Tremor Event: Between roughly December 3rd to the 15th, it appears that tremor in central Oregon has picked up. The last slow slip and tremor event was in February 2016, 22 months ago.
Tremor is the release of seismic noise from slow slip along the interface of the Juan de Fuca and North American plates and lasts for several weeks to months. This process is known as Episodic Tremor and Slip (ETS).
Slow slip happens down-dip of the locked zone. The locked zone is where tectonic stress builds up until it releases in a great earthquake or megaquake. The recurrence interval of slow slip and tremor varies at different regions along the Cascadia Subduction Zone.
The last ETS event in Cascadia started in February 2017 around the western edge of the Olympic Mountains. The duration was approximately 35 days with a two-week quiescent period. Prior ETS events in northern Washington/Vancouver Island area was approximately December 2015.
The last ETS event in central Oregon was 2016 and lasted just over a week before it stopped on March 1, 2016.
ETS events are still being studied to understand the processes about slow slip and megathrust earthquakes.
Slow slip tremors
Episodic tremor and slip (ETS) is a seismological phenomenon observed in some subduction zones. These events are characterized by non-earthquake seismic rumbling, or tremor, and slow slip along the plate interface. Slow slip events are distinguished from earthquakes by their propagation speed and focus.