Depth variations in seismic velocity in the subducting crust: Evidence for fluid‐related embrittlement for intermediate‐depth earthquakes

Shiina, Takahiro; Nakajima, Jun; Matsuzawa, Toru; Toyokuni, Genti; Kita, S.

doi:10.1002/2016gl071798

Cited by 35 publications

(17 citation statements)

References 31 publications

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“…The events that are above 80 and 60 km in the Japan and northern Chile subduction zones, respectively, seem located mostly in the oceanic crust according to global or regional slab models (Hayes et al, ; Kita et al, ) and may result from dehydration. This would be consistent with some observations, such as water infiltration in the trench outer rise (Ranero et al, ), positive correlation of seismicity rate and fault density (Boneh et al, ), and low velocity in the crust using guild waves (Shiina et al, ). Along with the low Vp/Vs in the lower layer in the double seismic zones, this may indicate that two mechanisms operate on the intermediate‐depth earthquakes (Florez & Prieto, ).…”

Section: Discussionsupporting

confidence: 91%

“…Dehydration embrittlement invokes the increase of pore pressure caused by metamorphic reactions (Hacker et al, 2003;Houston, 2007), which decreases effective friction and permits rocks to rupture. Many geophysical observations are consistent with this mechanism, such as pervasive normal faults in the outer rise of trenches that allow water to infiltrate before subduction (Grevemeyer et al, 2007;Ranero et al, 2003Ranero et al, , 2005, the positive correlation of seismicity rate and fault density on incoming plates (Boneh et al, 2019;Shillington et al, 2015), thermal-controlled metamorphic dehydration from thermal-chemical modeling (Chen et al, 2019;Kita et al, 2006;Wei et al, 2017), and highly conductive anomalies above intermediate-depth earthquakes imaged using magnetotellurics (Vargas et al, 2019) or very low wavespeed anomalies from guided 10.1029/2019GL085062 waves (Shiina et al, 2013(Shiina et al, , 2017. Laboratory experiments also show evidence for dehydration-related brittle deformation (Dobson, 2002;Jung et al, 2004Jung et al, , 2009Okazaki & Hirth, 2016), although cases exist where only slow slip events are observed (Chernak & Hirth, 2011).…”

Section: Introductionmentioning

confidence: 93%

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Earthquake Depth Phase Extraction With P Wave Autocorrelation Provides Insight Into Mechanisms of Intermediate‐Depth Earthquakes

Fang

Hilst

2019

Geophysical Research Letters

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Constraining the mechanism of earthquakes in subduction zones requires adequate estimates of source location and near-source elastic properties. In this study, we propose a P wave autocorrelation-based method to extract depth phase energy from teleseismic earthquakes with moment magnitude down to 4.0. We apply the method to improve location estimates of intermediate-depth earthquakes in the Japan and northern Chile subduction zones, which represent so-called cold and warm slabs, respectively, and which are both marked by double seismic zones. A positive correlation of slab age and double-seismic-zone width validates a thermally controlled model of slab morphology. The negative to normal differential times (and, thus, low or normal Vp/Vs) of the deep parts of the double seismic zones suggest that the intermediate-depth earthquakes considered here are not due to dehydration.Plain Language Summary Earthquake depth, which can be obtained by using depth phases, is critical for hazard mitigation and a better understanding of their mechanisms. Most techniques that use depth phases with teleseismic data require large events, even when array-based techniques are used, which limits comprehensive studies of subduction zone seismicity. Here we propose a new autocorrelation-based method to extract depth phase energy from teleseismic data generated by earthquakes with magnitudes as small as M w 4.0. The application to Japan and northern Chile validates the robustness of the method, and the more precise delineation of the double seismic zones reveals a positive correlation between their widths and the age of the slab when it approaches the trench. Moreover, we find the differential times of sP and pP energy, which are sensitive to the average Vp/Vs between the events and the surface, are different between the upper and the lower layers in the double seismic zone, with the lower layer mostly showing negative to normal values and thus low or normal Vp/Vs. This indicates a lack of water in the lower layer and suggests that other mechanisms are needed to interpret intermediate earthquakes rather than dehydration.

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Section: Discussionsupporting

confidence: 91%

Section: Introductionmentioning

confidence: 93%

Earthquake Depth Phase Extraction With P Wave Autocorrelation Provides Insight Into Mechanisms of Intermediate‐Depth Earthquakes

Fang

Hilst

2019

Geophysical Research Letters

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“…In all the cases except for Case 1 (where we consider the bulk viscosity to be too low for realistic slab interiors), we observe that a large amount of fluid is trapped in the high porosity region produced by the fluid source. It has been shown that in northern Japan (i.e., Tohoku and Hokkaido) the seismic P wave and S wave velocities in the oceanic crust beneath the fore-arc mantle are significantly lower than those expected for the fully hydrated mid-ocean ridge basalt, suggesting the presence of pore fluids (Shiina et al, 2013(Shiina et al, , 2017. These observations are broadly consistent with our results in that pore fluids are kept inside the slab.…”

