Anomalous V_p/V_s Ratios at Seismic Frequencies Might Evidence Highly Damaged Rocks in Subduction Zones

Abstract

Unusually high compressional (P) to shear (S) wave velocity ratios (V p /V s ) were measured at different subduction zones and interpreted as fluid-pressurized regions. Because no laboratory data reported such high values in isotropic rocks, mineralogical or anisotropic constrains were assumed. However, fluid-saturated rocks' V p /V s is a frequency-dependent property so that standard laboratory measurements cannot be directly upscaled to the field. Using a new methodology, we measured the property in the elastic regime relevant to field measurements for diverse lithologies. We obtained extreme V p /V s values, consistent with those reported at seismic frequency in the field. Consistently with a model, it shows that if high fluid pressure is a key factor, anomalous V p /V s values could evidence intense degrees of microfracturation in isotropic rocks, whichever its mineralogical content. The permeability of these regions could be larger than 10 À16 m 2 .Plain Language Summary Anomalous seismic properties measured at subduction zones across the globe have been linked to the occurrence of earthquakes or tremors. However, different physical causes were postulated to interpret such seismic properties, and none of the laboratory measurements quantitatively fitted with those anomalous values. From laboratory measurements dedicated to link with field ones, we report the first data set that fits with field measurements observed at subduction zones. We show that such values might evidence a large degree of microfracturing, opened by high pore fluid pressure, in isotropic rocks of any mineralogy.An additional complexity is seldom accounted for when comparing field and laboratory measurements (Müller et al., 2010): While field measurements are performed at the subsonic frequency (i.e., about 1 Hz), standard laboratory measurements are performed at ultrasonic frequencies (i.e., 1 MHz). Recent experimental studies (Pimienta, Fortin, & Guéguen, 2016b) showed that Poisson's ratio (or V p /V s ) of fluid-saturated rocks is a PIMIENTA ET AL.12,210

Direct Inversion of S‐P Differential Arrival Times for Ratio in SE Asia

“…Above 100 km depth, high V p V s ratios may also indicate the presence of fluids triggering high rates of seismicity in the mantle wedge (e.g., Halpaap et al, 2019;White et al, 2019) and promoting intraslab faulting (Chiarabba et al, 2008). Recent rock physics research demonstrates that highly pressurized fluids can produce high V p V s anomalies in subduction settings in the presence of intense microfracturing (Pimienta et al, 2018).…”

“…Upper crust Low (L) L L Presence of quartz (Chiarabba & Amato, 2003;Wagner et al, 2008) Crust-mantle High (H) H L Fluids/silica-orthopiroxene enrichment (Wagner et al, 2008;Zheng & Lay, 2006) Volcanic forearcs and mantle wedge L L H Overpressuring and partial melting-possible source of arc magmatism or hydrated continental mantle wedge (Graeber & Asch, 1999;Reyners et al, 2006;Zhao, 2009) Slab boundaries L L H Serpentinization (Hyndman & Peacock, 2003;Reyners et al, 2006) Passive Margins H L H Serpentinized peridotite (D'Alessandro et al, 2016) Slab core L H L Dehydration of serpentine in oceanic subducting slab (Wagner et al, 2005) Mantle L L -Temperature changes (Wiens & Smith, 2004;Wiens et al, 2006) Mantle H H L Subducted oceanic lithosphere (Zhao, 2009) Mantle H H L Anisotropy of peridotite (Hacker & Abers, 2012) High-pressure subduction setting --H Highly pressurized fluids, intense fracturing (Pimienta et al, 2018) Mantle H H L Cratonic xenoliths (Bastow et al, 2015;James et al, 2004)…”

“…Nevertheless, the high-compressional setting under the northern Banda Sea and, especially Seram, corresponds to the highest d(V p /V s ) in our model (+5%; Figure 10) and is consistent with strike-slip faulting produced by extreme horizontal extension driving mantle upwelling (Pownall et al, 2013), coupled with the likely presence of fluids sourced from depth. Fluid-pressurized regions have been recently recognized as a source of strong positive d(V p /V s ) variations in fault zones and subduction settings, independently of the accumulation of molten materials at depth (Pimienta et al, 2018). Fluid pressurized faulting in an extreme compressional setting is thus a possible explanation for the large positive d(V p /V s ) variations as well as for the complex spatial variation of the focal mechanisms determined in studies of the two-slab model for the Banda Sea (Das, 2004).…”

Constraints on the Geometry of the Subducted Gorda Plate From Converted Phases Generated by Local Earthquakes

“…Thus, the elevated Vp/Vs values in Models two and three that are consistent with the converted phase amplitudes likely indicate the existence of significant fluid-filled porosity in the subducted melange and oceanic crust. To reach Vp/Vs values in excess of ∼1.85 would likely correspond to fluid-filled porosities in excess of ∼3% (assuming 20-50 MPa effective normal stress) (Peacock et al, 2011;Pimienta et al, 2018). While the exact amount of fluid-filled porosity is difficult for us to interpret owing to the variety of factors that affect the converted phase amplitudes, our results are consistent with the interpretation of the 10-20 km depth range as being dominated by brittle deformation that produces significant porosity which is maintained throughout the interseismic peirod.…”

The Earth's Surface Controls the Depth‐Dependent Seismic Radiation of Megathrust Earthquakes

“…In light of this, we discuss three regions of possibly elevated V P /V S ratios. The first region is the subduction channel, the thin upper layer of the downgoing slab that is composed of fluid-rich seafloor sediments (Hicks et al, 2014;Naif et al, 2015;Saffer & Tobin, 2011;Zhu et al, 2020) and hydrated minerals in a mafic fractured crust (Bostock, 2013;Hicks et al, 2014;Nishimura et al, 2019;Pimienta et al, 2018;Shelly et al, 2006). The second region is the slope apron, the thin layer of the seafloor sediments that covers the wedge, which is best accessed by offshore drilling and active seismic surveys (Fujie et al, 2013;Peacock et al, 2010;Tsuji et al, 2011;Zhu et al, 2020).…”