Even-degree lateral variations in the Earth's mantle constrained by free oscillations and the free-air gravity anomaly

Abstract

The recent occurrence of several large earthquakes, in particular the 1994 June 9 Bolivia event, has motivated a re‐examination of the Earth's large‐scale heterogeneity from a normal‐mode or free‐oscillation perspective. Compared to earlier studies, the number of normal‐mode constraints on lateral variations in the mantle has increased five‐fold, and toroidal and cross‐coupled modes complement the traditional spheroidal mode data set. It is demonstrated that this large collection of mode data, combined with the free‐air gravity anomaly, can reliably constrain even‐degree lateral variations in wave velocities as well as density. We present the first whole‐mantle density model constrained by seismology. Our shear and compressional velocity models are consistent with existing models based upon traveltimes and waveforms, and are reasonably well correlated throughout the mantle. Shear and bulk sound velocity models exhibit a gradual decrease in correlation with depth, and are anti‐correlated near the core–mantle boundary. We find that lateral variations in density are poorly correlated with wave velocities, and are locally anti‐correlated with shear velocity in the lowermost mantle. The correlations between wave velocities and density suggest both a thermal and a compositional origin to lateral heterogeneity. In addition to traditional maps of lateral variations in wave velocity, we also present maps of lateral variations in shear and bulk moduli. The inversion puts weak constraints on even‐degree topographic variations on the core–mantle boundary, the 660 km discontinuity and dynamic free surface topography. Finally, we determine both radially and laterally varying scaling relationships, including Poisson's ratio.