Shallow Thrust and Outer Rise Earthquakes in Northwestern Pacific Subduction Zones and their Role in Subduction Zone Water Budgets with Special Focus on the Mariana Islands

Abstract

This dissertation utilizes accurate earthquake locations and focal mechanisms to examine two distinct regions within shallow subduction zones: the shallow plate interface and the subduction zone trench and outer rise. In particular, I focus on the shallow plate interface and outer rise of the Mariana Subduction Zone and then expand the focus to examine the outer rise at other Northern and Western Pacific Subduction Zones. By understanding where earthquakes occur in these regions, we hope to obtain a better understanding of the cycling of water through subduction zones, mineralogical changes in the presence of water, and the effects of water and hydrous minerals on faulting processes within the shallow subduction zone. The first project is focused on the Northern Mariana shallow plate interface and reveals that small plate interface earthquakes occur at greater depths than previously thought. I show that the earthquake magnitude varies with depth, which may reveal varying conditions of stress, hydration, structure, or mineralogy along the fault. For the second and third projects, I focus on the subduction outer rise; at this setting, extensional earthquakes near the top of the bending, incoming oceanic plate are thought to provide pathways for water to enter and hydrate the plate. The stresses within the plate may also be impacted by locking along the subduction plate interface. At the Mariana subduction zone, I observe differences in stress distributions within the incoming plate between the Southern and Central regions; this difference may be related to greater locking along the Southern Mariana plate interface. Our results for the Northern and Western Pacific show that extensional outer rise earthquakes occur to ~10-15 km within the incoming plate mantle at most subduction zones. If this entire depth range is hydrated, as much as ~10^9-10^10 Tg/Myr of water may be subducted at the Northwestern Pacific; however, lateral heterogeneities in outer rise faulting would result in reduced concentrations of input water