Episodic Tremor and Slip on the Cascadia Subduction Zone: The Chatter of Silent Slip

Abstract

We found that repeated slow slip events observed on the deeper interface of the northern Cascadia subduction zone, which were at first thought to be silent, have unique nonearthquake seismic signatures. Tremorlike seismic signals were found to correlate temporally and spatially with slip events identified from crustal motion data spanning the past 6 years. During the period between slips, tremor activity is minor or nonexistent. We call this associated tremor and slip phenomenon episodic tremor and slip (ETS) and propose that ETS activity can be used as a real-time indicator of stress loading of the Cascadia megathrust earthquake zone.

Teleseismic constraints on the geological environment of deep episodic slow earthquakes in subduction zone forearcs: A review

“…Associated with slow slip, a new source of seismic energy (called non-volcanic tremor) in the forearc was discovered by Obara (2002) off southwest Japan, which appeared as coherent noise propagating across arrays of seismograph stations. Rogers and Dragert (2003) then found similar signals in the forearc of the Cascadia subduction zone that occurred concurrently with slow slip events and both phenomena recurred episodically, which led to the term "Episodic Tremor and Slip", or…”

“…These so-called episodic tremor and slip (ETS) events were later found to occur all along Cascadia (Brudzinski and Allen, 2007) from the mapping of either or both slow slip and tremor (e.g., Kao et al, 2008;Szeliga et al, 2008;Wech et al, 2009). At the time the lack of precise tremor hypocenters fuelled the debate regarding the nature of the tremor signal, whether it was occurring within the overriding plate as the slow slip was progressing and changing the stress field to generate hydraulic fracturing (Kao et al, 2005;Rogers and Dragert, 2003), or via direct shear slip on the plate interface during slow slip (Shelly et al, 2006(Shelly et al, , 2007. Low Frequency earthquakes (LFE) families that form at least part of the tremor during slow slip have now been found all along the Cascadia margin (Plourde et al, 2015;Royer and Bostock, 2014;Thomas and Bostock, 2015) and are consistent with shear slip on the plate interface (see also Rubin and Armbruster, 2013;Armbruster et al, 2014, for tremor locations).…”

“…Rogers and Dragert (2003) characterized the recurrence of aseismic slow slip in conjunction with non-volcanic tremor beneath southern Vancouver Island that firmly established the presence of slow episodic megathrust fault rupture downdip of the seismogenic zone. These so-called episodic tremor and slip (ETS) events were later found to occur all along Cascadia (Brudzinski and Allen, 2007) from the mapping of either or both slow slip and tremor (e.g., Kao et al, 2008;Szeliga et al, 2008;Wech et al, 2009).…”

Array measurements of deep tremor signals in the Cascadia subduction zone

“…Similar seismic signals have been recently observed in the Cascadia subduction zone. Here periods of deep tremor are clearly correlated both in space and time with the slip episodes observed every 14 ± 2 months by continuous GPS measurement on Vancouver Island and northern Washington [Dragert et al, 2002;Rogers and Dragert, 2003;McCausland and Malone, 2003]. Data from the widely-spaced stations of the Pacific Northwest Seismic Network (PNSN) have been used to infer rough estimates about the source location, using either the signal's envelopes [McCausland and Malone, 2004] or a modified beam-forming technique [Kao and Shan, 2004].…”

Reply [to “Comment on ‘Coupling semantics and science in earthquake research’”]

“…Lay and Schwartz quoted the result at the Nicoya Peninsula that microseismicity occurs downdip of the region with the least fault slip .This result and the many other more intriguing observations [e.g., Freymueller et al, 2000;Ozawa et al, 2002;Rogers and Dragert, 2003;Uchida et al, 2003;Yagi et al, 2003] reflect the heterogeneous and transient nature of fault motion in the real world, but in our view they do not negate simple models that are designed to explain basic concepts.…”