Observations of Mantle Seismic Anisotropy Using Array Techniques: Shear‐Wave Splitting of Beamformed SmKS Phases. Issue 1 (30th December 2022)

• Record Type: Journal Article
• Authors: Wolf, Jonathan; Frost, Daniel A.; Long, Maureen D.; Garnero, Edward; Aderoju, Adeolu O.; Creasy, Neala; Bozdağ, Ebru
• Is Part Of: Journal of geophysical research. Volume 128:Issue 1(2023)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-12-30
• Subjects: seismic anisotropy -- shear‐wave splitting -- beamforming -- mantle dynamics -- computational seismology; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2022JB025556
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: Shear‐wave splitting measurements are commonly used to resolve seismic anisotropy in both the upper and lowermost mantle. Typically, such techniques are applied to SmKS phases that have reflected (m‐1) times off the underside of the core‐mantle boundary before being recorded. Practical constraints for shear‐wave splitting studies include the limited number of suitable phases as well as the large fraction of available data discarded because of poor signal‐to‐noise ratios (SNRs) or large measurement uncertainties. Array techniques such as beamforming are commonly used in observational seismology to enhance SNRs, but have not been applied before to improve SmKS signal strength and coherency for shear wave splitting studies. Here, we investigate how a beamforming methodology, based on slowness and backazimuth vespagrams to determine the most coherent incoming wave direction, can improve shear‐wave splitting measurement confidence intervals. Through the analysis of real and synthetic seismograms, we show that (a) the splitting measurements obtained from the beamformed seismograms (beams) reflect an average of the single‐station splitting parameters that contribute to the beam; (b) the beams have (on average) more than twice as large SNRs than the single‐station seismograms that contribute to the beam; (c) the increased SNRs allow the reliable measurement of shear wave splitting parameters from beams down to average single‐station SNRs of 1.3. Beamforming may thus be helpful to more reliably measure splitting due to upper mantle anisotropy. Moreover, we show that beamforming holds potential to greatly improve detection of lowermost mantle anisotropy by demonstrating differential SKS–SKKS splitting analysis using beamformed USArray data. Plain Language Summary: When earthquakes occur, seismic waves are produced that travel through the deep Earth to distant seismic stations. In some portions of the Earth, seismic waves traveling in different directions or with different vibration directions travel at different speeds. This phenomenon is known as seismic anisotropy and results from individual mineral crystals aligning with mantle flow. Therefore, by measuring seismic anisotropy, we can obtain insights into how Earth's mantle flows, a process called mantle convection. In this work, we show that seismic anisotropy can be inferred from recordings of seismic phases that are summed (or stacked) across a number of spatially separated stations (seismic arrays). The resulting stacks are also called beams. Beams have an increased signal clarity compared to single‐station seismograms, leading to several advantages for analyses of seismic anisotropy. For example, the increased signal strength in beams allows for the usage of weaker seismic phases, which are not commonly used for measuring seismic anisotropy. Moreover, measurements made on beamformed data are more robust. This new technique enables us to suggest new directions for lowermost mantle anisotropy analyses. Key Points: Major limitations for shear‐wave splitting measurements are the limited number of suitable phases and low signal‐to‐noise ratios (SNRs) Beamforming enhances the SNR, enabling us to use unusual seismic phases and a larger data fraction for shear‐wave splitting measurements This holds potential for investigations of mantle anisotropy, particularly in the lowermost mantle