Observations of PKP(DF) and PKP(BC) across the United Kingdom: implications for studies of attenuation in the Earth's core

We demonstrate that PKP(DF) and PKP(BC), recorded by a network of broad‐band stations in the United Kingdom from earthquakes in the southwest Pacific, show considerable spatial variation in amplitude and frequency content. We suggest that strong heterogeneity in apparent attenuation at the base of the mantle related to ultra‐low‐velocity zones (ULVZ) beneath the southwest Pacific and/or possibly to the east of Iceland may explain the anomalous observations of DF and BC across the UK. The length scale of attenuating ULVZs required to explain the observations is similar to that estimated recently for a southwest Pacific ULVZ by Wen & Helmberger (1998), that is, low‐wave‐speed (low apparent Qα) regions of 80 km height separated by steep‐sided ‘valleys’ of width 40 km with typical lowermost mantle wave speeds (high apparent Qα). If the apparent attenuation is due to anelasticity alone, disregarding the possible contribution from scattering, the minimum ULVZ Qα values required to explain the amplitude of 2 Hz BC are about 15–30, consistent with the interpretation of the ULVZs as zones of partial melt at the base of the mantle. This suggests that estimates of inner core anelastic attenuation, inferred from DF/BC spectral ratios, will be biased if DF and/or BC cross ULVZs at the base of the mantle under the source and/or receiver. Further, the presence of attenuating ULVZs may explain the large scatter in estimates of inner core Qα from global studies.