Seismic attenuation in the African LLSVP estimated from PcS phases

Seismic tomography models have revealed two broad regions in the lowermost mantle marked by ∼3% slower shear velocity than normal beneath the south central Pacific and southern Africa. These two regions are known as large-low-shear-velocity provinces (LLSVP). There is debate over whether the LLSVPs can be explained by purely thermal variations or whether they must be chemically distinct from normal mantle. Elastic properties alone, have been unable to distinguish the thermal from chemical interpretations. Anelastic structure, however, can help discriminate among models of the LLSVPs since intrinsic attenuation is more sensitive to temperature than to chemical variations. Here we estimate Qμ (the shear wave quality factor) in the African LLSVP using PcS waves generated from a Scotia Arc earthquake, recorded by broadband seismometers deployed in Southern Africa during the Kaapvaal experiment. The upward leg of the PcS waves sweeps from normal mantle into the African LLSVP across the array. We use the spectral ratio (SR) and instantaneous frequency matching (IFM) techniques to measure the differential attenuation (Δt⁎) between waves sampling the African LLSVP and the waves that sample normal lower mantle. Using both methods for estimating Δt⁎ we find that PcS waves sampling the LLSVP are more attenuated than the waves that miss the LLSVP yielding a Δt⁎ difference of more than 1 s. Using the Δt⁎ measurements we estimate the average Qμ in the LLSVP to be about 110. Using a range of activation enthalpy (H⁎) estimates, we find an average temperature anomaly within the LLSVP ranging from +250 to +800 K. Our estimated temperature anomaly range overlaps previous isochemical geodynamic studies that explain the LLSVP as a purely thermal structure although the large uncertainties cannot rule out chemical variations as well.