Non-volcanic tremor driven by large transient shear stresses

Abstract

Non-impulsive seismic radiation or 'tremor' has long been observed at volcanoes and more recently around subduction zones. Although the number of observations of non-volcanic tremor is steadily increasing, the causative mechanism remains unclear. Some have attributed non-volcanic tremor to the movement of fluids, while its coincidence with geodetically observed slow-slip events at regular intervals has led others to consider slip on the plate interface as its cause. Low-frequency earthquakes in Japan, which are believed to make up at least part of non-volcanic tremor, have focal mechanisms and locations that are consistent with tremor being generated by shear slip on the subduction interface. In Cascadia, however, tremor locations appear to be more distributed in depth than in Japan, making them harder to reconcile with a plate interface shear-slip model. Here we identify bursts of tremor that radiated from the Cascadia subduction zone near Vancouver Island, Canada, during the strongest shaking from the moment magnitude M(w) = 7.8, 2002 Denali, Alaska, earthquake. Tremor occurs when the Love wave displacements are to the southwest (the direction of plate convergence of the overriding plate), implying that the Love waves trigger the tremor. We show that these displacements correspond to shear stresses of approximately 40 kPa on the plate interface, which suggests that the effective stress on the plate interface is very low. These observations indicate that tremor and possibly slow slip can be instantaneously induced by shear stress increases on the subduction interface-effectively a frictional failure response to the driving stress.

Scaling Relationships of Source Parameters for Slow Slip Events

“…We find that the source zone of SSEs is consistent with an asperity model where small patches of locally high strength are distributed within a broader zone of low strength. We infer that the shear strength over the entire region is significantly less than seismogenic faults because of near-lithostatic pore pressure Rubinstein et al, 2007;Audet et al, 2009;Nadeau and Guilhem, 2009). The low shear strength also limits the static stress drop.…”

“…Seismic observations on subduction zones where SSEs occur suggest that the pore fluid pressure is near-lithostatic Audet et al, 2009), resulting in a very low effective normal stress. The low estimate of the effective normal stress limits the level of shear stress on the fault, which might be on the order of tens of kilopascals (e.g., Rubinstein et al, 2007;Nadeau and Guilhem, 2009). Thus, the stress drop during an event would be limited to a fraction of the shear stress.…”

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

“…The low effective normal stresses are consistent with the triggering and modulation of tremor by small dynamic stresses such as passing surface waves (Rubinstein et al, 2007) and tidal stresses (Royer et al, 2015;Rubinstein et al, 2008), as well as numerical modeling of slow slip (Liu and Rice, 2007;Rubin, 2008;Segall et al, 2010). Based on the spatial correspondence between the LVZ and the slow earthquake source region, it is tempting to conjecture that high pore-fluid pressures inferred from elevated Vp/Vs values are responsible for deep episodic slow earthquake activity.…”

Episodic slow slip events and rate‐and‐state friction

“…These include tomographic images of high P wave/S wave velocity ratios [Shelly et al, 2006], and increased tremor activity on tidal timescales [Shelly et al, 2007a] or during the passage of teleseismic surface waves [Miyazawa and Mori, 2006;Rubinstein et al, 2007], both of which impart small stress changes that are most likely to be effective triggers if the ambient effective stress is very low. Dehydration reactions in the downgoing plate are a plausible source of high fluid pressure, and tremor and slow slip appear to be absent where the P-T conditions of the downgoing slab are unfavorable for these reactions [Liu and Rice, 2007].…”