ALBERT

Description: A large data set of surface-wave phase anomalies, body-wave travel times, normal-mode splitting functions and long-period waveforms is used to investigate the scaling between shear velocity, density and compressional velocity in the Earth's mantle. We introduce a methodology that allows construction of joint models with various levels of scaling complexity ( , ), in order to detect seismological signatures of chemical heterogeneity. We demonstrate that the datasets considered cannot be fit concurrently with a uniform ν or a positive and uniform ϱ throughout the mantle. The variance reductions to P-wave travel times and v P -sensitive modes are up to 40 percent higher with our preferred model of anisotropic shear and compressional velocity than the recent anisotropic shear-velocity model S362ANI+M, which was constructed assuming a uniform ν throughout the mantle. Several features reported in earlier tomographic studies persist after the inclusion of new and larger data sets; anti-correlation between bulk-sound and shear velocities in the lowermost mantle as well as an increase in ν with depth in the lower mantle are largely independent of the regularization scheme. When correlations between density and shear-velocity variations are imposed in the lowermost mantle, variance reductions of several spheroidal and toroidal modes deteriorate by as much as 40 percent. Recent measurements of the splitting of 0 S 2 , in particular, are largely incompatible with perfectly correlated shear-velocity and density heterogeneity throughout the mantle. A way to significantly improve the fits to various data sets is by allowing independent density perturbations in the lowermost mantle. Our preferred joint model consists of denser-than-average anomalies (∼1 % peak-to-peak) at the base of the mantle roughly coincident with the low-velocity superplumes. The relative variation of shear velocity, density and compressional velocity in our study disfavors a purely thermal contribution to heterogeneity in the lowermost mantle, with implications for the long-term stability and evolution of superplumes.

Description: Author(s): A. Ekström, G. R. Jansen, K. A. Wendt, G. Hagen, T. Papenbrock, B. D. Carlsson, C. Forssén, M. Hjorth-Jensen, P. Navrátil, and W. Nazarewicz With the goal of developing predictive ab initio capability for light and medium-mass nuclei, two-nucleon and three-nucleon forces from chiral effective field theory are optimized simultaneously to low-energy nucleon-nucleon scattering data, as well as binding energies and radii of few-nucleon syste... [Phys. Rev. C 91, 051301] Published Fri May 01, 2015

Description: 〈span〉〈div〉SUMMARY〈/div〉We develop a procedure to improve estimates of relative earthquake locations using Rayleigh and Love wave arrivals for multiple earthquakes recorded at common stations. We fit predicted differential traveltimes to those measured using a cross-correlation technique, and correct the phases of the cross-correlation functions for phase delays that result from the surface-wave radiation patterns. We develop an empirical measure of uncertainty that provides realistic estimates of the errors in the earthquake locations. We investigate the effectiveness of the relocation procedure by first applying it to two suites of synthetic earthquakes. We then relocate real earthquakes in three separate regions: two ridge-transform systems and one subduction zone. We demonstrate that the inclusion of source corrections in the relocation procedure results in improved location estimates compared to relocations without source corrections. The source correction also allows for automated application of the relocation procedure, even in regions with a wide range of earthquake focal mechanisms.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉We present new anisotropic phase-velocity maps of the Pacific basin for Rayleigh and Love waves between 25 s and 250 s. The isotropic and anisotropic phase-velocity maps are obtained by inversion of a dataset of single-station surface-wave phase-anomaly measurements recorded for paths crossing the Pacific basin. We develop an age-dependent gradient-damping scheme that allows us to reduce the amount of smoothness damping required in the inversion. The observed isotropic phase velocities have a strong age dependence, and our results are consistent with models of halfspace cooling: simple phase-velocity models that depend only on seafloor age explain 40 − 97% of the data variance for Love waves and 20 − 97% for Rayleigh waves. These values represent a large fraction, ranging from 0.55-0.99, of the variance reduction of our best-fitting phase-velocity models. We find that 2ζ azimuthal anisotropy is required to fit our Rayleigh wave phase-anomaly dataset but that our data do not require Love wave anisotropy. Rayleigh wave anisotropy also exhibits a clear age dependence, with a large decrease in the magnitude of 2ζ azimuthal anisotropy for seafloor older than 70 Ma that cannot be explained simply as a change in anisotropy direction between the lithosphere and asthenosphere. Long-period Rayleigh wave anisotropy directions align well overall with absolute-plate-motion directions, with a median angular misfit of 20° at 125 s. However, we observe large areas within the Pacific basin with a small but consistent offset of 10° − 20° between the two directions. The disagreement between absolute plate motion and anisotropy for long-period waves suggests the presence of mantle flow beneath the base of the plate in a direction other than absolute plate motion.〈/span〉

