with the continuous GPS network Pacific Northwest Geodetic Array discovered periodic slow-slip across the Cascadia Subduction Zone. Previously undetected by seismic networks, these slip events exhibit regular recurrence intervals thus changing current understanding of earthquake behavior. Since this time, definitions for this newly discovered phenomenon have evolved. At first, the term "silent-earthquake" was employed to illustrate the absence of a seismic signature. Subsequent investigations and recent discoveries have led to a change in characterization. Now these slow-slip events are defined as eposodic tremor and slip (ETS).
In short, an ETS is a discreet time interval (episode) of relative tectonic plate movement (slip) coupled with high frequency seismic energy bursts (tremor). ETS usually last for around a few weeks duration as opposed to regular earthquakes where energy is released within seconds to minutes.
Stress between these two colliding plates builds since differential movement between the two zones is not entirely compensated from ETS displacement. Quick slip across the upper locked portion of a subduction zone occurs in large megathrust earthquakes when accumulated stresses surpass the upper region's locking threshold. In contrast, the subtle motion caused by ETS is so "slow" it's difficult to record at the surface. One might say "quiet" or possibly "silent" in nature, but definitely important since these events affect lithospheric plate interactions that are responsible for damaging "fast" earthquakes. Will the size of future large-scale megathrust earthquakes be reduced or will the time interval between these earthquakes increase with an ETS? A process with such imposing consequences is hardly "silent" in terms of relevance. In fact an ETS is not silent at all.