A Back‐Projection Method For Imaging Large‐Scale Lateral Variations of Lg Coda Q With Application to Continental Africa

Abstract

A new method is developed which uses back‐projection tomography to regionalize large‐scale lateral variations of coda Q for Lg waves which have traversed long continental paths. Successful use of this method requires precise and stable single‐trace measurements of Lg coda Q (Xie & Nuttli 1988). the method converges rapidly and requires minimal computer storage. It also allows quantitative estimation of resolution and error in imaging lateral variations of coda Q. the spatial resolution of this method is limited by the uneven spatial coverage of the data base, by our limited knowledge of Lg coda generation, and by the trade‐off between the stability and the spatial resolution of the coda Q inversion. the method is applied to a large set of digital Lg coda data from Africa, where large‐scale lateral variations of Lg coda Q are found to correlate well with major tectonic features. Most of Africa is stable, and like other stable regions, has relatively high coda Q values. the lowest values of coda Q are associated with the African rift system. Other regions of low‐Q values include the Atlas mountains and Cape Fold Belt, regions of Mesozoic and younger deformation. the lateral variation of frequency dependence of Lg coda Q correlates, in most regions, with that of Q at 1 Hz. Our analysis of the spatial resolution of this method indicates that the resolving power of Lg coda Q imaging is comparable to that of velocity tomography using long‐period surface waves. Using empirical approaches, we estimate that uncertainties in Lg coda Q and its power‐law frequency dependence parameter are less than about 60 and about 0.2, respectively, for most of Africa. Copyright © 1990, Wiley Blackwell. All rights reserved