资源环境科技发展态势分析平台

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 S1750P, 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 S1750P 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 S1750P is a robust observation and constrains the shear velocity anomaly delta V-S of the heterogeneity to be negative. The amplitude of S1750P indicates that delta V-S is in the range from - 1.6% to - 12.4%. The large uncertainty in delta V-S is due to the large variability in the recorded S1750P amplitude and simplifications in the modeling of S1750P waveforms. The lower end of our estimate for delta V-S is consistent with ab initio calculations by Tsuchiya (2011), who estimated that delta V-S of eclogite at lower mantle pressure is between 0 and - 2% due to shear softening from the poststishovite phase transition.