Outcrop to Microscale Field Observations of a Unique On-Land Exposure of an Active Long-Lived Tsunamigenic Splay Fault in South Central Alaska

In the 1964 Mw 9.2 megathrust earthquake in South Central Alaska, a megathrust splay fault on Montague Island had co-seismic surface rupture and up to 10 meters of . The offshore expression of this fault has been proposed to be the source of a in Seward. Splay faults can transfer seismic slip from a megathrust rupture to the surface and thus influence tsunami-genesis. The unique exposure of an active megathrust splay fault on Montague Island provides us with the opportunity to investigate the geometry and structure of the fault zone and document the mechanical properties and alteration from wall rock to fault core. Outcrop-scale investigations have identified a 150m wide fault zone, intensely fractured host rock and , an 8m wide continuous gouge zone, and large-scale deformation variability across the fault. Microstructural analysis has documented cataclasite formation, foliated fault gouge, directional shear sense, and clay alteration and formation due to faulting processes. These results insight into co-seismic or aseismic slip behaviors, faulting related mechanical or chemical alteration, and slip weakening/strengthening behaviors. This identification of structural variation and deformation mechanics can be used to constrain empirical constitutive equations that dictate faulting and rupture behavior, provide real-world constraints for large-scale numerical models, and inform interpretations of offshore splay faults imaged by seismic reflection data.