High Resolution Transition Zone Discontinuity Images across the Pacific Ocean from SS Precursors Using Local Slant Stack Filters

SS precursor observations are a powerful tool to study the topography and character of transition zone discontinuities, especially in regions, such as ocean basins, where few seismic stations exist, precluding other high resolution approaches. Still, the available coverage is limited by the distribution of sources and stations, but also by the level of noise and by the fact that, in some distance ranges, interfering seismic phases (e.g. the postcursors to S or S_diff, and the precursors to ScSScS) mask the weak signal from the SS precursors.

We introduce an array data processing tool, the Local Slant-Stack Filter (LSSF), to address the two last issues, and clean up the otherwise noisy SS precursor record sections. We show that these filters are a powerful tool for extracting the weak yet coherent SS precursor signals while removing interfering seismic phases as well as random noise, yielding robust precursor travel time measurements with spatial resolution higher than what can be achieved by the conventional common midpoint stacking method. The effectiveness of the filters are demonstrated by application to synthetic and real data. We systematically apply this filtering method to an SS precursor dataset recorded by the US Transportable Array, which consists of 47 high-quality events with magnitudes ranging from M_w 6.2 to 7.8 and samples a vast region of the Pacific Ocean and its northwest margin, and present maps of 410 and 660 discontinuity topography. We discuss correlations observed between our discontinuity images and several fine-scale heterogeneities revealed by mantle shear wave tomography in the vicinity of Hawaii and the Pacific Superswell.