Seismic scatterers in the mid-lower mantle

Recent seismological studies have revealed that rocks with significantly different elastic properties are juxtaposed in the mid-lower mantle and often scatter seismic waves efficiently enough to be detected by seismic array analyses. Seismic networks all over the world with various aperture and geometry have been utilized for detecting scattered waves. A large number of objects that act as scatterers in the mid-lower mantle have been mapped out by analyzing several different types of scattered waves, mostly of short period: S-to-P scattering, P-to-P scattering prior to PP waves, P-to-P scattering prior to P′P′, P-to-P scattering in P coda waves, and P-to-P scattering prior to PKP. The scatterers have a lesser size that is smaller than the wavelengths (∼10 km), and probably extend several tens of kilometers at least. The mid-mantle scatterers are most likely to represent basaltic rocks that subducted into the lower mantle. Revealing the elastic properties relative to the surrounding rocks and the geometry of the individual scatterers, as well as the global distribution of the scatterers, should shed new light on the style of mantle convection. Relevant observations of mid-lower mantle scatterers are reviewed, and the directions of future progress are suggested.