The Intersection of the Nootka Fault Zone with the Continental Margin off Vancouver Island

The Nootka Fault Zone extends from the margin offshore from Nootka Island, near Vancouver Island, at a fault-trench-trench triple junction, marking the intersection of the Juan de Fuca, Explorer and North American plates. The presence of the fault zone itself was initially inferred from magnetic anomaly analysis and substantiated by observations of focused seismicity, reflection seismic profiles, gravity and bathymetric data. Differential motion of the Explorer region seems to require its independence from the Juan de Fuca plate, though this has been subject to debate. The triple junction itself has not previously been examined in detail. Previously available field data at the Nootka fault zone itself are sparse. In fact the precise location and trend of the Nootka fault zone is not well confined. The Nootka fault is an example of an active fault subducting under an accretionary prism where the consequences of frequent earthquake shaking might be examined. For example, there is evidence of several mud volcanoes present in the basin and active venting on the margin. Further study is imperative in preparation for the NEPTUNE ocean floor observatory and potential future IODP drill holes. Furthermore, more accurate velocity models are required for improving earthquake location in this region since all current monitoring stations are land-based and to the east of the fault zone leading to systematic biases in epicenter locations, of tens of kilometers to the northeast, in routine locations. Here we present new seismic reflection data integrated with previous work for a new characterization of the seafloor approaching and on the deformation front.