Significant geometric variation of the subducted plate beneath the northernmost Cascadia subduction zone and its tectonic implications as revealed by the 2014 M 6.4 earthquake sequence

Complex Structure in the Nootka Fault Zone Revealed by Double‐Difference Tomography and a New Earthquake Catalog

“…In general, the distribution of relocated earthquakes around the NFZ is similar to those recovered in previous studies (Hutchinson et al, 2019(Hutchinson et al, , 2020Savard et al, 2020), with the exception that the deepest events within the 2014 cluster locate 5-10 km deeper than those of Hutchinson et al (2020). Furthermore, we do not observe a suite of shallow (<15 km) hypocenters above the 2014 aftershocks as in Hutchinson et al (2020).…”

“…In general, the distribution of relocated earthquakes around the NFZ is similar to those recovered in previous studies (Hutchinson et al, 2019(Hutchinson et al, , 2020Savard et al, 2020), with the exception that the deepest events within the 2014 cluster locate 5-10 km deeper than those of Hutchinson et al (2020). Furthermore, we do not observe a suite of shallow (<15 km) hypocenters above the 2014 aftershocks as in Hutchinson et al (2020). These discrepancies, 2012) is not defined, but strong indications of a dipping oceanic plate exist in the form of hypocentral locations and a northeast-dipping Vp/Vs contact between high (>1.8) and low (<1.65) regions.…”

“…The former possibility is attractive as it confines faulting to a single unit (the NFZ lithosphere) and is consistent with the distribution of aftershocks. The latter is less well substantiated by the results of this study as a shallow suite of aftershocks are not present in our relocated hypocenters as in Hutchinson et al (2020). This remains a possibility however, since shallow earthquake locations can be recovered depending on the selection of velocity model used in initial localizations (Figures S22 and S23 in the Supporting Information S1).…”

“…Several large (M > 6) right-lateral strike-slip events have occurred in or near the NFZ in 2004NFZ in , 2011NFZ in , and 2014 with aftershocks that reveal structures sub-parallel to the continental margin. Recent studies of seismicity around the NFZ enabled in part by temporary deployments of ocean bottom seismometers (OBSs) document strong geometric variations in the oceanic lithosphere both along-strike (Hutchinson et al, 2020) and along-dip (Savard et al, 2020).…”

“…For example, aftershocks from the 2004 Mw 6.3 event locate at depths as shallow as 20 km and were interpreted to lie beneath the top of a bending JdF plate (Savard et al, 2020). Less than 50 km to the northwest, the deepest aftershocks of the 2014 Mw 6.6 event locate at depths >35 km and suggest that locally the oceanic plate also dips to the northwest (Hutchinson et al, 2020). How a >15 km vertical offset over a short lateral distance is accommodated within oceanic lithosphere is unclear, and neither model attempts to fully reconcile the structure of the oceanic lithosphere around the NFZ with earthquake hypocenters beneath the continental slope.…”