On the Importance of Using Directional Information in the Search for Lower Mantle Reflectors

Abstract

The SS precursor signals are a powerful tool for mapping topography of mantle discontinuities, which are sensitive to the thermal and compositional structure of the mantle. The depth of mantle discontinuities is usually estimated using the differential travel time between the main arrival and its precursor. However, this method ignores potential travel path deviations that influence the travel time of precursor signals. Here, we use an approach that considers directivity information as well as travel-time measurements. Applying seismic array techniques, we measure slowness, back azimuth, and travel time of the signals, and use this information to backproject to the point of reflection. In our test dataset, we observe deviations from the predicted values in slowness and back azimuth in the range of 0.1–2.3 s/° and 1–20°, respectively. These values lead to reflection locations that can differ considerably from theoretical reflection points calculated with great circle plane paths as well as depths different from the depth calculated for in-plane propagation, with differences up to ∼150 km. Our results indicate that the travel-path deviations should be considered to avoid misinterpretation of mantle discontinuities and potentially reduce previously observed scatter in discontinuity depth.