A strongly negative shear velocity gradient and lateral variability in the lowermost mantle beneath the Pacific

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Abstract
A new approach for constraining the seismic shear velocity structure above the core‐mantle boundary is introduced, whereby SH‐SKS differential travel times, amplitude ratios of SV/SKS, and Sdiff waveshapes are simultaneously modeled. This procedure is applied to the lower mantle beneath the central Pacific using da.ta from numerous deep‐focus southwest Pacific earthquakes recorded in North America. We analyze 90 broadband and 248 digitized analog recordings for this source‐receiver geometry. SH‐SKS times are highly variable and up to 10 s larger than standard reference model predictions, indicating the presence of laterally varying low shear velocities in the study area. The travel times, however, do not constrain the depth extent or velocity gradient of the low‐velocity region. SV/SKS amplitude ratios and SH waveforms are sensitive to the radial shear velocity profile, and when analyzed simultaneously with SH‐SKS times, rnveal up to 3% shear velocity reductions restricted to the lowermost 190±50 km of the mantle. Our preferred model for the central‐eastern Pacific region (Ml) has a strong negative gradient (with 0.5% reduction in velocity relative to the preliminary reference Earth model (PREM) at 2700 km depth and 3% reduction at 2891 km depth) and slight velocity reductions from 2000 to 2700 km depth (0–0.5% lower than PREM). Significant small‐scale (100–500 km) shear velocity heterogeneity (0.5%–1%) is required to explain scatter in the differential times and amplitude ratios.

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