ALBERT

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.

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.

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.