Wiens Douglas A.

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Last Name
Wiens
First Name
Douglas A.
ORCID
0000-0002-5169-4386

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Now showing 1 - 17 of 17
  • Article
    Seismicity of the incoming plate and forearc near the Mariana Trench recorded by ocean bottom seismographs
    (American Geophysical Union, 2020-04-06) Eimer, Melody ; Wiens, Douglas A. ; Cai, Chen ; Lizarralde, Daniel ; Jasperson, Hope
    Earthquakes near oceanic trenches are important for studying incoming plate bending and updip thrust zone seismogenesis, yet are poorly constrained using seismographs on land. We use an ocean bottom seismograph (OBS) deployment spanning both the incoming Pacific Plate and the forearc to study seismicity near the Mariana Trench. The yearlong deployment in 2012–2013 consisted of 20 broadband OBSs and 5 suspended hydrophones, with an additional 59 short period OBSs and hydrophones recording for 1 month. We locate 1,692 earthquakes using a nonlinear method with a 3D velocity model constructed from active source profiles and surface wave tomography results. Events occurring seaward of the trench occur to depths of ~35 km below the seafloor, and focal mechanisms of the larger events indicate normal faulting corresponding to plate bending. Significant seismicity emerges about 70 km seaward from the trench, and the seismicity rate increases continuously towards the trench, indicating that the largest bending deformation occurs near the trench axis. These plate‐bending earthquakes occur along faults that facilitate the hydration of the subducting plate, and the lateral and depth distribution of earthquakes is consistent with low‐velocity regions imaged in previous studies. The forearc is marked by a heterogeneous distribution of low magnitude (<5 Mw) thrust zone seismicity, possibly due to the rough incoming plate topography and/or serpentinization of the forearc. A sequence of thrust earthquakes occurs at depths ~10 km below seafloor and within 20 km of the trench axis, demonstrating that the megathrust is seismically active nearly to the trench.
  • Article
    Lithospheric erosion in the Patagonian slab window, and implications for glacial isostasy
    (American Geophysical Union, 2022-01-18) Mark, Hannah F. ; Wiens, Douglas A. ; Ivins, Erik ; Richter, Andreas ; Mansour, Walid Ben ; Magnani, M. Beatrice ; Marderwald, Eric ; Adaros, Rodrigo ; Barrientos, Sergio
    The Patagonian slab window has been proposed to enhance the solid Earth response to ice mass load changes in the overlying Northern and Southern Patagonian Icefields (NPI and SPI, respectively). Here, we present the first regional seismic velocity model covering the entire north-south extent of the slab window. A slow velocity anomaly in the uppermost mantle indicates warm mantle temperature, low viscosity, and possibly partial melt. Low velocities just below the Moho suggest that the lithospheric mantle has been thermally eroded over the youngest part of the slab window. The slowest part of the anomaly is north of 49°S, implying that the NPI and the northern SPI overlie lower viscosity mantle than the southern SPI. This comprehensive seismic mapping of the slab window provides key evidence supporting the previously hypothesized connection between post-Little Ice Age anthropogenic ice mass loss and rapid geodetically observed glacial isostatic uplift (≥4 cm/yr).
  • Article
    Complex and diverse rupture processes of the 2018 Mw 8.2 and Mw 7.9 Tonga-Fiji deep earthquakes
    (American Geophysical Union, 2019-02-20) Fan, Wenyuan ; Wei, S. Shawn ; Tian, Dongdong ; McGuire, Jeffrey J. ; Wiens, Douglas A.
    Deep earthquakes exhibit strong variabilities in their rupture and aftershock characteristics, yet their physical failure mechanisms remain elusive. The 2018 Mw 8.2 and Mw 7.9 Tonga‐Fiji deep earthquakes, the two largest ever recorded in this subduction zone, occurred within days of each other. We investigate these events by performing waveform analysis, teleseismic P wave backprojection, and aftershock relocation. Our results show that the Mw 8.2 earthquake ruptured fast (4.1 km/s) and excited frequency‐dependent seismic radiation, whereas the Mw 7.9 earthquake ruptured slowly (2.5 km/s). Both events lasted ∼35 s. The Mw 8.2 earthquake initiated in the highly seismogenic, cold core of the slab and likely ruptured into the surrounding warmer materials, whereas the Mw 7.9 earthquake likely ruptured through a dissipative process in a previously aseismic region. The contrasts in earthquake kinematics and aftershock productivity argue for a combination of at least two primary mechanisms enabling rupture in the region.
