Geographic variations in lowermost mantle structure from the ray parameters and decay constants of core‐diffracted waves

We introduce an array‐based approach for constraining seismic velocity structure in the lowermost mantle by measuring the frequency dependence of the ray parameter p and decay constant γ for core‐diffracted waves (Pdiff and SHdiff). The approach uses an iterative multichannel cross‐correlation algorithm that solves for relative arrival times and amplitudes of core‐diffracted waveforms from multiple peaks in normalized correlograms from pairs of coazimuthal stations. The approach is applied to 60 mb ≥ 5.8 earthquakes for Pdiff and 36 for SHdiff during 2001–2002, with nearly 50,000 unique profile station pairs with epicentral‐distance differences of Δd ≥ 10° and azimuth differences of Δφ ≤ 10°, sampling a significant portion of the lowermost mantle. Cap‐averaging the resulting ray parameter estimates produces geographic variations that are largely consistent with the distribution of lowermost mantle large low‐velocity provinces and seismically fast slab‐accumulation regions seen in seismic tomography models, whose locations also strongly influence the geographic distribution of heat flow out of the core. Geographic variations in station‐pair diffracted‐wave decay constants differ from those of the ray parameters, suggesting that variations in the decay of core‐diffracted waves are more linked to lowermost mantle seismic velocity gradients than absolute values of seismic velocity. The ray parameters and decay constants of core‐diffracted waves are strongly frequency‐dependent, and these frequency variations also vary significantly with geographic location. Combining lateral and vertical seismic‐velocity variations with mineral‐physics data on elasticity and conductivity of lowermost mantle species can provide constraints on D″ composition and CMB heat flux. Key Points Core‐diffracted P and S waves provide a means of quantifying the long‐wavelength global velocity structure of the lowermost mantle The ray parameter and amplitude decay rate, as functions of frequency, are both sensitive to the vertical velocity structure of the CMBR The dispersion of Pdiff and SHdiff waves can ultimately be used to determine regional variations in CMB heat flow rates Plain Language Summary We present a means of quantifying the global P‐ and S‐velocity structure of the lowermost mantle using seismic waves that diffract around Earth's core, with an eventual aim toward determining how much heat passes from the core into the mantle. Measuring the rate of heat flow out of the core r