Global adjoint tomography: first-generation model

Abstract

We present the first-generation global tomographic model constructed based on adjoint tomography, an iterative full-waveform inversion technique. Synthetic seismograms were calculated using GPU-accelerated spectral-element simulations of global seismic wave propagation, accommodating effects due to 3-D anelastic crust & mantle structure, topography & bathymetry, the ocean load, ellipticity, rotation, and self-gravitation. Fréchet derivatives were calculated in 3-D anelastic models based on an adjoint-state method. The simulations were performed on the Cray XK7 named 'Titan', a computer with 18 688 GPU accelerators housed at Oak Ridge National Laboratory. The transversely isotropic global model is the result of 15 tomographic iterations, which systematically reduced differences between observed and simulated three-component seismograms. Our starting model combined 3-D mantle model S362ANI with 3-D crustal model Crust2.0. We simultaneously inverted for structure in the crust and mantle, thereby eliminating the need for widely used 'crustal corrections'. We used data from 253 earthquakes in the magnitude range 5.8 ≤ M w ≤ 7.0. We started inversions by combining ∼30 s body-wave data with ∼60 s surface-wave data. The shortest period of the surface waves was gradually decreased, and in the last three iterations we combined ∼17 s body waves with ∼45 s surface waves. We started using 180 min long seismograms after the 12th iteration and assimilated minor-and major-arc body and surface waves. The 15th iteration model features enhancements of well-known slabs, an enhanced image of the Samoa/Tahiti plume, as well as various other plumes and hotspots, such as Caroline, Galapagos, Yellowstone and Erebus. Furthermore, we see clear improvements in slab resolution along the Hellenic and Japan Arcs, as well as subduction along the East of Scotia Plate, which does not exist in the starting model. Point-spread function tests demonstrate that we are approaching the resolution of continentalscale studies in some areas, for example, underneath Yellowstone. This is a consequence of our multiscale smoothing strategy in which we define our smoothing operator as a function of the approximate Hessian kernel, thereby smoothing gradients less wherever we have good ray coverage, such as underneath North America.

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).…”

Pillars of the Mantle

“…Recently generated results [4] already indicate several prominent features reported in high-resolution continental studies, such as major slabs (Hellenic, Japan, Bismarck, Sandwich, etc.) and enhancement in plume structures (the Pacific superplume, the Hawaii hot spot, etc.).…”

Linear Array Ambient Noise Adjoint Tomography Reveals Intense Crust‐Mantle Interactions in North China Craton

“…Once Fréchet derivatives related to all virtual sources are computed, the adjoint tomography workflow continues to a postprocessing phase and then a model update (Bozdag et al, 2016).…”

“…However, most of these studies employed the so-called traditional ambient noise tomography method based on ray theory, which does not consider complex wave propagation phenomena in heterogeneous media. Utilizing these seismic wave simulations, adjoint-state methods can efficiently incorporate the full nonlinearity of wave propagation in iterative seismic inversions (e.g., Bozdag et al, 2016;Chen et al, 2015Chen et al, , 2017Fichtner & Villaseñor, 2015;Fichtner et al, 2009Fichtner et al, , 2010Liu & Gu, 2012;Liu & Tromp, 2006Tape et al, 2009Tape et al, , 2010Tromp et al, 2005;Zhu et al, 2012;Zhu & Tromp, 2013, Zhu et al, 2015. Recent advances in numerical methods for the wave equation combined with developments in high-performance computation (HPC) have enabled routine simulation of seismic wave propagation in realistic 2-D and 3-D Earth models based on the spectral-element method (SEM) (Komatitsch et al, 2004;Komatitsch & Tromp, 2002a, 2002b.…”