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  • 1
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    In:  Science, Philadelphia, 4, vol. 296, no. 5567, pp. 485-489, pp. B05318, (ISSN: 1340-4202)
    Publication Date: 2002
    Keywords: Seismology ; Anisotropy ; Plate tectonics
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  • 2
    Publication Date: 2005
    Keywords: Earthquake ; Tsunami(s) ; Earthquake hazard ; Indonesia ; Banda ; Aceh ; seismic Moment ; Surface waves ; Source ; Modelling ; Crustal deformation (cf. Earthquake precursor: deformation or strain) ; slow ; red ; silent ; contribution ; Camp
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  • 3
    Publication Date: 2016-07-21
    Description: Kawakatsu and Abe [2016] have highlighted the potential complicating effect of sediment reverberations on the analysis and interpretation of crust and mantle phases inferred from receiver functions analyzed from ocean-bottom seismograms. In their comment, they identify resonant peaks in the power spectrum at one of the stations, T06 , in the analysis of [ Olugboji et al ., 2016], and demonstrate with synthetic modeling how sediment-induced resonances can cause instability in the recovered receiver-function (RF) traces. They also request a detailed explanation of how LQT rotation is conducted, and why its use leads to stable receiver functions in the analysis of Olugboji et al . [2016]. We welcome this query as an opportunity to highlight certain technical aspects of the data-analysis procedures used in Olugboji et al [2016]. Our methods derive partly from methods recommended by previous studies of receiver functions estimated from seismic seafloor data [ Bostock and Trehu , 2012; Janiszewski and Abers , 2015; Audet , 2016], particularly the use of the modal wavefield decomposition [e.g., Reading et al , 2003]) (which we approximated by the LQT rotation) to suppress reverberation signals in the overlying water column [ Bostock and Trehu , 2012]. This article is protected by copyright. All rights reserved.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 4
    Publication Date: 2016-03-08
    Description: Receiver-function observations in the oceanic upper mantle can test causal mechanisms for the depth, sharpness and age-dependence of the seismic wavespeed decrease thought to mark the lithosphere-asthenosphere boundary (LAB). We use a combination of frequency-dependent harmonic decomposition of receiver functions and synthetic forward-modeling to provide new seismological constraints on this “seismic LAB” from 17 ocean-bottom stations and 2 borehole stations in the Philippine Sea and northwest Pacific Ocean. Underneath young oceanic crust, the seismic LAB depth follows the ∼1300 K isotherm but a lower isotherm (∼1000 K) is suggested in the Daito ridge, the Izu-Bonin-Mariana trench and the northern Shikoku basin. Underneath old oceanic crust, the seismic LAB lies at a constant depth ∼70 km. The age-dependence of the seismic LAB depth is consistent with either a transition to partial-melt conditions or a sub-solidus rheological change as the causative factor. The age-dependence of interface sharpness provides critical information to distinguish these two models. Underneath young oceanic crust, the velocity gradient is gradational, while for old oceanic crust a sharper velocity gradient is suggested by the receiver functions. This behavior is consistent with the prediction of the sub-solidus model invoking anelastic relaxation mediated by temperature and water-content, but is not readily explained by a partial-melt model. The Ps conversions display negligible two-lobed or four-lobed back-azimuth dependence in harmonic stacks, suggesting that a sharp change in azimuthal anisotropy with depth is not responsible for them. We conclude that these ocean-bottom observations indicate a sub-solidus elastically-accommodated grain-boundary sliding (EAGBS) model for the seismic LAB. Because EAGBS does not facilitate long-term ductile deformation, the Seismic LAB may not coincide with the conventional transition from lithosphere to asthenosphere. This article is protected by copyright. All rights reserved.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 5
    Publication Date: 2011-06-14
    Description: Orogeny in the geologically young northern Apennines is explained by the eastward retreat of a convergence zone in which Adriatic lithosphere subducts to the west. It is unclear, however, whether all of the lithosphere or only the lower portion has subducted since 15 Ma, when the convergence zone lay near Corsica. We combine teleseismic P and S wave arrival time data from the RETREAT seismic network (2003–2006) and surrounding permanent stations to estimate tomographic images of the upper mantle structure beneath the northern Apennines. We image a vertically oriented slab that that extends to only ∼300 km depth. Our slab termination is shallower than previous studies but is confirmed by resolution tests. Furthermore, our images resolve the southern edge of the northern Apennines slab at ∼43°N, with no deep continuity with any slab segment to the south, as earlier proposed. Our results suggest that only a 300–400 km strip of lithosphere has subducted since 15 Ma beneath the northern Tyrrhenian Sea, a length roughly equivalent to the distance from the present-day west coast of Corsica to the crest of the northern Apennines. Although not a definitive indicator, the largely aseismic vertical slab configuration and its limited extent, coupled with other volcanic and geophysical indicators, suggests the delamination scenario of Bird (1979), in which only the lower portion of the continental lithosphere subducts.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 6
    Publication Date: 2015-07-01
    Description: Nature Geoscience 8, 509 (2015). doi:10.1038/ngeo2462 Authors: Shun-ichiro Karato, Tolulope Olugboji & Jeffrey Park
    Print ISSN: 1752-0894
    Electronic ISSN: 1752-0908
    Topics: Geosciences
    Published by Springer Nature
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  • 7
    Publication Date: 2013-02-13
