Estimate of the Rigidity of Eclogite in the Lower Mantle From Waveform Modeling of Broadband S ‐to‐ P Wave Conversions

Broadband USArray recordings of the 21 July 2007 western Brazil earthquake ( M w =6.0; depth = 633 km) include high‐amplitude signals about 40 s, 75 s, and 100 s after the P wave arrival. They are consistent with S wave to P wave conversions in the mantle beneath northwestern South America. The signal at 100 s, denoted as S 1750 P , has the highest amplitude and is formed at 1,750 km depth based on slant‐stacking and semblance analysis. Waveform modeling using axisymmetric, finite difference synthetics indicates that S 1750 P is generated by a 10 km thick heterogeneity, presumably a fragment of subducted mid‐ocean ridge basalt in the lower mantle. The negative polarity of S 1750 P is a robust observation and constrains the shear velocity anomaly δ V S of the heterogeneity to be negative. The amplitude of S 1750 P indicates that δ V S is in the range from −1.6% to −12.4%. The large uncertainty in δ V S is due to the large variability in the recorded S 1750 P amplitude and simplifications in the modeling of S 1750 P waveforms. The lower end of our estimate for δ V S is consistent with ab initio calculations by Tsuchiya (2011), who estimated that δ V S of eclogite at lower mantle pressure is between 0 and −2% due to shear softening from the poststishovite phase transition.