SKS and SPdKS sensitivity to two-dimensional ultralow-velocity zones

Seismic wave propagation through two-dimensional core-mantle boundary (CMB) ultralow-velocity zones (ULVZs) is modeled using a global pseudospectral algorithm. Seismograms are synthesized for several types of ULVZ models to investigate the effect of these structures on SKS and SPdKS phases. One-dimensional models and two-dimensional models with different, quasi-1-D velocity structures on the source and receiver sides of the CMB provide a baseline for comparison with other 2-D models. Models with finite length ULVZs are used to test the sensitivity of the SPdKS travel time and waveform to different portions of the P diffracted wave path. This test shows that SPdKS waves are only sensitive to ULVZs with lengths >100 km. Our results give three tools for identifying and characterizing 2-D ULVZ structures. First, dual SPdKS pulses indicate exposure to at least two separate CMB velocity structures, either different source and receiver-side CMB velocities or different adjacent velocity regions for which P-diff inception occurs outside of and propagates into a ULVZ. Second, high-amplitude SKS precursors indicate a very strong (i.e., thick and/or large velocity perturbations) ULVZ. Hence, the absence of SKS precursors in most previous ULVZ studies indicates that very strong, sharp ULVZs are not common. Third, mean SPdKS delays relative to PREM constrain the minimum ULVZ strength and length combinations required to produce a given travel time delay.