Lower mantle heterogeneity beneath Eurasia imaged by parametric migration of shear waves

Physics

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Scientific paper

Lower mantle structure beneath central Eurasia is examined using a recently developed technique for migrating parametric representations of shear waves from teleseismic events. The migration method of Lay and Young [Lay, T., Young, C.J., 1996. Imaging scattering structures in the lower mantle by migration of long period shear waves. J. Geophys. Res., 101: 20023-20040.] assumes isotropic scattering from discrete heterogeneities to account for S wave coda arrivals, and provides resolution at scale lengths of about 500 km. The migration is applied to 15 s period shear wave data from 21 western Pacific earthquakes recorded in Europe and the Middle East that exhibit extra arrivals between S and ScS on the transverse components. Migrations are performed using PREM as well as model SGLE [Gaherty, J.B., Lay, T., 1992. Investigation of laterally heterogeneous shear velocity structure in D'' beneath Eurasia. J. Geophys. Res., 97: 417-435.], which has a discontinuity at 2605 km, 286 km above the core-mantle boundary. The migration images for the most coherent coda arrival (Scd) resemble those expected for a lower mantle discontinuity model and simulations are performed for models with discontinuities at various depths. Quantitative correlation of the images for the data and synthetic migrations show the optimal average depth of the discontinuity to be 2605 km. Migrations of subsets of the data suggest that the discontinuity depth varies from 2605 km in the northern end of the study area to 2620 km in the southern end. An additional intermediate arrival (Scd2) is observed in a limited portion of the dataset, and migrations indicate this phase is generated by localized discrete scatterers located at depths of 2750 to 2770 km under north-central Eurasia. These results are generally consistent with previous forward modeling studies, but the migration approach allows lateral variations to be modeled more systematically. The heterogeneous structure near the base of the mantle appears to be a manifestation of a complex boundary layer.

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