    Description: ABSTRACT [1]  We explore possible models for the seismological signature of the oceanic lithosphere-asthenosphere boundary (LAB) using the latest mineral-physics observations. The key features that need to be explained by any viable model include (1) a sharp (〈20 km width) and a large (5-10%) velocity drop, (2) LAB depth at ~70 km in the old oceanic upper mantle and (3) an age-dependent LAB depth in the young oceanic upper mantle. We examine the plausibility of both partial melt and sub-solidus models. Because many of the LAB observations in the old oceanic regions are located in areas where temperature is ~1000-1200 °K, significant partial melting is difficult. We examine a layered model and a melt accumulation model (at the LAB) and show that both models are difficult to reconcile with seismological observations. A sub-solidus model assuming absorption-band physical dispersion is inconsistent with the large velocity drop at the LAB. We explore a new sub-solidus model, originally proposed by Karato [2012], that depends on grain-boundary sliding. In contrast to the previous model where only the absorption-band behavior was assumed, the new model predicts an age-dependent LAB structure including the age-dependent LAB depth and its sharpness. Strategies to test these models are presented.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 8
    Publication Date: 2016-01-29
    Description: In the Japan subduction zone, a locally-depressed 660-discontinuity has been observed beneath northeast Asia, suggesting downwelling of materials from the mantle transition zone (MTZ). Vertical transport of water-rich MTZ-materials across the major mineral phase changes could lead to water release and to partial melting in surrounding mantle regions, causing seismic low-velocity anomalies. Melt layers implied by low-velocity zones (LVZs) above the 410-discontinuity have been detected in many regions, but seismic evidence for partial melting below the 660-discontinuity has been limited. High-frequency migrated Ps receiver-functions indicate LVZs below the depressed 660-discontinuity and above the 410-discontinuity in the deep Japan subduction zone, suggesting dehydration melting induced by water transport out of the MTZ. Our results provide insights into water circulation associated with dynamic interactions between the subducted slab and surrounding mantle.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 9
    Publication Date: 2016-01-21
    Description: ABSTRACT Crustal anisotropy beneath ocean islands can be attributed to preferentially aligned minerals, cracks, or dike structures. Stacked with harmonic weighting, receiver functions from permanent ocean-island stations display evidence of strong and distinct anisotropy parameters in the underlying crust and underplated layer. We analyze data for eleven IRIS-GSN stations in the Pacific Ocean. We observe the prevalence of two-lobed receiver function (RF) amplitude variations with back-azimuth, consistent with “slow” tilted-axis anisotropy. In most cases the anisotropy is accommodated in the underplated crust. Synthetic modeling of a representative station indicates that the strength of anisotropy of Vp=10% and Vs=5% is possible. The strike direction of the inferred symmetry axis tends to align with plate motion, with some scatter. At stations in the northwest Pacific i.e. KWAJ, TARA, and WAKE, the strike direction of the symmetry axis aligns with plate motion at the time of volcano emplacement. Beneath station POHA and the closest stations to the present-day Hawaiian hotspot, alignment of the symmetry axis is almost orthogonal to the plate motion. We attribute the crustal anisotropy to the preferred alignment of dike structures that transported asthenospheric magma toward the seafloor volcanic edifice. Our results suggest that the thermal-plume origin for ocean islands must be supplemented by tectonic-stress heterogeneities that allow magma to penetrate the lithosphere via fractures. Magma-transport fractures should align normal to the least-compressive direction, which are predicted by theoretical models to align approximately with plate motion at the time of emplacement. This article is protected by copyright. All rights reserved.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 10
    Publication Date: 2019
    Description: Abstract High‐frequency harmonic regression (2.0‐4.0‐Hz cutoff) of receiver functions at two long‐running seismic observatories at mid‐Pacific hotspot islands confirms earlier detections of this underplated material with seismic velocities intermediate to crust and mantle, and reveals it to be multilayered and anisotropic within ~30 km of the surface. Magmatic underplating beneath the oceanic Moho has been proposed to accompany basaltic melt that erupts at the seafloor and (eventually) atop a subaerial volcano. An alternate hypothesis is “metasomatic underplating” whereby crustal fractures developed during magma ascent allow seawater to infiltrate and to serpentinize the sub‐Moho mantle partially. Metasomatic underplating would lower seismic wavespeeds, promote the buoyancy of the hotspot swell, and induce textural anisotropy as metamorphic expansion of olivine‐rich peridotite promotes a crack network along which serpentinization spreads. Differential expansion of mantle peridotite and crustal gabbro promotes cracks in the crust that offer new pathways for seawater to descend to the Moho, allowing metasomatic underplating to expand laterally and to contribute anisotropy to the underplated layer. Rare serpentinized mantle xenoliths confirm that crack textures can develop during serpentinization at depth. The discovery of iron‐oxidizing microbial mats on the seafloor flank of the Loihi volcano, and many locations of diffuse low‐temperature venting worldwide, is consistent with the circulation of metasomatic fluids with reducing chemistry, sourced from serpentinization at depth. Post‐eruptive uplift of Santa Maria Island (Azores), and asymmetry of the Hawaiian swell, suggests that underplating requires 2‐4 Myr to complete, suggesting that fluid infiltration is slow, subject to cycles of blockage and fresh fracturing.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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