Searching for Mid-Mantle Water with Multitaper-Correlation SS Precursors

Earth’s mantle transition zone (MTZ) is a possible global water reservoir and may be responsible for long-term (~100 Ma) ocean-mass regulation, driven by plate tectonics and mantle convection. Estimates of MTZ mineral water capacity exceed 1 wt%, far greater than that of rocks of either the upper- or lower-mantle. When water-rich material from the transition zone penetrates the upper or lower mantle, partial melting occurs due to the decrease in water capacity after phase transition, generating a reduction in seismic velocities. This process can add an additional low-velocity zone (LVZ) that can be imaged above(below) the 410(660)-km discontinuities, if melt is present. Depending on the melt fraction and wetting angle at mineral-grain boundaries, decreases in shear velocity of 0.5-2.6% can be generated. Seismic receiver functions have detected velocity reductions both above and below the MTZ, interpreted to be partial melting induced by high water content under the Alpine orogeny, in the deep Japan-slab subduction zone, and across the western United States. Since much of Earth lacks seismic stations, we apply SS precursors to conduct a global survey for such LVZs. The precursors to the SS phase reflect off interfaces near the source-receiver midpoint, so seismic stations are not required to be located above the target region. Detection and interpretation of LVZs surrounding sharp positive-velocity gradients, such as the 410- or 660-km discontinuity, is often complicated by side lobes, an artifact of common signal-processing routines. To address this challenge, we develop an SS precursor method based on the multitaper-correlation (MTC) technique. MTC allows for analysis at higher frequency, leading to finer depth resolution, and can increase the number of useful data records. We conduct MTC SS-precursor analysis for seismic waveforms recorded on the ~125 stations of the Global Seismographic Network to benchmark our new method and search for LVZs. Results will be compared to LVZs interpreted from previous seismic analysis.