  • Article
    Teleseismic earthquake wavefields observed on the ross ice shelf
    (Cambridge University Press, 2020-10-14) Baker, Michael G. ; Aster, Richard C. ; Wiens, Douglas A. ; Nyblade, Andrew A. ; Bromirski, Peter D. ; Gerstoft, Peter ; Stephen, Ralph A.
    Observations of teleseismic earthquakes using broadband seismometers on the Ross Ice Shelf (RIS) must contend with environmental and structural processes that do not exist for land-sited seismometers. Important considerations are: (1) a broadband, multi-mode ambient wavefield excited by ocean gravity wave interactions with the ice shelf; (2) body wave reverberations produced by seismic impedance contrasts at the ice/water and water/seafloor interfaces and (3) decoupling of the solid Earth horizontal wavefield by the sub-shelf water column. We analyze seasonal and geographic variations in signal-to-noise ratios for teleseismic P-wave (0.5–2.0 s), S-wave (10–15 s) and surface wave (13–25 s) arrivals relative to the RIS noise field. We use ice and water layer reverberations generated by teleseismic P-waves to accurately estimate the sub-station thicknesses of these layers. We present observations consistent with the theoretically predicted transition of the water column from compressible to incompressible mechanics, relevant for vertically incident solid Earth waves with periods longer than 3 s. Finally, we observe symmetric-mode Lamb waves generated by teleseismic S-waves incident on the grounding zones. Despite their complexity, we conclude that teleseismic coda can be utilized for passive imaging of sub-shelf Earth structure, although longer deployments relative to conventional land-sited seismometers will be necessary to acquire adequate data.
  • Article
    Mantle flow pattern associated with the Patagonian slab window determined from azimuthal anisotropy
    (American Geophysical Union, 2022-09-12) Ben‐Mansour, Walid ; Wiens, Douglas A. ; Mark, Hannah F. ; Russo, Raymond M. ; Richter, Andreas ; Marderwald, Eric ; Barrientos, Sergio
    Geological processes in Southern Patagonia are affected by the Patagonian slab window, formed by the subduction of the Chile Ridge and subsequent northward migration of the Chile Triple Junction. Using shear wave splitting analysis, we observe strong splitting of up to 2.5 s with an E‐W fast direction just south of the triple junction and the edge of the subducting Nazca slab. This region of strong anisotropy is coincident with low uppermost mantle shear velocities and an absence of mantle lithosphere, indicating that the mantle flow occurs in a warm, low‐viscosity, 200–300 km wide shallow mantle channel just to the south of the Nazca slab. The region of flow corresponds to a volcanic gap caused by depleted mantle compositions and absence of slab‐derived water. In most of Patagonia to the south of this channel, splitting fast directions trend NE‐SW consistent with large‐scale asthenospheric flow.Plain Language SummarySlab windows represent openings or gaps in the downgoing slab, allowing the mantle to flow through the plane of the slab from one side of the subduction zone to the other. The subduction of a spreading ridge beneath South America forms a gap in the subducting slab below Patagonia, presenting an opportunity to investigate the influence of slab windows on mantle flow and geological processes. Although this region has been poorly instrumented in the past, the deployment of new seismic instruments and available data allow us to study how the mantle seismic velocity varies with direction in the region. From the directional dependence of seismic velocity, we can infer the direction of mantle flow. We observe a change from N‐S to E‐W mantle flow in the northern part of the slab window, near the edge of the subducting Nazca plate. The flow occurs in a warm, low viscosity shallow mantle channel corresponding to a gap in activity along the volcanic arc.Key PointsShear wave splitting indicates strong anisotropy with an E‐W fast direction just south of the Chile Triple Junction and the edge of the subducting Nazca slabThe splitting and shear wave velocity structure suggest eastward shallow mantle flow in a 200–300 km wide channel around the edge of the Nazca slabIn most of southernmost Patagonia, splitting shows NE‐SW fast directions consistent with large‐scale asthenospheric flow.
  • Article
    Ross ice shelf vibrations
    (John Wiley & Sons, 2015-09-16) Bromirski, Peter D. ; Diez, Anja ; Gerstoft, Peter ; Stephen, Ralph A. ; Bolmer, S. Thompson ; Wiens, Douglas A. ; Aster, Richard C. ; Nyblade, Andrew A.
