Evaluation of seismological evidence for decadal changes in Earth’s lowermost mantle and outer core

Abstract

Recent reports of decadal-scale structural changes in the lowermost mantle and transport of strong heterogeneity in the outer core may offer important insights into core-mantle structure and dynamics. These reports are derived from waveform comparisons of earthquake pairs or repeating earthquakes, particularly using differential travel-time measurements made with cross-correlation techniques. Here we show that the interpretation can contain several overlooked traps, particularly imperfect source repeatability and station-related issues. We systematically examine the earthquake-pair waveforms reported by two recent studies, and provide independent investigations on the source repeatability, along with its associated travel time shifts. We also examine the quality of related stations, using comparisons between co-located sensors. We demonstrate that the reported temporal changes in Earth’s deep interior can be largely explained by artifacts from imperfect source repeatability and station-related issues. Our findings caution that the outer core or lowermost mantle should not be considered as a likely source of detectable temporal variability unless alternative explanations have been exhaustively excluded.