Seismic Anisotropy, Intermediate-Depth Earthquakes, and Mantle Flow in the Chile-Argentina Flat-Slab Subduction Zone

by Anderson, Megan Louise.

Subduction zone structure and kinematics are topics of ongoing investigation with broadband seismology. Recent advances in experimental observations of mantle materials at high temperatures and pressure, expanded broadband seismic datasets, new seismic analysis methods, and advances in computational modeling are ever increasing our capacity for synthesized investigation of tectonic environments. With the resulting expanded capability for interpretation, the geophysical community is in a position to build more refined and detailed models of subduction zone processes. This study takes part in these advances by refining structural observations of the subducting slab and making new observations of mantle kinematics in a part of the South American subduction zone in Chile and Argentina (between 30° and 36°S). First, I investigate the utility of multiple-event earthquake relocation algorithms for accurate locations using a regional dataset for seismic events in Nevada and then I apply the observations resulting from this study in the determination of new Wadati-Benioff zone seismicity hypocenter locations for the study area in South America. I interpolate new contours of the top of the subducting slab from this seismicity that, when interpreted with focal mechanism solutions for these events, are consistent with its deformation due to slab pull. I use shear wave splitting of teleseismic earthquake waves and s-waves from local earthquakes to characterize mantle strain within the mantle wedge and within and below the subducting slab. From these observations, I conclude that mantle flow in subduction zones is quite responsive to local changes in slab geometry as well as the thermal state of and shear stresses in the mantle wedge. 12