Global High-Resolution mid-Mantle Imaging with Multiple-Taper SS-Precursor Estimates

Abstract

SS precursor imaging has long been used to detect sharp interfaces within Earth’s mid-Mantle. The topography of the 410- and 660-km discontinuities, the major interfaces in the mantle transition zone (MTZ), can provide valuable insight into the temperature of and material flow within the mantle. Additionally, negative velocity gradients and possible partial melt surrounding the MTZ in some regions provide evidence for a hydrogen-enriched mid-mantle, a feature that may have implications for global water circulation and long-term (~100 Ma) ocean-mass regulation. Here, we apply a novel SS-precursor deconvolution technique based on multiple-taper correlation (MTC). Typical SS-precursor techniques require tightly bandpassed signals (e.g., 0.02-0.1 Hz), limiting both vertical and horizontal resolution. Higher-frequency content allows for the detection of finer structure in and around the MTZ. MTC-based SS-precursor estimates can increase the frequency cutoff to above 0.5 Hz, thereby increasing vertical resolution to under 10 km. We conduct this analysis on a global data set of over 300,000 SS waveforms recorded on the permanent GSN, GEOSCOPE, and GEOFON networks as well as the temporary EarthScope TA and AlpArray. Such a large dataset provides unprecedented bounce-point density, particularly in the North Pacific Ocean. Preliminary results suggest a global average depth of ~409 km and ~665 km for the 410- and 660-km discontinuities respectively. In this work we used time-delays calculated for the 1-D ak135 velocity model. In general, we find moderate agreement with previous low-frequency SS precursor analysis. Additionally, we identify a sharp feature above the MTZ, north of the Hawaiian Islands, that was interpreted previously from an asymmetry in sidelobe amplitudes, suggesting a low-velocity zone with a sharp interface (<10-km thickness), rather than a thick wavespeed gradient. Further results will include corrections for 3-D structure with various mantle tomography models and focus on potential impacts to the solid-Earth water cycle.