Seismic Attenuation and S‐Velocity Structures in Beneath Central America Using 1‐D Full‐Waveform Inversion

Abstract

Because the Earth's mantle is not perfectly elastic, seismic waves are slightly attenuated as they travel through it. Attenuation is mostly sensitive to temperature, with larger temperature leading to stronger attenuation, and can be measured from the analysis of seismic waveforms. More specifically, attenuation is parameterized with the quality factor, Q, which is inversely proportional to the attenuation, that is, lower Q indicates stronger attenuation. Together with the velocity of seismic shear waves (V S ), Q provides key information on the deep mantle thermo-chemical structure. Here, we measure radial profiles of V S and Q in the mantle lowermost 480 km in 11 corridors located beneath Central America, where a subducted slab is believed to be present. Our results show that V S and Q are overall larger than their average values at these depths, and that the D″ discontinuity, which is believed to be the signature of the phase transition from bridgmanite to post-perovskite (pPv), is elevated. In the central region, however, V S is slower, attenuation larger, and the D″ discontinuity is deeper than in surrounding corridors, suggesting a higher temperature. A careful examination of our results suggest that this temperature increase is accompanied by a thinning of the pPv lens.

Estimating core-mantle boundary temperature from seismic shear velocity and attenuation

“…Because it is based on the fact that seismic velocity is affected by lateral changes in the depth of the phase transition from bridgmanite to post-perovskite (pPv), and therefore by lateral changes in the amount of pPv at a given location, the pdfs of T ref deduced from our approach depend on the transition temperature from bridgmanite to pPv close to the CMB, T pPv . Applying our method to measurements of V S and Q S beneath Central America (Borgeaud and Deschamps, 2021) and the Northern Pacific (Deschamps et al, 2019), we found that for T pPv = 3500 K the CMB temperature should be in the range 3,470-3880 K with a 95% likelihood. Because in our approach the value of T CMB depends on the value of T pPv right above the CMB, which remains poorly known, comparison between our results and available estimates is not straightforward.…”

“…Another constraint on local and horizontally averaged temperatures may be obtained from seismic attenuation, which is a thermally activated process (Minster and Anderson, 1981;Anderson and Given, 1982), implying that its amplitude depends on temperature. In the reminder of this paper, we detail this method, and we perform a preliminary application using models of V S and Q S obtained beneath Central America (Borgeaud and Deschamps, 2021) and the Northern Pacific (Deschamps et al, 2019).…”

“…Our goal is not to provide a conclusive value for the CMB temperature, as more measurements of V S and Q S together with a transition temperature to pPv more precise than current estimates may be needed for this, but rather to test our method. The measurements of V S and Q S we used are the lowermost layer of 1D radial profiles obtained beneath the Northern Pacific (Deschamps et al, 2019) and beneath Central America (Borgeaud and Deschamps, 2021) by full-waveform inversions of seismic data. In both cases, the inversion method is similar to that used in Konishi et al (2017), and the radial resolution of the 1D models is 50 km, i.e.…”

“…the V S -anomalies and Q S in Table 1 sample a 50 km thick layer above the CMB. Note that the method used to recover Central America profiles further includes travel-time corrections for the 3D mantle structure beneath Central America, spectral amplitude misfit to better constrain Q S , and corrections for focusing effects in the lowermost mantle (Borgeaud and Deschamps, 2021). We converted V S to relative anomalies dlnV S with respect to PREM (Dziewonski and Anderson, 1981) shear-wave velocity, which, in the lowermost 50 km, is equal to 7.26 km/s.…”

“…Alternatively, T CMB may be lower than predicted by our approach (Figure 8), in which case pPv may be present all around the CMB. This, however, is difficult to reconcile with the values of V S and Q S observed beneath Central America, which cannot be explained by Frontiers in Earth Science frontiersin.org temperature changes only and instead require changes in the depth of the pPv lens lower boundary (Borgeaud and Deschamps, 2021). Other modelling parameters, most particularly the frequency exponent and activation enthalpy of the quality factor, α and H, are still poorly known (Section 2.2).…”