Ridge-like lower mantle structure beneath South Africa

Abstract

Recent (ScS-S) results from probing the deep structure beneath southern Africa display strong delays of up to 10 s at distances beyond 90°. Such delays could be explained by long-period tomographic models containing smooth (weak) features with the addition of rough (strong) D″ structure (3–9% drops in shear velocities). However, these structures cannot explain the (SKS-S) differentials sampling the same region. To explain the (SKS-S) and (ScS-S) data sets simultaneously requires instead a large-scale ridge-like structure with a relatively uniform 3% reduction in shear velocity. The structure is about 1000 km wide and extends at least 1200 km above D″. It is orientated roughly NW-SE and leans toward the east at latitudes from 15° to about 30°. It proves difficult to explain such sharp features with thermal effects alone and, thus, the importance of high-resolution waveform modeling to establish their existence. To derive the above results, we developed a special algorithm by matching simulated synthetics to observed broadband waveforms. This is achieved by computing the various arrivals separately using generalized ray theory for a reference model and allowing the arrivals to shift in relative times to match the data. Tomographic models can then be constructed, or existing tomographic models can be altered, to match these data, and new 2-D synthetics can be constructed as well to better fit the waveform data. These updated synthetics can again be decomposed and reassembled, and the process can be repeated. This algorithm is applied to a combination of analog and digital data along a corridor from South America, producing the high-resolution 2-D model described above.