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

摘要

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 gamma for core-diffracted waves (P-diff 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 P-diff and 36 for SHdiff during 2001-2002, with nearly 50,000 unique profile station pairs with epicentral-distance differences of Delta d >= 10 degrees and azimuth differences of Delta phi <= 10 degrees, 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 stationpair 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.