As waves produced by earthquakes reverberate through the solid Earth, they can be reflected or scattered from discontinuities within and between the mantle and core. Changes in the composition and temperature of mantle minerals can cause the waves to speed up, slow down, or bend and even reverse their paths. By monitoring earthquakes occurring within 10° of a seismic receiver array in Alaska, Tkalčić et al. have spotted a new phase of seismic pressure wave. These waves appeared to travel directly through the center of the Earth and inner core, and bounced back after scattering off the underside of a discontinuity in the upper mantle, 150 to 220 km below Antarctica. Because these waves were back-scattered just below the surface, they arrived at the receiver about a minute ahead of similar waves reflected from the antipodal surface itself; hence the authors termed them P′P′ near-podal precursors. The scatterers could be lenses of partially melted minerals or could comprise local concentrations of material different in composition than the rest of the upper mantle. -- JB
Geophys. Res. Lett. 33, 10.1029/2005GL024626 (2006).
