Varying fault patterns and the role of proto-thrusts in strain accommodation along the northern Cascadia subduction zone

Large subduction earthquakes and associated tsunamis pose major hazards to coastal regions worldwide. The Cascadia subduction zone, where the Juan de Fuca plate subducts beneath North America, has historically experienced several devastating megathrust earthquakes (MW > 8.5), most recently in 1700 CE. Nowadays, the Cascadia margin is considered to be in a late stage of the interseismic period and therefore one of the regions in the world, that is most prone to a major subduction earthquake in the foreseeable future. However, despite its high seismic potential, deformation processes at the Cascadia deformation front remain incompletely understood. In this study, we focus on the northern Cascadia margin and perform a systematic analysis of structural variations and fault patterns along the margin, using 14 newly acquired high-resolution 2D multichannel seismic data oriented perpendicular to the deformation front offshore Vancouver Island. These data are complemented by legacy seismic profiles that provide improved velocity constraints as well as high-resolution bathymetric data. Our results show that the deformation front is well marked by a series of bathymetric ridges and is segmented into sections of 4–10 km length. The main frontal thrust is traceable throughout the profiles and changes vergence from mostly seaward verging in the north to a landward verging segment, before swapping back to seaward verging in the south. The high-resolution data enables us to image proto-thrust zones in northern Cascadia for the first time and reveals a link between the occurrence and geometry of these proto-thrusts and the frontal thrust vergence. Landward-verging frontal thrusts are associated with wide proto-thrust zones characterized by mixed vergence, whereas seaward-verging frontal thrusts exhibit sparse, predominantly seaward-verging proto-thrusts both landward and seaward of the main frontal thrust. Velocity analyses reveal compaction-related velocity increases seaward of the deformation front exclusively in areas with well-developed proto-thrust zones, indicating that the proto-thrusts have accommodated a significant amount of strain prior to frontal accretion and play an important role in frontal deformation processes. We will compare our findings to other subduction zones and discuss them in the context of hazard potential expected from potential future megathrust earthquakes in the area.