Modeling shear wave structure of the subducting kurile slab using both travel time and amplitude residuals

Three-dimensional imaging of subducting slabs is a primary approach to quantifying mantle flow processes. Most slab imaging studies have utilized P wave data. To provide independent information about slab velocity structure, long-period shear wave data for events in the Kurile slab were analyzed. Travel times and amplitudes were measured for long-period horizontally polarized S and ScS phases for 10 intermediate and deep focus earthquakes. The major challenge of working with this data was isolating the slab component from the overall residual. Empirical path corrections were calculated for both the amplitude and travel times by inverting a total of 28 northwestern Pacific events using S, ScS, sS, and sScS measurements. In addition to the empirical corrections, differential residual spheres were calculated for the travel times. The travel time data had to be relocated, and the amplitude residuals had to be corrected for source radiation pattern, geometric spreading, and attenuation. The travel time data was modeled using a three-dimensional finite difference algorithm. The modeling suggests either a 100 km tabular slab or a 100 km slab that widens to 200 km below 500 km depth. Both penetrate to at least 800 km depth and average 5% faster than PREM ambient mantle above 500 km and slow to 2--3% below 500 km. The amplitudes were modeled using a three-dimensional Gaussian beam method. The resulting amplitude model is a slab that averages 4% faster than PREM ambient mantle, penetrates to only 670 km depth, and in the southern part of the subduction zone is deflected at 670 km depth. Both the travel time and amplitude models treated the subducting slab as a thermal anomaly. While this treatment fits large portions of both data sets, there remain inconsistencies. Therefore, theoretical slabs including the olivine - spinel phase transformations were generated. The synthetic amplitude residuals for these models indicate focusing and defocusing patterns similar to the data. These results suggests that metastable olivine is present in the subducting Kurile slab, and indicate the necessity of including internal structure to slab modeling.