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Analyzing Recent Splay Fault Activity in the Cascadia Accretionary Wedge Using High-Resolution Seismic Reflection Data

Description: 

Splay faults, which branch from the main plate boundary fault in the accretionary wedge with a
steeper dip angle and a closer proximity to shore, can result in larger tsunamis with shorter
warning times upon rupture compared to megathrust-only fault ruptures, posing an increased
hazard to coastal communities. Slip occurred on splay faults during several of the last century’s
biggest subduction zone earthquakes, but our understanding of potential active splay faults and
their hazards in Cascadia remains limited. In order to identify the most recently active and
therefore potentially seismogenic splay faults in the Cascadia accretionary wedge offshore
Washington state, we conduct a detailed stratigraphic and structural interpretation of near-surface
deformation in the wedge and in shallow slope basins. We use high-frequency sparker seismic
data to examine the record of deformation in these basins and the upper ~1 km of the
surrounding accreted wedge sediment packages. Observations of folded and faulted strata allow
us to determine the history of deformation and develop an integrated stratigraphic and structural
sequence of events for each basin. We find numerous basins that display near-surface
deformation of the youngest sediments, indicative of recently active splay faulting in portions of
the accretionary wedge. Several basins also contain splay faults which break to the surface with
observable scarps on multibeam bathymetry. By observing basins that are crossed by multiple
seismic profiles, specific prominent splay faults can be traced along the margin. We make a
preliminary estimation of total displacement on prominent splay faults using average
sedimentation rates to quantify how compression is partitioned to various portions of the
accretionary wedge. This study connects deep fault structure in the accretionary wedge to near
surface deformation, bridging the two scales of most previous work, and improving our overall
understanding of splay fault activity in Cascadia.

Session: Constraining Seismic Hazard in the Cascadia Subduction Zone

Type: Oral

Date: 4/20/2023

Presentation Time: 05:00 PM (local time)

Presenting Author: Anna Ledeczi

Student Presenter: Yes

Invited Presentation: 

Authors