Imaging subduction zones: slab structure, fluids, and fault slip

Where and how water is stored within the oceanic lithosphere controls at what depth it is released during eventual plate subduction. The location of this fluid release has a profound impact on subduction zone processes, modulating megathrust fault slip, driving the production of arc magmas, and controlling the deep Earth water cycle. The magnitude of hydration depends heavily on the tectonic structure of the incoming plate, which can vary significantly over short along-strike distances, with regions of increased faulting and internal deformation enhancing fluid storage capacity. However, how these structural variations, and their subsequent fluid release, manifest at depth within subduction zones is currently poorly constrained, in part due to the sparsity of marine geophysical observations that cross from the incoming plate to the forearc at multiple locations along a subduction zone. To help address this gap, this thesis uses passive and active seismic imaging to derive 2-D and 3-D structural models of the Alaska Peninsula and Cascadia subduction zones. These structural observations are directly linked to the process of fluid storage and release using rock physics models, with implications for the rupture size of great earthquakes in Alaska and seafloor fluid seeps and arc magma production in Cascadia. Furthermore, this thesis emphasizes the use of fault mechanics models to bridge the gap between structural observations of the megathrust fault and the mechanisms behind earthquakes and aseismic slip globally. In Chapter 2, I present 3-D compressional (Vp) and shear-wave (Vs) velocity models, and their ratio Vp/Vs, using local earthquake tomography in the Alaska Peninsula subduction zone. Low velocity anomalies along the slab spatially correlate with the coseismic slip of the 2020 Mw7.8 Simeonof, 2021 Mw8.2 Chignik, and 2023 Mw7.2 earthquakes. From this correlation, I propose a fault mechanics model based on earthquake nucleation length that links high pore-pressure zones along the megathrust, resulting from slab dehydration, to rupture barriers that arrest seismic slip. In Chapter 3, I present two 2-D Vp models inside the Blanco shear zone and Rogue propagator wake in the southern Cascadia subduction zone, derived using traveltime tomography from wide-angle seismic profiles collected on ocean bottom seismometers. These models help constrain how propagator wakes and shear zones enhance hydration in the relatively dry incoming Juan de Fuca and Gorda plates. Finally, in Chapter 4, I use fault mechanics to explain maximum earthquake depth along the megathrust across the global subduction system. I demonstrate that a temperatureinsensitive transition from frictional failure to viscous deformation is most consistent with observations of both the downdip earthquake limit and a weak fluid-rich fault. This thesis elucidates the role of water in subduction zone dynamics and draws direct connections between tectonic structures that form on geologic time-scales and subduction zone processes, such as earthquakes, that are experienced by humanity in the present day.

Grantor dc:publisher

Massachusetts Institute of Technology and Woods Hole Oceanographic Institution

Year dc:date.issued

2026

Author dc:creator

• Moser, Liam P.

Advisor dc:contributor.advisor

• Canales, J. Pablo

• Imaging
• Subduction zones

DOI

10.1575/1912/73000

OAI identifier oai:identifier

oai:darchive.mblwhoilibrary.org:1912/73000