Constraining the Radial Structure of Seismic Attenuation using Multiple S-Wave Datasets

Abstract

Seismic attenuation, quantified by the quality factor Q, provides crucial constraints on Earth's thermal and chemical structure due to its strong sensitivity to temperature and presence of volatiles. This study investigates the radial Q structure of Earth's mantle using amplitude measurements from core-diffracted S waves (Sdiff) and core-reflected S waves (ScS), along with direct S and multiply reflected S waves (SSSS).We analyze 7,598 Sdiff, 463 ScS, 5,940 S, and 1,726 SSSS measurements from 448 earthquakes recorded globally between 2009-2017, significantly improving the ray path coverage and the resolution of the seismic quality factor (Q) structure in the lowermost mantle compared to previous studies. Our results reveal that Sdiff amplitudes steadily increase with epicentral distance and exceed those predicted by the Preliminary Reference Earth Model (PREM), indicating structural complexities in the lowermost mantle not captured by existing 1-D Q models. Linear inversion of amplitude measurements unaffected by mantle triplications confirms a high-Q region in the uppermost lower mantle (600-900 km depth), consistent with previous findings from Zhu et al. (2022). However, forward modeling demonstrates that attenuation variations alone cannot explain the observed distance-dependent Sdiff amplitude increase, even when assuming purely elastic conditions (Q = ∞) in the lowermost 500 km. Instead, our modeling indicates that a thin (∼25 km) low-velocity layer with approximately 4% velocity reduction at the base of the mantle best explains the observations. This layer acts as a waveguide, trapping diffracted energy and producing the observed amplitude enhancement. These findings reveal significant structural complexity in the lowermost mantle, suggesting a more heterogeneous core-mantle boundary region than previously recognized. The thin low-velocity layer may reflect enriched iron content and unique physical conditions associated with high temperatures, phase transitions, and chemical interactions with the outer core.