    Broadband seismic stations were deployed across the Ross Ice Shelf (RIS) in November 2014 to study ocean gravity wave-induced vibrations. Initial data from three stations 100 km from the RIS front and within 10 km of each other show both dispersed infragravity (IG) wave and ocean swell-generated signals resulting from waves that originate in the North Pacific. Spectral levels from 0.001 to 10 Hz have the highest accelerations in the IG band (0.0025–0.03 Hz). Polarization analyses indicate complex frequency-dependent particle motions, with energy in several frequency bands having distinctly different propagation characteristics. The dominant IG band signals exhibit predominantly horizontal propagation from the north. Particle motion analyses indicate retrograde elliptical particle motions in the IG band, consistent with these signals propagating as Rayleigh-Lamb (flexural) waves in the ice shelf/water cavity system that are excited by ocean wave interactions nearer the shelf front.
  • Article
    Tidal and thermal stresses drive seismicity along a major Ross Ice Shelf rift
    (American Geophysical Union, 2019-05-23) Olinger, Seth D. ; Lipovsky, Bradley P. ; Wiens, Douglas A. ; Aster, Richard C. ; Bromirski, Peter D. ; Chen, Zhao ; Gerstoft, Peter ; Nyblade, Andrew A. ; Stephen, Ralph A.
    Understanding deformation in ice shelves is necessary to evaluate the response of ice shelves to thinning. We study microseismicity associated with ice shelf deformation using nine broadband seismographs deployed near a rift on the Ross Ice Shelf. From December 2014 to November 2016, we detect 5,948 icequakes generated by rift deformation. Locations were determined for 2,515 events using a least squares grid‐search and double‐difference algorithms. Ocean swell, infragravity waves, and a significant tsunami arrival do not affect seismicity. Instead, seismicity correlates with tidal phase on diurnal time scales and inversely correlates with air temperature on multiday and seasonal time scales. Spatial variability in tidal elevation tilts the ice shelf, and seismicity is concentrated while the shelf slopes downward toward the ice front. During especially cold periods, thermal stress and embrittlement enhance fracture along the rift. We propose that thermal stress and tidally driven gravitational stress produce rift seismicity with peak activity in the winter.
  • Article
    Near-surface environmentally forced changes in the Ross Ice Shelf observed with ambient seismic noise
    (John Wiley & Sons, 2018-10-16) Chaput, Julien ; Aster, Richard C. ; McGrath, Daniel ; Baker, Michael G. ; Anthony, Robert E. ; Gerstoft, Peter ; Bromirski, Peter D. ; Nyblade, Andrew A. ; Stephen, Ralph A. ; Wiens, Douglas A. ; Das, Sarah B. ; Stevens, Laura A.
    Continuous seismic observations across the Ross Ice Shelf reveal ubiquitous ambient resonances at frequencies >5 Hz. These firn‐trapped surface wave signals arise through wind and snow bedform interactions coupled with very low velocity structures. Progressive and long‐term spectral changes are associated with surface snow redistribution by wind and with a January 2016 regional melt event. Modeling demonstrates high spectral sensitivity to near‐surface (top several meters) elastic parameters. We propose that spectral peak changes arise from surface snow redistribution in wind events and to velocity drops reflecting snow lattice weakening near 0°C for the melt event. Percolation‐related refrozen layers and layer thinning may also contribute to long‐term spectral changes after the melt event. Single‐station observations are inverted for elastic structure for multiple stations across the ice shelf. High‐frequency ambient noise seismology presents opportunities for continuous assessment of near‐surface ice shelf or other firn environments.
  • Article
    Ross ice shelf icequakes associated with ocean gravity wave activity
    (American Geophysical Union, 2019-08-01) Chen, Zhao ; Bromirski, Peter D. ; Gerstoft, Peter ; Stephen, Ralph A. ; Lee, Won Sang ; Yun, Sukyoung ; Olinger, Seth D. ; Aster, Richard C. ; Wiens, Douglas A. ; Nyblade, Andrew A.
