Seismic scatterers at the shallowest lower mantle beneath subducted slabs

Data from a western United States short-period seismic network are analyzed in order to investigate anomalous later phases within a time window from about 10 s to 120 s after direct P waves for deep and intermediate-depth earthquakes in circum-Pacific subduction zones. The anomalous phases are best interpreted as S-to-P scattered waves (wavelengths of ~ 10 km) from heterogeneities in the shallow lower mantle (depths ≤ 950 km). Several S-to-P scatterers where elastic properties of the rocks must substantially change within several kilometers are detected in the shallowest 300 km of the lower mantle beneath four circum-Pacific regions: Kuril, Bonin, Fiji, and Peru. Around most of the observed scatterers the seismic tomography models have delineated high seismic velocity anomalies which are associated with recently subducted Pacific slab or Nazca slab. The most likely origin of the scatterers would be basalt which used to form the oceanic crust. Interestingly enough the majority of the scatterers are located near the bottom boundaries of the slabs. Given rather moderate degrees of folding and/or buckling of the Pacific and Nazca slabs in the shallowest lower mantle at the study areas, the existence of the oceanic crust associated with the most recent slab subduction at the scattering sites is unlikely. Alternative and more likely explanations include: the presence of ancient basaltic rocks, localized dehydration of hydrous minerals, or a sharp boundary between different rock fabrics such as isotropic and anisotropic lower mantle rocks associated with mantle flow dragged by the slab subduction.