SEMUCB WMQ : 3D global upper mantle shear velocity, radial anisotropy and attenuation model developed using full waveforms
Inferring global upper-mantle shear attenuation structure by waveform tomography using the spectral element method
Haydar Karaoglu, Barbara Romanowicz
Geophysical Journal International, Volume 213, Issue 3, 1 June 2018, Pages 1536–1558, https://doi.org/10.1093/gji/ggy030
Reprints can be requested from either author.
Abstract: We present a global upper-mantle shear wave attenuation model that is built through a hybrid full-waveform inversion algorithm applied to long-period waveforms, using the spectral element method for wavefield computations. Our inversion strategy is based on an iterative ap-proach that involves the inversion for successive updates in the attenuation parameter (δQ−1) μ and elastic parameters (isotropic velocity VS, and radial anisotropy parameter ξ) through a Gauss–Newton-type optimization scheme that employs envelope- and waveform-type misfit functionals for the two steps, respectively. We also include source and receiver terms in the inversion steps for attenuation structure. We conducted a total of eight iterations (six for at-tenuation and two for elastic structure), and one inversion for updates to source parameters. The starting model included the elastic part of the relatively high-resolution 3-D whole mantle seismic velocity model, SEMUCB-WM1, which served to account for elastic focusing effects. The data set is a subset of the three-component surface waveform data set, filtered between 400 and 60 s, that contributed to the construction of the whole-mantle tomographic model SEMUCB-WM1. We applied strict selection criteria to this data set for the attenuation itera-tion steps, and investigated the effect of attenuation crustal structure on the retrieved mantle attenuation structure. While a constant 1-D Qμ model with a constant value of 165 throughout the upper mantle was used as starting model for attenuation inversion, we were able to recover, in depth extent and strength, the high-attenuation zone present in the depth range 80–200 km. The final 3-D model, SEMUCB-UMQ, shows strong correlation with tectonic features down to 200–250 km depth, with low attenuation beneath the cratons, stable parts of continents and regions of old oceanic crust, and high attenuation along mid-ocean ridges and backarcs. Below 250 km, we observe strong attenuation in the southwestern Pacific and eastern Africa, while low attenuation zones fade beneath most of the cratons. The strong negative correlation of Q−1 and VS anomalies at shallow upper-mantle depths points to a common dominant origin for the two, likely due to variations in thermal structure. A comparison with two other global upper-mantle attenuation models shows promising consistency. As we updated the elastic 3-D model in alternate iterations, we found that the VS part of the model was stable, while the ξ structure evolution was more pronounced, indicating that it may be important to include 3-D attenuation effects when inverting for ξ, possibly due to the influence of dispersion corrections on this less well-constrained parameter.
Download SEMUCB_WMQ model and codes to read it and plot it: (sometimes known as SEMUCB-WMQ)
This code currently supports evaluation of the model:
1) at a user-specified radius and (lon, lat) tuple; 2) at a user-specified radius and list of (lon, lat) tuples; or 3) on a regular lat-lon grid with user-specified spacing and radius.
In either case, the code exports, the relative Voigt-average shear velocity, Xi (= Vsh ** 2 / Vsv ** 2) and Qmu perturbations (relative to the reference 1D model data/model.ref), as well as the associated absolute Vsv and Vsh values.
Please report bugs to Haydar Karaoglu (haydarkaraoglu at gmail dot com)
Please find the model, the code and README file here
Caveats about A3d models
Although the A3d model format uses a spline basis to express perturbations relative to a "reference" 1D model (data/model.ref), it is important to note that the latter is not designed to be a "good" 1D reference model from the standpoint of fitting normal mode eigenfrequencies or other spherically averaged data (unlike REF or PREM, for example). It is simply a convenience in terms of parameterization.
To put this another way, although this code is capable of computing model values in terms of percent perturbation relative to the global 1D reference model, it is important to note that the latter represents the spherically averaged part of the model at *most* depths. Above ~ 60 km depth, the reference is not truly spherically averaged, as the average omits regions within the homogenized crustal model. This is one of the reasons why the reference model is not truly a spherically symmetric reference earth model (and is not intended to be).
The above code is an extension for a model including 3D Q of a code developed previously for reading and plotting of SEMUCB_WM1: as can be found [here]
We are working on providing a code to evaluate the model in 3D including its 3D crust for the purpose of computing synthetics. Please note that the crust is not a standard crust model as crust2.0 or crust1.0, it is a smooth crustal model developed at the same time as the mantle model. Both need to be used together to predict the wavefield, you cannot mix the mantle model with another crustal model.
