Spatial-temporal fault movement analysis around a propagator wake on the Juan de Fuca plate

Robert Perrin, Jeff Beeson, Anne Trehu, Rachel Lauer, Glenn Spinelli, Rob Harris

This study investigates fault movement on the Juan de Fuca (JdF) plate near the southern end of the Cascadia Subduction Zone (CSZ), focusing on the interaction between tectonic processes, fluid migration, and differential sediment compaction. Using a novel semi-automated fault offset analysis method applied to seismic reflection data, we identified fault movement patterns within the sedimentary section overlying the crust across a 68 km by 80 km survey area located ~85 km west of the Cascadia deformation front. Our method was validated against manual horizon picks, demonstrating strong agreement, with discrepancies in only 8% of cases. The fault offsets generally increase with increasing depth between the 0.76 Ma and 0.90 Ma horizons. However, decreasing offset with depth is sometimes observed in the older sediments below the 0.90 Ma horizon. We speculate that this is due to fluid release and compaction of the lower sedimentary package after a large increase in sedimentation rate after 0.90 Ma. Our results reveal subtle variations in fault movement rates that align with a depression in the crustal surface, suggesting possible tectonic control. However, the high volume of fluids trapped within the lower sedimentary layers may also influence fault behavior, especially as the plate approaches subduction. Uneven fluid release and differential compaction in these layers could contribute to variability in the mechanical properties of the down-going plate and may affect segmentation in seismic hazards along the margin. This complex interplay between tectonic forces, sediment compaction, and fluid dynamics underscores the need for a deeper understanding of the factors influencing seismic risk in the CSZ.