A wide depth distribution of seismic tremors along the northern Cascadia margin

Abstract

The Cascadia subduction zone is thought to be capable of generating major earthquakes with moment magnitude as large as M(w) = 9 at an interval of several hundred years. The seismogenic portion of the plate interface is mostly offshore and is currently locked, as inferred from geodetic data. However, episodic surface displacements-in the direction opposite to the long-term deformation motions caused by relative plate convergence across a locked interface-are observed about every 14 months with an unusual tremor-like seismic signature. Here we show that these tremors are distributed over a depth range exceeding 40 km within a limited horizontal band. Many occurred within or close to the strong seismic reflectors above the plate interface where local earthquakes are absent, suggesting that the seismogenic process for tremors is fluid-related. The observed depth range implies that tremors could be associated with the variation of stress field induced by a transient slip along the deeper portion of the Cascadia interface or, alternatively, that episodic slip is more diffuse than originally suggested.

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

“…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).…”

“…First, tremor and LFEs appear to occur near the top of the LVZ, whereas regular Wadati-Benioff seismicity is mostly restricted to the lower oceanic crust and upper mantle. Although tremor was originally thought to reflect hydrofracturing within the overlying continental crust (Kao et al, 2005), source mechanisms of repeating LFEs during tremor indicate that they represent shear slip on a fault (Bostock et al, 2012;Shelly et al 2007). Second, deep ETS events occur downdip of the locked zone around the transition from unstable to stable slip, near the intersection of the downgoing plate with the overlying Moho and where the plate interface is ~30 to 40 km deep (Fig.…”

A precise hypocenter determination method using network correlation coefficients and its application to deep low-frequency earthquakes

“…tribution ranges from 20 to 50 km ( Fig. 1) and resembles the tremor source distribution determined by Kao et al (2005) in the Cascadia subduction zone. Since the wide depth distribution suggests a different physical interpretation, such as fluid movement in the overriding plate, the important question is whether or not the apparent wide depth distribution of LFEs along the Nankai subduction zone is real.…”

Geophysical and geochemical constraints on the evolution of oceanic lithosphere from formation to subduction

“…Episodic nonvolcanic tremors [Rogers and Dragert, 2003] associated with slow slip along the subduction interface [Dragert et al [2001] are abundant in the Cascadia subduction zone, and their occurrence is thought to be facilitated by slab-derived fluids based on the location of the tremor in close proximity to strong seismic reflectors above the plate interface [Kao et al, 2005]. Intraslab seismicity located at intermediate depths is…”

“…Audet et al, 2009;Bostock et al, 2002;Hacker et al, 2003;Kao et al, 2005;Preston et al, 2003;Rogers and Dragert, 2003;Walowski et al, 2015]. Prior studies within the interior of the JdF plate provide evidence for hydration of the upper crust associated with ridge flank hydrothermal circulation and spatially correlated with local zones of ridge propagation [McClymont and Clowes, 2005;Nedimović et al, 2009;Nedimović et al, 2008;Newman et al, 2011] but the full extent of when and how water is incorporated into the lithosphere during the evolution of the JdF plate, and how water is distributed at depth within the plate prior to subduction remains unknown.…”