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

Abstract

More than a decade after the discovery of deep episodic slow slip and tremor, or slow earthquakes, at subduction zones, much research has been carried out to investigate the structural and seismic properties of the environment in which they occur. Slow earthquakes generally occur on the megathrust fault some distance downdip of the great earthquake seismogenic zone in the vicinity of the mantle wedge corner, where three major structural elements are in contact: the subducting oceanic crust, the overriding forearc crust and the continental mantle. In this region, thermo-petrological models predict significant fluid production from the dehydrating oceanic crust and mantle due to prograde metamorphic reactions, and their consumption by hydrating the mantle wedge. These fluids are expected to affect the dynamic stability of the megathrust fault and enable slow slip by increasing pore-fluid pressure and/or reducing friction in fault gouges.Resolving the fine-scale structure of the deep megathrust fault and the in situ distribution of fluids where slow earthquakes occur is challenging, and most advances have been made using A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPTteleseismic scattering techniques (e.g., receiver functions). In this paper we review the teleseismic structure of six well-studied subduction zones (three hot, i.e., Cascadia, southwest Japan, central Mexico, and three cool, i.e., Costa Rica, Alaska, and Hikurangi) that exhibit slow earthquake processes and discuss the evidence of structural and geological controls on the slow earthquake behavior. We conclude that changes in the mechanical properties of geological materials downdip of the seismogenic zone play a dominant role in controlling slow earthquake behavior, and that near-lithostatic pore-fluid pressures near the megathrust fault may be a necessary but insufficient condition for their occurrence.

Subduction zone megathrust earthquakes

“…Audet and Schwartz (2013) identified similar correlations among high Vp/Vs, higher fluid pressures, and slow slip in the southeastern Nicoya segment of the Costa Rica subduction zone. The high fluid pressures, and the related decrease in the effective normal stress are commonly invoked as necessary for these slow-slip processes in the both the shallow (e.g., Saffer and Wallace, 2015) and deep portions of the seismogenic zone (e.g., Audet and Kim, 2016).…”

“…Based on determination of seismic velocities in the location of 2011 Tohoku rupture, Zhao et al (2011) found that high coseismic slip and foreshock locations are concentrated in the high-seismic-velocity areas; the nearby low-seismic-velocity patches, inferred to be sediments and high fluid content, host aseismic creep and slow thrust events. Beyond the sediment thickness and composition, fluids expelled from subducted sediments also appear to be an important component in the slow-slip process (e.g., Peacock, 2009;Peng and Gomberg, 2010;Audet and Kim, 2016).…”

Migrations and Clusters of Shallow Very Low Frequency Earthquakes in the Regions Surrounding Shear Stress Accumulation Peaks Along the Nankai Trough

“…Their ruptures occur along plate boundaries and propagate with slower velocities than regular earthquakes. The activities of slow earthquakes are concentrated at the shallower and deeper extensions of megathrust rupture zones, and their characteristics may be linked to stress accumulation, frictional properties, and the presence of pore fluids at the plate boundaries (e.g., Audet & Kim, ; Matsuzawa et al, ; Saffer & Wallace, ). By using dense onshore seismic and geodetic networks, the characteristics of slow earthquakes occurring at deeper depths, such as the deep low frequency tremor (LFT), very low frequency earthquake (VLFE), slow slip event (SSE), and episodic tremor and slip (ETS), have been extensively investigated (e.g., Brudzinski & Allen, ; Ide et al, ; Ide & Yabe, ; Ito et al, ; Obara, ; Rogers & Dragert, ; Shelly et al, ).…”

Earthquake Swarms and Slow Slip on a Sliver Fault in the Mexican Subduction Zone

“…The fluids are thought to have originated from the oceanic crust when blueschist dehydrates to eclogite, hence leading to the serpentinization of the overlying mantle wedge corner (77,78). Therefore, the contrast in permeability between the impermeable gabbroic lower continental crust and the serpentinite below may trap fluids, contributing to high pore-fluid pressures and thus episodic tremor and slip generation (77,79).…”