Investigation of laterally heterogeneous shear velocity structure in D″ beneath Eurasia

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Abstract
Seismic waves are used to explore large‐scale radial structure and lateral heterogeneity of shear velocity in the D″ region, the chemical and thermal boundary layer at the base of the mantle. We analyze long‐period, horizontally polarized (SH) shear waveforms from earthquakes in the northwest Pacific recorded in Europe and the Middle East for evidence of proposed laterally varying shear velocity stratification in D″ beneath Eurasia. Specifically, our data sample the lower mantle extending from beneath the Arctic Ocean near Novaya Zemlya, USSR, to under northern India. Using a reflectivity technique to model the SH waveforms, we determine a locally stratified shear velocity model, SGLE, that provides a good average travel time and waveform fit to over 80% of the data sensitive to D″ structure. SGLE is characterized by a rapid 2.75% increase in shear velocity approximately 290 km above the core‐mantle boundary, similar to previously proposed models for this region. Unlike for other regions analyzed using similar waveform modeling techniques, the percentage of data inconsistent with this average radial model is fairly substantial, indicating that laterally heterogeneous D″ velocity structure is important in this region. Some waveforms lack the reflected arrival expected for a stratified model, while others show evidence of two arrivals, both possibly originating within D″. While our data are insufficient for direct inversion of the three‐dimensional structure, we utilize several simple parameterizations of the waveforms to gain some understanding of the region. Both the relative amplitude and differential travel time variations of the waveforms indicate that D″ beneath Eurasia is characterized by (1) broad regions in which the structure has large‐scale stratification giving coherent long‐period reflections, with superimposed velocity variations that scatter shorter‐wavelength waves; and (2) smaller regions which are sufficiently heterogeneous to scatter even the long‐period energy. The scale length of the lateral heterogeneity in D″ that produces these variations is inferred to be somewhat less than 500 km, which contrasts with weaker lateral heterogeneity that is observed in the central region of the lower mantle, where heterogeneity scale lengths of approximately 500–1000 km appear to dominate. These observations are consistent with a model of the lower mantle in which distributed heterogeneity is concentrated in D″, generating a locally stratified layer in some regions. The effects of a diffuse or topographically variable D″ discontinuity must be further explored using two‐ and three‐dimensional synthetic calculations before inversions for the detailed structure can be undertaken.