Next ETS Expected any time now
The next 14-month Magnitude 6+ slow earthquake (Episodic Tremor and Slip) event is expected after a quiet period in Cascadia... or has it been that quiet.. (skip to: Winter 2017 ETS page)
Tremor throughout Cascadia seems to have been relatively quiet over the past four months; only 5,600 tremor locations from the wech-o-meter compared to an average of 15,300 for the six previous four month periods. In fact the whole past year seems quieter than previous years.
From Central Washington University's Pacific Northwest Geodetic Array comes a nice explanation of ETS and what it means to us:
Slow Earthquakes, ETS, and Cascadia
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.
During an ETS relative plate motion occurs within a transition region of a subducting lithospheric plate. This transition delineates an area between the upper-locked and lower-slipping interface of a subduction zone. 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.
A lot more at the site - basically, tension is building and the release will happen with a magnitude 9 unzipping of the zone between these two plates. This happens every 300 to 500 years and the last one happened over 300 years ago. This zone is located off the California, Oregon and Washington coastlines - communities on the coastline will be devestated. Further inland, there will be major damage but not as bad.