    Gravity waves impacting ice shelves illicit a suite of responses that can affect ice shelf integrity. Broadband seismometers deployed on the Ross Ice Shelf, complemented by a near‐icefront seafloor hydrophone, establish the association of strong icequake activity with ocean gravity wave amplitudes (AG) below 0.04 Hz. The Ross Ice Shelf‐front seismic vertical displacement amplitudes (ASV) are well correlated with AG, allowing estimating the frequency‐dependent transfer function from gravity wave amplitude to icefront vertical displacement amplitude (TGSV(f)). TGSV(f) is 0.6–0.7 at 0.001–0.01 Hz but decreases rapidly at higher frequencies. Seismicity of strong icequakes exhibits spatial and seasonal associations with different gravity wave frequency bands, with the strongest icequakes observed at the icefront primarily during the austral summer when sea ice is minimal and swell impacts are strongest.
  • Article
    Ocean-excited plate waves in the Ross and Pine Island Glacier ice shelves
    (Cambridge University Press, 2018-09-12) Chen, Zhao ; Bromirski, Peter D. ; Gerstoft, Peter ; Stephen, Ralph A. ; Wiens, Douglas A. ; Aster, Richard C. ; Nyblade, Andrew A.
    Ice shelves play an important role in buttressing land ice from reaching the sea, thus restraining the rate of grounded ice loss. Long-period gravity-wave impacts excite vibrations in ice shelves that can expand pre-existing fractures and trigger iceberg calving. To investigate the spatial amplitude variability and propagation characteristics of these vibrations, a 34-station broadband seismic array was deployed on the Ross Ice Shelf (RIS) from November 2014 to November 2016. Two types of ice-shelf plate waves were identified with beamforming: flexural-gravity waves and extensional Lamb waves. Below 20 mHz, flexural-gravity waves dominate coherent signals across the array and propagate landward from the ice front at close to shallow-water gravity-wave speeds (~70 m s−1). In the 20–100 mHz band, extensional Lamb waves dominate and propagate at phase speeds ~3 km s−1. Flexural-gravity and extensional Lamb waves were also observed by a 5-station broadband seismic array deployed on the Pine Island Glacier (PIG) ice shelf from January 2012 to December 2013, with flexural wave energy, also detected at the PIG in the 20–100 mHz band. Considering the ubiquitous presence of storm activity in the Southern Ocean and the similar observations at both the RIS and the PIG ice shelves, it is likely that most, if not all, West Antarctic ice shelves are subjected to similar gravity-wave excitation.
  • Article
    Seasonal and spatial variations in the ocean-coupled ambient wavefield of the Ross Ice Shelf
    (Cambridge University Press, 2019-09-30) Baker, Michael G. ; Aster, Richard C. ; Anthony, Robert E. ; Chaput, Julien ; Wiens, Douglas A. ; Nyblade, Andrew A. ; Bromirski, Peter D. ; Gerstoft, Peter ; Stephen, Ralph A.
    The Ross Ice Shelf (RIS) is host to a broadband, multimode seismic wavefield that is excited in response to atmospheric, oceanic and solid Earth source processes. A 34-station broadband seismographic network installed on the RIS from late 2014 through early 2017 produced continuous vibrational observations of Earth's largest ice shelf at both floating and grounded locations. We characterize temporal and spatial variations in broadband ambient wavefield power, with a focus on period bands associated with primary (10–20 s) and secondary (5–10 s) microseism signals, and an oceanic source process near the ice front (0.4–4.0 s). Horizontal component signals on floating stations overwhelmingly reflect oceanic excitations year-round due to near-complete isolation from solid Earth shear waves. The spectrum at all periods is shown to be strongly modulated by the concentration of sea ice near the ice shelf front. Contiguous and extensive sea ice damps ocean wave coupling sufficiently so that wintertime background levels can approach or surpass those of land-sited stations in Antarctica.
  • Article
    Ice shelf structure derived from dispersion curve analysis of ambient seismic noise, Ross Ice Shelf, Antarctica
    (Oxford University Press, 2016-02-16) Diez, Anja ; Bromirski, Peter D. ; Gerstoft, Peter ; Stephen, Ralph A. ; Anthony, Robert E. ; Aster, Richard C. ; Cai, Chen ; Nyblade, Andrew A. ; Wiens, Douglas A.