Section: Implications For Geophysical Observationssupporting

confidence: 91%

“…It has been shown that in northern Japan (i.e., Tohoku and Hokkaido) the seismic P wave and S wave velocities in the oceanic crust beneath the fore-arc mantle are significantly lower than those expected for the fully hydrated mid-ocean ridge basalt, suggesting the presence of pore fluids (Shiina et al, 2013(Shiina et al, , 2017. It has been shown that in northern Japan (i.e., Tohoku and Hokkaido) the seismic P wave and S wave velocities in the oceanic crust beneath the fore-arc mantle are significantly lower than those expected for the fully hydrated mid-ocean ridge basalt, suggesting the presence of pore fluids (Shiina et al, 2013(Shiina et al, , 2017.…”

Section: Implications For Geophysical Observationsmentioning

confidence: 99%

Fluid Migration in a Subducting Viscoelastic Slab

Morishige

Keken

2018

Geochem Geophys Geosyst

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Metamorphic dehydration reactions in a subducting slab release fluids that trigger arc volcanism and are thought to be responsible for intermediate‐depth seismicity. The fluid flow from the source is controlled by buoyancy and compaction pressure which is modified by viscous and elastic effects. In this paper, we investigate how fluid migrates in viscoelastic slab by using 2‐D and 3‐D numerical models based on a theory of two‐phase flow. When bulk viscosity is sufficiently low, viscosity plays a dominant role and fluid goes up almost vertically soon after its release producing porosity waves. When a higher bulk viscosity is assumed, a large amount of fluid is trapped in a high porosity region produced by the fluid source and migrates along the source except for a case where the ratio of permeability (K) to fluid viscosity (μ) is relatively low. We also find that porosity increases in the deeper part of the fluid source in cases with intermediate and low values of K/μ. In 3‐D, fluid focusing occurs where the slab bends away from the trench causing a local increase in porosity and compaction pressure. These findings may help us explain several types of observations in subduction zones including slow earthquakes at the plate interface, low seismic wave velocities in the oceanic crust, double seismic zones in the slab, and shallow subduction angle at the bend of the slab.

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“…One explanation is that the slab dehydration has started to occur at a depth of <50 km. Another possibility is that a considerable amount of water, released from dehydration of the subducted oceanic crust in the deep part, migrates upward to the shallow part of the slab (Halpaap et al, ; Shiina et al, ). Therefore, we propose that the inferred dry forearc mantle is more affected by the possible low permeability of the slab interface (Halpaap et al, ; Katayama et al, ) than the possible low slab hydration.…”

Section: Discussionmentioning

confidence: 99%

Seismic Evidence for Water Transportation in the Forearc off Northern Japan

Zhao

2020

JGR Solid Earth

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The water cycle plays an essential role in arc volcanism, earthquake generation, mantle rheology, and thermal structure of subduction zones. Previous seismic studies have revealed strong structural heterogeneities in the megathrust zone in Northeast Japan and Hokkaido. However, water transportation in the forearc region remains poorly understood due to the lack of long-term seismic observatories at the seafloor. Using high-quality data recently recorded by the permanent ocean-bottom-seismometer network (S-net) in the Pacific Ocean off Northern Japan, we study the fine three-dimensional P wave velocity (V p ) structure of the crust and upper mantle beneath the Kuril and Tohoku forearc region. Our results reveal high velocities in the mantle-wedge corner, implying a low degree of serpentinization there. We suggest that the forearc mantle in the study region is cold and anhydrous. The slab interface under the forearc area may have a low permeability, which controls the fluid flux to the mantle wedge and the overriding plate. A low-V p layer atop the subducting Pacific plate is interpreted as the subducted oceanic crust. Dehydration of the subducted oceanic crust occurs at depths of 80-120 km, providing a large volume of water to the overriding mantle wedge to produce arc volcanoes, and part of the water may migrate upward to the shallow area. Large megathrust earthquakes (M w ≥ 6.0) mainly occurred around low-V p patches in the megathrust zone. Destruction of the low-permeability slab interface would result in fluid flow upward, which may trigger large megathrust earthquakes and seismicity in the mantle wedge under the forearc.

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Depth variations in seismic velocity in the subducting crust: Evidence for fluid‐related embrittlement for intermediate‐depth earthquakes

Cited by 35 publications

References 31 publications

Earthquake Depth Phase Extraction With P Wave Autocorrelation Provides Insight Into Mechanisms of Intermediate‐Depth Earthquakes

Earthquake Depth Phase Extraction With P Wave Autocorrelation Provides Insight Into Mechanisms of Intermediate‐Depth Earthquakes

Fluid Migration in a Subducting Viscoelastic Slab

Seismic Evidence for Water Transportation in the Forearc off Northern Japan

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