Lowermost mantle anisotropy near the eastern edge of the Pacific LLSVP: constraints from SKS–SKKS splitting intensity measurements

Abstract

Seismic anisotropy has been documented in many portions of the lowermost mantle, with particularly strong anisotropy thought to be present along the edges of large low shear velocity provinces (LLSVPs). The region surrounding the Pacific LLSVP, however, has not yet been studied extensively in terms of its anisotropic structure. In this study, we use seismic data from southern Peru, northern Bolivia and Easter Island to probe lowermost mantle anisotropy beneath the eastern Pacific Ocean, mostly relying on data from the Peru Lithosphere and Slab Experiment and Central Andean Uplift and Geodynamics of High Topography experiments. Differential shear wave splitting measurements from phases that have similar ray paths in the upper mantle but different ray paths in the lowermost mantle, such as SKS and SKKS, are used to constrain anisotropy in D. We measured splitting for 215 same station-event SKS-SKKS pairs that sample the eastern Pacific LLSVP at the base of the mantle. We used measurements of splitting intensity(SI), a measure of the amount of energy on the transverse component, to objectively and quantitatively analyse any discrepancies between SKS and SKKS phases. While the overall splitting signal is dominated by the upper-mantle anisotropy, a minority of SKS-SKKS pairs (∼10 per cent) exhibit strongly discrepant splitting between the phases (i.e. the waveforms require a difference in SI of at least 0.4), indicating a likely contribution from lowermost mantle anisotropy. In order to enhance lower mantle signals, we also stacked waveforms within individual subregions and applied a waveform differencing technique to isolate the signal from the lowermost mantle. Our stacking procedure yields evidence for substantial splitting due to lowermost mantle anisotropy only for a specific region that likely straddles the edge of Pacific LLSVP. Our observations are consistent with the localization of deformation and anisotropy near the eastern boundary of the Pacific LLSVP, similar to previous observations for the African LLSVP.

Lowermost Mantle Anisotropy Beneath Africa From Differential SKS‐SKKS Shear‐Wave Splitting

“…Raypaths for each of these 896 station‐event pairs are shown in Figure . Compared to the total number of waveforms examined in this study, 896 pairs seems like a small number, but this yield is comparable to other studies of SKS ‐ SKKS splitting discrepancies (e.g., Long, ; Lynner & Long, ; Deng et al, ). Because most of the data we use came from temporary deployments (with typical run times between 1 and 3 years), we found that for a given station, it was often impossible to find even one event for which both SKS and SKKS phases were clearly recorded and had stable splitting measurements.…”

“…As in other SKS ‐ SKKS discrepancy studies, we also observe a considerable amount of scatter of individual nondiscrepant and discrepant measurements with similar raypaths (Lynner & Long, ; Deng et al, ). However, a comparison between Figure , which shows the entire SKS ‐ SKKS data set, and Figure , which shows our measurement density as well as mean splitting intensity values per grid point, demonstrates that despite the scatter, there are clear and convincing spatial patterns.…”

“…Understanding the deformation geometry at the base of the mantle, as reflected in shear‐wave splitting measurements, can help to decipher mantle flow patterns and eventually lead to insight into the origin and dynamics of structures such as LLSVPs. Previous studies have shown a substantial spatial correlation between regions with relatively strong lowermost mantle seismic anisotropy and the borders of LLSVPs, including both the African (Cottaar & Romanowicz, ; Lynner & Long, ; Wang & Wen, ) and the Pacific (Deng et al, ). Therefore, we also examine our observations in relation to the African LLSVP geometry.…”

“…The splitting intensity relates to the splitting parameters φ and δt by where β is the angle between the backazimuth and the fast polarization direction φ . As discussed in detail by Deng et al (), the splitting intensity has some important advantages over the transverse energy minimization method for SKS ‐ SKKS splitting discrepancy studies. Individual splitting intensity measurements are more robust than traditional methods (e.g., Monteiller & Chevrot, ), and differences in splitting for SKS and SKKS phases can be expressed in a straightforward way via splitting intensity differences (Deng et al, ).…”

“…We carried out our analysis using the SplitRacer package of Reiss and Rümpker (), which in its original form only implemented the transverse energy minimization method. For this study, we incorporated splitting intensity measurements into the code, following Deng et al (). We calculated 95% confidence intervals for all parameters based on the F test (Silver & Chan, ), with an adjustment for the number of degrees of freedom given by the equations of Walsh et al (), who showed that the original error estimates in Silver and Chan () were too low.…”