    An L-configured, three-component short period seismic array was deployed on the Ross Ice Shelf, Antarctica during November 2014. Polarization analysis of ambient noise data from these stations shows linearly polarized waves for frequency bands between 0.2 and 2 Hz. A spectral peak at about 1.6 Hz is interpreted as the resonance frequency of the water column and is used to estimate the water layer thickness below the ice shelf. The frequency band from 4 to 18 Hz is dominated by Rayleigh and Love waves propagating from the north that, based on daily temporal variations, we conclude were generated by field camp activity. Frequency–slowness plots were calculated using beamforming. Resulting Love and Rayleigh wave dispersion curves were inverted for the shear wave velocity profile within the firn and ice to ∼150 m depth. The derived density profile allows estimation of the pore close-off depth and the firn–air content thickness. Separate inversions of Rayleigh and Love wave dispersion curves give different shear wave velocity profiles within the firn. We attribute this difference to an effective anisotropy due to fine layering. The layered structure of firn, ice, water and the seafloor results in a characteristic dispersion curve below 7 Hz. Forward modelling the observed Rayleigh wave dispersion curves using representative firn, ice, water and sediment structures indicates that Rayleigh waves are observed when wavelengths are long enough to span the distance from the ice shelf surface to the seafloor. The forward modelling shows that analysis of seismic data from an ice shelf provides the possibility of resolving ice shelf thickness, water column thickness and the physical properties of the ice shelf and underlying seafloor using passive-source seismic data.
  • Article
    The crust and upper mantle structure of central and West Antarctica from Bayesian inversion of Rayleigh Wave and receiver functions
    (John Wiley & Sons, 2018-09-22) Shen, Weisen ; Wiens, Douglas A. ; Anandakrishnan, Sridhar ; Aster, Richard C. ; Gerstoft, Peter ; Bromirski, Peter D. ; Hansen, Samantha E. ; Dalziel, Ian W. D. ; Heeszel, David S. ; Huerta, Audrey D. ; Nyblade, Andrew A. ; Stephen, Ralph A. ; Wilson, Terry J. ; Winberry, J. Paul
    We construct a new seismic model for central and West Antarctica by jointly inverting Rayleigh wave phase and group velocities along with P wave receiver functions. Ambient noise tomography exploiting data from more than 200 seismic stations deployed over the past 18 years is used to construct Rayleigh wave phase and group velocity dispersion maps. Comparison between the ambient noise phase velocity maps with those constructed using teleseismic earthquakes confirms the accuracy of both results. These maps, together with P receiver function waveforms, are used to construct a new 3‐D shear velocity (Vs) model for the crust and uppermost mantle using a Bayesian Monte Carlo algorithm. The new 3‐D seismic model shows the dichotomy of the tectonically active West Antarctica (WANT) and the stable and ancient East Antarctica (EANT). In WANT, the model exhibits a slow uppermost mantle along the Transantarctic Mountains (TAMs) front, interpreted as the thermal effect from Cenozoic rifting. Beneath the southern TAMs, the slow uppermost mantle extends horizontally beneath the traditionally recognized EANT, hypothesized to be associated with lithospheric delamination. Thin crust and lithosphere observed along the Amundsen Sea coast and extending into the interior suggest involvement of these areas in Cenozoic rifting. EANT, with its relatively thick and cold crust and lithosphere marked by high Vs, displays a slower Vs anomaly beneath the Gamburtsev Subglacial Mountains in the uppermost mantle, which we hypothesize may be the signature of a compositionally anomalous body, perhaps remnant from a continental collision.
  • Article
    Near-surface seismic anisotropy in Antarctic glacial snow and ice revealed by high-frequency ambient noise
    (Cambridge University Press, 2022-12-19) Chaput, Julien ; Aster, Rick ; Karplus, Marianne ; Nakata, Nori ; Gerstoft, Peter ; Bromirski, Peter D. ; Nyblade, Andrews A. ; Stephen, Ralph A. ; Wiens, Douglas A.
    Ambient seismic recordings taken at broad locations across Ross Ice Shelf and a dense array near West Antarctic Ice Sheet (WAIS) Divide, Antarctica, show pervasive temporally variable resonance peaks associated with trapped seismic waves in near-surface firn layers. These resonance peaks feature splitting on the horizontal components, here interpreted as frequency-dependent anisotropy in the firn and underlying ice due to several overlapping mechanisms driven by ice flow. Frequency peak splitting magnitudes and fast/slow axes were systematically estimated at single stations using a novel algorithm and compared with good agreement with active source anisotropy measurements at WAIS Divide determined via active sources recorded on a 1 km circular array. The approach was further applied to the broad Ross Ice Shelf (RIS) array, where anisotropy axes were directly compared with visible surface features and ice shelf flow lines. The near-surface firn, depicted by anisotropy above 30 Hz, was shown to exhibit a novel plastic stretching mechanism of anisotropy, whereby the fast direction in snow aligns with accelerating ice shelf flow.
