Small scale heterogeneity in the mid-lower mantle beneath the circum-Pacific area

Abstract

Data from a western United States short-period seismic network are investigated in order to detect anomalous later P-wave arrivals within a time window from 50s to 170s after direct P waves for deep and intermediate-depth earthquakes at circum-Pacific subduction zones. We interpret the anomalous arrivals as S-to-P scattered waves from heterogeneities in the mid-lower mantle. The searched regions (depths from ∱/4800km to 2200km) cover approximately 2% of the entire lower mantle. The heterogeneous scattering objects are located by array processing of the seismic waves using semblance, with careful investigations of arrival time, slowness and azimuth of the later phases relative to direct P waves. We usually observe two or three later phases which are separated in arrival time by 5•20s and have similar arrival azimuth and slowness. This observation suggests that a few scatterers often form a cluster with a typical spacing of 100•200km vertically. More than ten clusters or groups of S-to-P scattering objects are detected in the mid-lower mantle beneath the circum-Pacific regions; northern Kuril, Izu-Bonin, Mariana, northern Fiji, and west coast of South America. The S-to-P scattering objects are found in a depth range from 1100 to 1800km, and the most prominent objects of each region tend to be located around the middle of the depth range. The amplitudes of the scattered waves range from 1% to 10% of the direct P waves, with exceptionally large values as large as 40%. The associated magnitudes of the changes in elasticity is not tightly constrained but probably are nearly 1% and may occasionally exceed 4% of shear velocity. Considering their high efficiency at converting short-period waves (wavelengths of 5•10km), the changes in elastic properties at the heterogeneous objects appear to occur within several kilometers. Seismic tomography studies have not detected prominent seismic wave speed anomalies except for those beneath South America which appear to be located in the former Farallon plate and those at north of Fiji which appear to be located around the Pacific superplume. The mid-lower mantle heterogeneity should represent sharp chemical variations in major element composition of the lower mantle rocks. The most plausible tectonic explanation of the variations is remnant of subducted and folded former oceanic crust which preceded the current subduction of the Pacific plate. The extreme estimates of low shear velocity (>4%) may indicate the prominent drops in the shear modulus due to ferro-elastic phase transition in stishovite (SiO2). Our observations imply that heterogeneity of kilometer-scale persists for billions of years despite of stirring due to convection and therefore is ubiquitous in the lower mantle. It is qualitatively consistent with some recent convection models.