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.
*graphic courtesy of Steve Malone
Due to continuous plate motion, the daily solution of any GPS measurement is recorded as a velocity; reflecting the overall strain accumulating in the upper crust. During an ETS, GPS velocities change direction until the event passes. This minute signal would go unnoticed without extended timeseries. With the proper corrections and long-term stablizations, GPS allows accurate measurements for each day. Since ETS usually last for over 10 days, this provides a nice measurement of offset that could not be recorded on the longest period seismometer.
When slow earthquakes were first described by Dragert and others (2001) as well as Miller and others (2002), the motion of the Juan De Fuca and overriding North American plates was thought to occur in the absence of seismic energy. Since this time, Obara (2002) discovered seismic band energy that correlates with these slip signatures in Japan. This corresponding seismic energy is the tremor of an ETS. Typically between 1-6 vibrations per second, slow earthquake related tremor has since been detected in Cascadia as well. Although separating significant tremor from noise in a seismic dataset proves difficult, once an ETS is delineated by GPS, a clear trend becomes evident.
Since we cannot measure offsets at depth directly, we infer motion on the fault surface with surface measurements. Before GPS, seismometers were the primary tool to measure earthquake type activity in the earth's crust. Besides the vague tremor associated with an ETS, the weeks-long motion of an ETS goes undetected by any seismometer. Today, with vast GPS arrays, we can now measure events at these time scales. In Cascadia, cumulative measurable surface offsets are less than 0.6cm (Szeliga et al., 2008); well within detectable range of GPS data over an extended time interval. While at the surface this deformation is easily resolved by GPS; unique source location of energy at depth is not.
Tremor allows better determination of source location. In both Japan and Cascadia, the depth of ETS tremor (and presumably slip) locates to 25-40km depth (Kao et al., 2005; McCausland et al., 2005; Obara, 2002), directly within the transition of a subduction zone. These independent tremor measurements substantiate arguments for a tectonic source and fine-tune the timescale of ETS activity. In the region of Washington state's Puget Sound, unlike a normal earthquake, a regular 14.5 month ETS periodicity has been proposed by Miller and others (2002) while a shorter period of 10.9 months seems to exist in northern California (Szeliga, 2004). In other subduction zones apart from Cascadia, ETS events exhibit no discernible periodicity although average horizontal offsets are comparable at around 5mm.
In general, vertical GPS uncertainties are large, and in Cascadia vertical ETS offsets are often small. Vertical offsets, therefore, require independent measurements since fault slip models are highly sensitive to this component. To address this issue, PANGA constructed a Very Long Baseline Tiltmeter Array (VLBT) in collaboration with UC Boulder. Not only does the Cascadia Tiltmeter Array resolve the vertical field to better than 1,000 times that of GPS, these tiltmeters also increase temporal precision with a sampling rate 100 times that of reliable GPS solutions; thus providing tighter constraints for fault models.
Below are 9 years of GPS data from the Cascadia Subduction Zone along the convergent margin from northern California to southwest B.C., Canada. ETS events well recorded are delineated with blue lines and total slip-time is indicated on right plot by brackets. Most events last 3 to 4 weeks with amplitude between 2 and 7mm (Szeliga et al. 2008).
Like any ordinary earthquake, an ETS has a measure of energy released during the event. This is calculated as moment magnitude (Mw). Cascadia ETS events average 6.7Mw (almost equivalent to the 2001 6.8Mw Nisqually Earthquake). This would represent around 2-3cm of slip across the plates at depth if measured ETS surface deformations are in fact caused by integrated slips at depth and tremor is simply the artifact of each individual slip of two portions of lithospheric plates in this subduction zone (Szeliga et al. 2008).