P and S velocity tomography of the Mariana subduction system from a combined land-sea seismic deployment

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We investigate the upper mantle seismic structure in the Tonga‐Lau‐Fiji region by jointly fitting the phase velocities of Rayleigh waves from ambient‐noise and two‐plane‐wave tomography. The results suggest a wide low‐velocity zone beneath the Lau Basin, with a minimum SV‐velocity of about 3.7 ± 0.1 km/s, indicating upwelling hot asthenosphere with extensive partial melting. The variations of velocity anomalies along the Central and Eastern Lau Spreading Centers suggest varying mantle porosity filled with melt. In the north where the spreading centers are distant from the Tonga slab, the inferred melting commences at about 70 km depth, and forms an inclined zone in the mantle, dipping to the west away from the arc. This pattern suggests a passive decompression melting process supplied by the Australian plate mantle from the west. In the south, as the supply from the Australian mantle is impeded by the Lau Ridge lithosphere, flux melting controlled by water from the nearby slab dominates in the back‐arc. This source change results in the rapid transition in geochemistry and axial morphology along the spreading centers. The remnant Lau Ridge and the Fiji Plateau are characterized by a 60–80 km thick lithosphere underlain by a low‐velocity asthenosphere. Our results suggest the removal of the lithosphere of the northeastern Fiji Plateau‐Lau Ridge beneath the active Taveuni Volcano. Azimuthal anisotropy shows that the mantle flow direction rotates from trench‐perpendicular beneath Fiji to spreading‐perpendicular beneath the Lau Basin, which provides evidence for the southward flow of the mantle wedge and the Samoan plume.

Section: Clsc-elsc-vfr Tofua Arc and Tonga Ridgementioning

confidence: 93%

Upper mantle structure of the Tonga‐Lau‐Fiji region from Rayleigh wave tomography

Wei

Zha

Shen³

et al. 2016

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“…The seismicity along the Mariana megathrust fault is characterized by a heterogeneous distribution, which has been attributed to subducting topography and/or partial serpentinization of the forearc (Emry et al, 2011). There is strong evidence for significant mantle wedge serpentinization, on the basis of active serpentine seamounts on the outer forearc (Fryer, 1996) and seismic imaging of the mantle wedge (Barklage et al, 2015; Cai et al, 2018; Pyle et al, 2010; Tibi et al, 2008), though the distribution may be heterogeneous. These characteristics make it an ideal location to investigate megathrust microseismicity and slip properties that may help explain the absence of larger events.…”

Section: Introductionmentioning

confidence: 99%

Seismicity of the Incoming Plate and Forearc Near the Mariana Trench Recorded by Ocean Bottom Seismographs

Eimer

Wiens

Cai

et al. 2020

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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 M w ) 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. Plain Language Summary Studying earthquakes near oceanic trenches is important forunderstanding subduction zones but can be difficult using only distant land-based instruments. This study uses seismographs designed to work on the seafloor to study earthquakes near the central Mariana Trench. As the subducting plate bends, faults form due to the increased extensional stress. These faults can become pathways for water to penetrate into this subducting plate. Understanding the amount of water stored in the plate is essential for constraining the global water cycle, and the earthquake distribution allows us to determine the distribution of active faults. We found that the earthquakes occur shallower than 35-km depth and within 70 km seaward of the trench. We also study earthquakes occurring along the megathrust, which is the interface between the subducting plate and the overriding plate that is prone to seismic activity. We found that the earthquakes show a patchy distribution, indicating that the megathrust interface is not uniform. This could be related to topographic features on the subducting plate interacting with the plate above. We also observed earthquakes at depths shallower than 10 km below the seafloor, indicating that the megathrust can rupture close to the trench.

“…Seismic tomography is an imaging technique used to map three-dimensional variations in the seismicvelocity structure of the Earth's interior at global (e.g., French & Romanowicz, 2014;Grand, 2002;Ritsema et al, 2004;Van der Hilst et al, 1997) or regional scales (e.g., Barklage et al, 2015;Bedle & Van der Lee, 2009;Burdick et al, 2017;Schmandt & Lin, 2014;Van der Lee & Frederiksen, 2005). Regional variations in velocity structure are always relative to one-dimensional seismic velocity profiles that change only with depth.…”

Section: Introductionmentioning

confidence: 99%

HyMaTZ: A Python Program for Modeling Seismic Velocities in Hydrous Regions of the Mantle Transition Zone

Wang

Barklage

Lou

et al. 2018

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Mapping the spatial distribution of water in the mantle transition zone (MTZ, 410‐ to 660‐km depth) may be approached by combining thermodynamic and experimental mineral physics data with regional studies of seismic velocity and seismic discontinuity structure. HyMaTZ (Hydrous Mantle Transition Zone) is a Python program with graphical user interface, which calculates and displays seismic velocities for different scenarios of hydration in the MTZ for comparison to global or regional seismic‐velocity models. The influence of water is applied through a regression to experimental data on how H2O influences the thermoelastic properties of (Mg,Fe)2SiO4 polymorphs: olivine, wadsleyite, and ringwoodite. Adiabatic temperature profiles are internally consistent with dry phase proportion models; however, modeling hydration in HyMaTZ affects only velocities and not phase proportions or discontinuity structure. For wadsleyite, adding 1.65 wt% H2O or increasing the iron content by 7 mol% leads to roughly equivalent reductions in VS as raising the temperature by 160 K with a pyrolite model in the upper part of the MTZ. The eastern U.S. low‐velocity anomaly, which has been interpreted as the result of dehydration of the Farallon slab in the top of the lower mantle, is consistent with hydration of wadsleyite to about 20% of its water storage capacity in the upper MTZ. Velocity gradients with depth in absolute shear velocity models are steeper in all seismic models than all mineralogical models, suggesting that the seismic velocity gradients should be lowered or varied with depth and/or an alternative compositional model is required.