Small-scale heterogeneities near the upper and lower mantle boundary beneath subduction zones

Abstract

Small-scale compositional heterogeneity near the upper and lower mantle boundary below circum-Pacific subduction zones is investigated. We array-process seismograms of deep earthquakes recorded by large-aperture seismic networks at western US, Japan, and Alaska with the aim of examining anomalous signals in the P coda until about 35 s after direct P wave. The signals arrive from the directions close to the direct P wave. The signal amplitudes decay with delay time after the direct P wave and the decay feature apparently depends on focal depth. The signals most likely arise from S-to-P and P-to-P scattering within the lower and/or upper mantle around the foci. We show by applying a stochastic scattering theory for randomly heterogeneous media that the observations are matched well by density and rigidity anomalies of spatial scale on the order of 10 km, which diminish below about 700 km depth. The observations are matched if the rigidity anomaly is of the same sign and three to four times as large as the density anomaly. These properties of the scattering objects could be consistent with the predicted elastic anomalies of basalt near the upper and lower mantle boundary relative to the surrounding mantle of pyrolytic composition. The estimated weak degree of density heterogeneity, on the order of 0.01 to 0.1 %, may reflect sparse distribution of basaltic material of 10 km scale in the uppermost 100 km of the lower mantle possibly resulting from the significantly smaller density of basalt than the surrounding rock.