Tectonic evolution of the Nootka fault zone and deformation of the shallow subducted Explorer plate in northern Cascadia as revealed by earthquake distributions and seismic tomography

At the northern Cascadia subduction zone, the subducting Explorer and Juan de Fuca plates interact across a transform deformation zone, known as the Nootka fault zone (NFZ). This study continues the Seafloor Earthquake Array Japan Canada Cascadia Experiment to a second phase (SeaJade II) consisting of nine months of recording of earthquakes using ocean-bottom and land-based seismometers. In addition to mapping the distribution of seismicity, including an M_W 6.4 earthquake and aftershocks along the previously unknown Nootka Sequence Fault, we also conducted seismic tomography, which delineates the geometry of the shallow subducting Explorer plate (ExP). We derived hundreds of high-quality focal mechanism solutions from the SeaJade II data. The mechanisms manifest a complex regional tectonic state, with normal faulting of the ExP west of the NFZ, left-lateral strike-slip behaviour of the NFZ, and reverse faulting within the overriding plate above the subducting Juan de Fuca plate. Using data from the combined SeaJade I and II catalogs, we have performed double-difference hypocentre relocations and found seismicity lineations to the southeast of, and oriented 18° clockwise from, the subducted NFZ, which we interpret to represent less active small faults off the primary faults of the NFZ. These lineations are not optimally oriented for shear failure in the regional stress field, which we inferred from averaged focal mechanism solutions, and may represent paleo-configurations of the NFZ. Further, active faults interpreted from seismicity lineations within the subducted plate, including the Nootka Sequence Fault, may have originated as conjugate faults within the paleo-NFZ.

Three-dimensional variations of the slab geometry correlate with earthquake distributions at the Cascadia subduction system

Significant along-strike variations of seismicity are observed at subduction zones, which are strongly influenced by physical properties of the plate interface and rheology of the crust and mantle lithosphere. However, the role of the oceanic side of the plate boundary on seismicity is poorly understood due to the lack of offshore instrumentations. Here tomographic results of the Cascadia subduction system, resolved with full-wave ambient noise simulation and inversion by integrating dense offshore and onshore seismic datasets, show significant variations of the oceanic lithosphere along strike and down dip from spreading centers to subduction. In central Cascadia, where seismicity is sparse, the slab is imaged as a large-scale low-velocity feature near the trench, which is attributed to a highly hydrated and strained oceanic lithosphere underlain by a layer of melts or fluids. The strong correlation suggests that the properties of the incoming oceanic plate play a significant role on seismicity.

Variations in seismicity are observed at subduction zones, but the oceanic sides remain poorly resolved. Here, the author presents tomographic results of the Cascadia subduction system demonstrating that there are significant variations of the oceanic lithosphere along the subduction zone.