Examining sediment consolidation state and hydrothermal circulation at and near buried seamounts on the Cascadia Margin and its relation to earthquake generation

Various mechanical and hydrological characteristics of subducting seamounts can influence megathrust rupture processes. For example, numerical modelling of seamount subduction suggests spatial variations in plate boundary seismicity due to the impacts of a subducting seamount on stresses within the upper plate (Sun et al., 2020). Downdip of the seamount on its leading flank, models predict that sediments are over consolidated with reduced porosity promoting increased effective fault-normal strength at the plate interface and earthquakes, while above and behind the seamount a "stress shadow" develops where sediments are under-consolidated and porosity is anomalously high, promoting low fault strength and stable sliding. Other studies have found evidence for increased hydrothermal circulation and fluid transfer from subducting seamounts to the overlying plate with the potential to contribute about 3-4 times more water than other regions without a seamount to the forearc and mantle wedge and linked to aseismic slow slip (Chesley et al., 2021).

In this study, we focus on the Cascadia Subduction Zone, utilizing multichannel seismic data collected aboard the R/V Marcus G. Langseth during the CASIE21 expedition. We image and characterize 12 fully to partially buried seamounts on the Juan de Fuca plate that range in size (0.5-1.5 km tall, 3-20 km wide), all located seaward of the deformation front (DF, by 12-75 km). Following the approach of Han et al. (2017), high-resolution Vp models from pre-stack depth migration imaging will be used to examine sediment consolidation state around these seamounts relative to a normal consolidation reference. In their study, Han et al. (2017) found anomalous sediment consolidation due to horizontal compression associated with subduction started ~10-15km from the Cascadia DF. In the CASIE21 dataset, seamounts are found within and outside this range, providing the opportunity to evaluate how sediment consolidation is impacted by seamount proximity and if there is evidence for the development of stress shadows as seamounts near the DF. Through comparison of sediment properties and velocity structure surrounding the seamount, we will also examine whether seamounts are sites of more altered crust and whether they may be sites of active hydrothermal circulation within this near DF zone.