J362D28: a new joint model of compressional and shear velocity in the Earth's mantle

Abstract

SUMMARY The discrimination between chemical and thermal heterogeneity in the Earth's mantle remains one of the most important and challenging questions to be answered by observational and theoretical geophysics. To answer this question requires a thorough knowledge of the ratio between compressional and shear velocity anomalies. We describe results of a joint inversion for compressional and shear velocity in the mantle using a large and diverse data set consisting of traveltimes, complete waveforms and surface wave dispersion measurements. A horizontal tessellation consisting of 362 spherical splines is used to parametrize the model, which is approximately equivalent to a spherical harmonic of degree 18 in resolution. The model contains peak variations (from PREM) of up to ±7 per cent in S velocity and ±2.5 per cent in P velocity in the upper mantle. These variations decrease to ±1.5 and ±0.6 per cent, respectively, at 1000 km depth and reach ±2.5 and ±1.0 per cent, respectively, in the D″ region. The rms ratio of S to P velocity perturbations is fairly constant between 2.0 and 2.5 in the lower mantle, but a local minimum in this ratio occurs at a depth of approximately 1700 km. Resolution tests show that the recovery of P and S velocity is not geographically uniform, but also show that this amplitude ratio is well resolved between 670 and 2700 km depth. We also observe a persistent negative correlation between bulk‐sound and shear velocity throughout the same region of the mantle. In addition, the model contains a minimum correlation between P and S velocity between 670 and ∼1100 km. This feature is supported by both the favourable outcome of resolution experiments and the poor fit provided by the starting model from the inversion (in which the P and S velocities are perfectly correlated) to our data set of P‐wave traveltimes. The power spectra of both P and S velocity heterogeneity are similar, although we note a slightly larger dominance of degree two in the spectra for P velocity in the mid‐mantle where resolution is highest.

Large-Scale Thermo-chemical Structure of the Deep Mantle: Observations and Models

“…In the lowermost mantle, the seismic tomographic images are dominated by two large low-shear-wave velocity provinces (LLSVPs) located along the equator beneath Africa and the Pacific. These regions were first observed by global tomographic models in the 1990s (Tanimoto 1990;Su et al 1994;Li and Romanowicz 1996), and more recent models provided refined images of these structures (e.g., Antolik et al 2003;Houser et al 2008;Kustowski et al 2008;Ritsema et al 2011). Tomographic models of the mantle beneath Africa (Ritsema et al 1999) show that the African LLSVP has globally the structure of a ridge with a NW/SE direction and is locally tilted toward the east.…”

Multiscale Modeling and Process Optimization for Engineered Microstructural Complexity

“…Hence, the proper choice of the underlying velocity model to calculate travel-times becomes important, especially for regional phases. To account for the bulk of 3D velocity heterogeneities in the Earth we use the global upper-mantle CUB2 and the global whole-mantle J362D28 (Antolik et al, 2003) 3-D models to obtain travel-time predictions for regional and teleseismic phases, respectively. Both models have been shown to improve seismic locations and achieve significant variance reduction in travel-time predictions (e.g.…”

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“…The explosion is considered GT1 (Fisk, 2002) and recorded by some 400 stations at teleseismic distances. In the experiment we used travel-time predictions obtained from the global 3-D model J362D28 (Antolik et al, 2003) for P phases between 28 and 90 degrees, and located the event with increasing number of stations. At each step we identified the 20 unique subnetworks that provided near-optimal azimuthal coverage for the selected number of stations (starting from 6 to 400) and calculated the median mislocation, area of error ellipse and actual coverage.…”

The Prospect of Using Three-Dimensional Earth Models to Improve Nuclear Explosion Monitoring and Ground-motion Hazard Assessment

“…Although a number of studies have demonstrated the utility of using 3-dimensional models for determining seismic locations (Antolik et al, 2003;Yang et al, 2004;Flanagan et al, 2007), the broader application of 3-dimensional models in operational monitoring systems has yet to occur. Changing operational practice is a serious matter for any monitoring agency, because operational changes are costly, and there are risks to operational robustness.…”