Description: Author(s): S. Binder, A. Ekström, G. Hagen, T. Papenbrock, and K. A. Wendt We develop interactions from chiral effective field theory (EFT) that are tailored to the harmonic oscillator basis. As a consequence, ultraviolet convergence with respect to the model space is implemented by construction and infrared convergence can be achieved by enlarging the model space for the … [Phys. Rev. C 93, 044332] Published Mon Apr 25, 2016

Description: Author(s): A. Bansal, S. Binder, A. Ekström, G. Hagen, G. R. Jansen, and T. Papenbrock We compute the medium-mass nuclei O 16 and Ca 40 using pion-less effective field theory (EFT) at next-to-leading order (NLO). The low-energy coefficients of the EFT Hamiltonian are adjusted to experimental data for nuclei with mass numbers A = 2 and 3, or alternatively to results from lattice quantum ch... [Phys. Rev. C 98, 054301] Published Thu Nov 01, 2018

Description: Author(s): A. Ekström, G. Baardsen, C. Forssén, G. Hagen, M. Hjorth-Jensen, G. R. Jansen, R. Machleidt, W. Nazarewicz, T. Papenbrock, J. Sarich, and S. M. Wild We optimize the nucleon-nucleon interaction from chiral effective field theory at next-to-next-to-leading order (NNLO). The resulting new chiral force NNLO opt yields χ 2 ≈1 per degree of freedom for laboratory energies below approximately 125 MeV. In the A =3, 4 nucleon systems, the contributions of th... [Phys. Rev. Lett. 110, 192502] Published Tue May 07, 2013

Description: Author(s): G. Baardsen, A. Ekström, G. Hagen, and M. Hjorth-Jensen Background: Coupled-cluster (CC) theory is a widely used many-body method for studying strongly correlated many-fermion systems. It allows for systematic inclusions of complicated many-body correlations beyond a mean field. Recent applications to finite nuclei have shown that first-principles approa... [Phys. Rev. C 88, 054312] Published Thu Nov 14, 2013

Description: 〈span〉〈div〉Summary〈/div〉We develop a procedure to improve estimates of relative earthquake locations using Rayleigh and Love wave arrivals for multiple earthquakes recorded at common stations. We fit predicted differential travel times to those measured using a cross-correlation technique, and correct the phases of the cross-correlation functions for phase delays that result from the surface-wave radiation patterns. We develop an empirical measure of uncertainty that provides realistic estimates of the errors in the earthquake locations. We investigate the effectiveness of the relocation procedure by first applying it to two suites of synthetic earthquakes. We then relocate real earthquakes in three separate regions: two ridge-transform systems and one subduction zone. We demonstrate that the inclusion of source corrections in the relocation procedure results in improved location estimates compared to relocations without source corrections. The source correction also allows for automated application of the relocation procedure, even in regions with a wide range of earthquake focal mechanisms.〈/span〉

Description: Author(s): A. Ekström, G. Hagen, T. D. Morris, T. Papenbrock, and P. D. Schwartz We construct a nuclear interaction in chiral effective field theory with explicit inclusion of the Δ -isobar Δ ( 1232 ) degree of freedom at all orders up to next-to-next-to-leading order (NNLO). We use pion-nucleon ( π N ) low-energy constants (LECs) from a Roy-Steiner analysis of π N scattering data, opti... [Phys. Rev. C 97, 024332] Published Mon Feb 26, 2018