Anomalous SmKS induced by postcritical reflection and refraction at the core-mantle boundary

Earth's outer core is generally thought to be a well-mixed liquid consisting mostly of iron and a small amount of lighter elements. Recent seismic studies using SmKS waves show that the top a few hundred kilometers of the outer core possess a P-wave velocity slightly lower than the PREM model, which cannot be explained by self-compression of a chemically homogeneous outer core. We investigated the SmKS waveforms of a deep earthquake occurring beneath South America recorded by a large and dense seismic array in China, and measured the differential arrival times of the SmKS pairs. We found significant waveform distortion of the SmKS caused by postcritical refraction and reflection at the core-mantle boundary. This waveform distortion can introduce significant bias to the measured differential times, leading to incorrect estimate of P-wave velocity of the outer core. Whether stable stratification is occurring in outer core or not requires further seismic investigations.