Possible control of subduction zone slow-earthquake periodicity by silica enrichment

Abstract

Seismic and geodetic observations in subduction zone forearcs indicate that slow earthquakes, including episodic tremor and slip, recur at intervals of less than six months to more than two years. In Cascadia, slow slip is segmented along strike and tremor data show a gradation from large, infrequent slip episodes to small, frequent slip events with increasing depth of the plate interface. Observations and models of slow slip and tremor require the presence of near-lithostatic pore-fluid pressures in slow-earthquake source regions; however, direct evidence of factors controlling the variability in recurrence times is elusive. Here we compile seismic data from subduction zone forearcs exhibiting recurring slow earthquakes and show that the average ratio of compressional (P)-wave velocity to shear (S)-wave velocity (vP/vS) of the overlying forearc crust ranges between 1.6 and 2.0 and is linearly related to the average recurrence time of slow earthquakes. In northern Cascadia, forearc vP/vS values decrease with increasing depth of the plate interface and with decreasing tremor-episode recurrence intervals. Low vP/vS values require a large addition of quartz in a mostly mafic forearc environment. We propose that silica enrichment varying from 5 per cent to 15 per cent by volume from slab-derived fluids and upward mineralization in quartz veins can explain the range of observed vP/vS values as well as the downdip decrease in vP/vS. The solubility of silica depends on temperature, and deposition prevails near the base of the forearc crust. We further propose that the strong temperature dependence of healing and permeability reduction in silica-rich fault gouge via dissolution-precipitation creep can explain the reduction in tremor recurrence time with progressive silica enrichment. Lower gouge permeability at higher temperatures leads to faster fluid overpressure development and low effective fault-normal stress, and therefore shorter recurrence times. Our results also agree with numerical models of slip stabilization under fault zone dilatancy strengthening caused by decreasing fluid pressure as pore space increases. This implies that temperature-dependent silica deposition, permeability reduction and fluid overpressure development control dilatancy and slow-earthquake behaviour.

Mantle hydration and Cl-rich fluids in the subduction forearc

“…It has been shown that foliated serpentine has anisotropic permeability that could focus fluid flow parallel to the slab up to the tip of the forearc mantle wedge (Kawano, et al 2011). The plate interface could also be sealed by precipitation of solutes such as silica, and episodically opened by large seismic events, as suggested for Cascadia (Hyndman, et al 2015) and other subduction zones (Audet and Burgmann 2014). The presence of fluids in the subducting oceanic crust is also suggested by high-conductivity anomalies parallel to the slab (Wannamaker, et al 2014), and is substantiated by extensive geochemical investigations of high-pressure ophiolitic terranes (Bebout and Penniston-Dorland 2016;.…”

Along-fault pore-pressure evolution during a slow-slip event in Guerrero, Mexico

“…The two most simple mechanisms to explain the weakening of the plate interface and the transition to stable-sliding with increasing distance from the trench are increasing temperatures with depth and increased pore-pressure from fluids released through metamorphic dehydration. Recent studies (Shelly et al, 2006;Audet et al, 2009;Song et al, 2009;Kim et al, 2010;Audet and Bürgmann, 2014) have suggested that trapped fluids at the plate interface and the consequent high pore pressures are the most important factor in creating the weak plate interface necessary for slow earthquakes.…”

Fault Zone Dynamic Processes

“…Systematic micro-and macrostructural field studies [e.g., Faulkner et al, 2006;Dor et al, 2006;Mitchell and Faulkner, 2009;Savage and Brodsky, 2011], as well as seismic surveys [e.g., Li et al, 2006;Cochran et al, 2009;Froment et al, 2014], have recently been performed on fault zones, a key component to understanding the energy balance of earthquakes [e.g., Rice, 2002;Kanamori, 2006]. Relating fault mechanics to fault zone structure, several authors have underlined the importance of combining field observations with geodetic and seismological measurements to understand what controls the seismic and aseismic slip behavior [e.g., Biegel and Sammis, 2004;Thomas et al, 2014;Audet and Burgmann, 2014]. Recent studies have successfully bridged the gap between rocks physics, laboratory experiments, and seismic observations of dynamic processes and fault zone evolution [e.g., Schubnel et al, 2006;Brantut, 2015].…”