  • Article
    Tsunami and infragravity waves impacting Antarctic ice shelves
    (John Wiley & Sons, 2017-07-20) Bromirski, Peter D. ; Chen, Zhao ; Stephen, Ralph A. ; Gerstoft, Peter ; Arcas, Diego R. ; Diez, Anja ; Aster, Richard C. ; Wiens, Douglas A. ; Nyblade, Andrew A.
    The responses of the Ross Ice Shelf (RIS) to the 16 September 2015 8.3 (Mw) Chilean earthquake tsunami (>75 s period) and to oceanic infragravity (IG) waves (50–300 s period) were recorded by a broadband seismic array deployed on the RIS from November 2014 to November 2016. Here we show that tsunami and IG-generated signals within the RIS propagate at gravity wave speeds (∼70 m/s) as water-ice coupled flexural-gravity waves. IG band signals show measureable attenuation away from the shelf front. The response of the RIS to Chilean tsunami arrivals is compared with modeled tsunami forcing to assess ice shelf flexural-gravity wave excitation by very long period (VLP; >300 s) gravity waves. Displacements across the RIS are affected by gravity wave incident direction, bathymetry under and north of the shelf, and water layer and ice shelf thicknesses. Horizontal displacements are typically about 10 times larger than vertical displacements, producing dynamical extensional motions that may facilitate expansion of existing fractures. VLP excitation is continuously observed throughout the year, with horizontal displacements highest during the austral winter with amplitudes exceeding 20 cm. Because VLP flexural-gravity waves exhibit no discernable attenuation, this energy must propagate to the grounding zone. Both IG and VLP band flexural-gravity waves excite mechanical perturbations of the RIS that likely promote tabular iceberg calving, consequently affecting ice shelf evolution. Understanding these ocean-excited mechanical interactions is important to determine their effect on ice shelf stability to reduce uncertainty in the magnitude and rate of global sea level rise.
  • Article
    Upper mantle hydration indicated by decreased shear velocity near the Southern Mariana Trench from Rayleigh wave tomography
    (American Geophysical Union, 2021-07-26) Zhu, Gaohua ; Wiens, Douglas A. ; Yang, Hongfeng ; Lin, Jian ; Xu, Min ; You, Qingyu
    Reduction of seismic velocities has been employed to study the hydration of incoming plates and forearc mantle in recent years. However, few constraints have been obtained in the Southern Mariana Trench. We use an ocean bottom seismograph (OBS) deployment to conduct Rayleigh wave tomographic studies to derive the SV-wave velocity structure near the Southern Mariana Trench. Measured group and phase velocities as a function of period are inverted to determine the SV-wave velocity using a Bayesian Monte Carlo algorithm. The incoming Pacific Plate is characterized by low velocities (3.6–4.1 km/s) within the upper ∼25 km of the mantle near the trench, indicating extensive mantle hydration of the incoming plate in southern Mariana. The velocity reduction in the forearc mantle is not as large as in central Mariana, most likely indicating a lower forearc serpentinization in this region, which is consistent with the absence of serpentinite mud volcanoes.
  • Article
    Constraints on bend‐faulting and mantle hydration at the Marianas Trench from seismic anisotropy
    (American Geophysical Union, 2023-05-13) Mark, Hannah F. ; Lizarralde, Daniel ; Wiens, Douglas A.
    Subduction zones are a key link between the surface water cycle and the solid Earth, as the incoming plate carries pore water and hydrous minerals into the subsurface. However, water fluxes from surface to subsurface reservoirs over geologic time are highly uncertain because the volume of water carried in hydrous minerals in the slab mantle is poorly constrained. Estimates of slab mantle hydration based on seismic tomography assume bulk serpentinization, representing an upper bound on water volume. We measure azimuthal seismic anisotropy near the Marianas Trench, use spatial variations in anisotropy to constrain the extent and geometry of bend‐related faulting, and place a lower bound on slab mantle water content for the case where serpentinization is confined within fault zones. The seismic observations can be explained by a minimum of ∼0.85 wt% water in the slab mantle, compared to the upper bound of ∼2 wt% obtained from tomography.