Seismic anisotropy in the lowermost mantle beneath the Galapagos Islands from SKS-SKKS splitting

Abstract

Seismic anisotropy measurements in the D” layer, the lowermost part of Earth's mantle, provide insight into its heterogeneity and associated mantle dynamics. In this study, we analyse shear wave splitting in SKS and SKKS waveforms to investigate anisotropy in the D” layer beneath the Galapagos Islands. Our dataset is based on 16 events recorded by the BOLIVAR array between 2003 and 2005 in the Tonga Trench, recreated from Vanacore and Niu (2011), who identified lowermost mantle heterogeneity beneath the Galapagos Islands using differential travel-time residuals.

We measure shear wave splitting in 364 SKS and SKKS phases using the eigenvalue minimization method (Silver & Chan, 1991; Teanby et al., 2004). Of these we identify 16 paths (i.e., event-station combinations) for which we have robust measurements of both SKS and SKKS phases. To resolve D” anisotropy, we use the splitting intensity method (Deng et al., 2017) and the error limits method from Silver & Chan (1991) to compare SKS and SKKS splitting. We also investigate the amplitude ratios of the radial and transverse components of SKS and SKKS waveforms as a proxy for lowermost mantle anisotropy, following Vanacore and Niu (2011), and extend on this method by proposing a signal-to-noise dependent threshold for discrepancy using synthetic modelling.

We use our dataset and synthetic modelling to evaluate the splitting intensity and amplitude ratios methodologies. We find that while both do resolve differential shear wave splitting in some circumstances, neither (either individually or together) provide a perfect proxy. Finally, the implications of our results validate some of the discrepancy identified in Vanacore & Niu (2011) generated by a plume root, and through evidence from multiple tomography models, we see a correlation in the location of our results with transitional parts of the lowermost mantle.

Date of Award	7 May 2024
Original language	English
Awarding Institution	University of Bristol
Supervisor	James M Wookey (Supervisor) & Oliver T Lord (Supervisor)