An automatically updated S‐wave model of the upper mantle and the depth extent of azimuthal anisotropy

Abstract

We present 3D2015_07Sv, an S wave model of the upper mantle based on the waveform modeling of 1,359,470 Rayleigh waves recorded since 1976. The use of approximate forward theory and modeling allows updating the model with new data on a regular basis. 3D2015_07Sv contains azimuthal anisotropy, achieves a lateral resolution of ∼600 km, and is consistent with other recent models up to degree 60 in the uppermost 200 km and degree 15 in the transition zone. Although radial anisotropy has been found to extend deeper beneath continents than beneath oceans, we find no such difference for azimuthal anisotropy, suggesting that beneath most continents, the alignment of olivine crystal is preferentially horizontal and azimuthally random at large scale. As most continents are located on slow moving plates, this supports the idea that azimuthal anisotropy aligns at large scale with the present plate motion only for plates faster than ∼4 cm yr−1.

Aseismic Deep Slab and Mantle Flow Beneath Alaska: Insight From Anisotropic Tomography

“…The trench‐normal FVDs below the slab may reflect the entrained mantle flow due to the downgoing Pacific slab as suggested by the previous studies (Christensen & Abers, ; Hanna & Long, ; Perttu et al, ), but a larger‐scale flow related to the Pacific plate motion is also possible due to the consistency of the FVDs and the absolute plate motion (Perttu et al, ). The global AAN models also indicate a good fit between the FVD in the asthenosphere (at 200‐km depth) and the absolute plate motion of the near‐trench incoming Pacific plate (Becker et al, ; Debayle et al, ; Schaeffer et al, ), which may indicate a large‐scale mantle flow in the NW‐SE direction relevant to the plate motion.…”

“…Considering the orientation of the symmetry axis of anisotropy within the Pacific plate due to the northwestward bending, the original anisotropy would have an NW‐SE (trench‐normal) FVD before the plate subduction, if we assume that the present anisotropy we obtained has not been overprinted by the subduction‐related processes. Recent global AAN models also revealed trench‐normal FVDs in the lithosphere of the Pacific plate before subduction (e.g., Debayle et al, ; Schaeffer et al, ). The consistency of these AAN results in the Pacific plate may indicate that the original frozen‐in anisotropy in the slab is well preserved.…”

Numerical Modeling of Mantle Flow Beneath Madagascar to Constrain Upper Mantle Rheology Beneath Continental Regions

“…Scholz et al () also suggest that most of their anisotropy observations surrounding Madagascar and in the Indian Ocean poorly align with the present‐day motion of the Somalian plate. They argue that the Somalian plate motion is too slow, 2.7 cm/yr (e.g., Argus et al, ), such that the shear induced LPOs are expected to be weak (Debayle et al, ).…”

Automated Large‐Scale Full Seismic Waveform Inversion for North America and the North Atlantic

“…The color scale is identical for all plots, and the deviations are in reference to the lateral mean of each model at any given depth. Models range from regional to global scales and have been generated using full‐waveform inversion with adjoint techniques similar to this study ( GLAD‐M15 , Bozdağ et al, ; US 22 , Zhu et al, ), full‐waveform inversion with asymptotic gradients ( SEMum_NA14 , Yuan et al, ), by inverting for fundamental and higher‐mode Rayleigh waveforms from vertical component seismograms using partitioned waveform inversion ( NA07 , Bedle & Van Der Lee, ), by inverting for Rayleigh wave phase velocities ( Porter, Liu, Holt , Porter et al, ; US.2016 ; Shen & Ritzwoller, additionally inverted for group velocities), and by inverting for path‐averaged shear velocities ( 3D2017_09Sv , Debayle et al, , has its own reference model).…”