Presentation by Anna Ledeczi.
Geophysical and geological perspectives on accretionary wedge evolution, splay faulting, and megathrust earthquake processes at the Cascadia subduction zone
Abstract:
Because splay faults branch at a steep dip angle from the plate-boundary décollement in an accretionary wedge, their coseismic displacement can potentially result in larger tsunamis with distinct characteristics compared to megathrust-only fault ruptures, posing an enhanced hazard to coastal communities. Elsewhere, there is evidence of coseismic slip on splay faults during many of the largest subduction zone earthquakes, but our understanding of potentially active splay faults and their hazards at the Cascadia subduction zone remains limited. I use nested scales of seismic reflection imaging to identify active splay faults in the widest, most completely locked portion of Cascadia. I present evidence for widespread evidence for active splay faulting up to 30 km landward of the deformation front and diminished fault activity landward outside of this zone. The abundance of surface-deforming splay faults in the active outer wedge domain suggests Cascadia megathrust events may commonly host distributed shallow rupture on multiple splay faults located within 30 km of the deformation front.
For seismological and modeling purposes, the plate boundary is often treated as a discrete, thin fault. However, direct evidence of the structure of megathrust faults from the natural rock record suggests it is rather an anastomosing, braided fault network with multiple simultaneously active strands. I present the first observations from a site in the Olympic Mountains, showing that it is a paleomegathrust fault that records both fast slip and slow distributed deformation during the seismic cycle through coeval coseismic brittle-frictional and interseismic viscous deformation. The absence of basalt indicates that the megathrust fault was localized within the incoming plate stratigraphic sequence, facilitating sediment underthrusting, similar to offshore structures observed via seismic reflection imaging in Cascadia and elsewhere today. This exposure presents an exhumed model for plate interface systems hosted entirely in sedimentary rock even at mid-seismogenic depths.
