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Subduction forearcs are subject to seismic hazard from upper plate faults that are often invisible to instrumental monitoring networks. Identifying active faults in forearcs therefore requires integration of geomorphic, geologic, and paleoseismic data. We demonstrate the utility of a combined approach in a densely populated region of Vancouver Island, Canada, by combining remote sensing, historical imagery, field investigations, and shallow geophysical surveys to identify a previously unrecognized active fault, the XEOLXELEK‐Elk Lake fault, in the northern Cascadia forearc, ∼10 km north of the city of Victoria. Lidar‐derived digital terrain models and historical air photos show a ∼2.5‐m‐high scarp along the surface of a Quaternary drumlinoid ridge. Paleoseismic trenching and electrical resistivity tomography surveys across the scarp reveal a single reverse‐slip earthquake produced a fault‐propagation fold above a blind southwest‐dipping fault. Five geologically plausible chronological models of radiocarbon dated charcoal constrain the likely earthquake age to between 4.7 and 2.3 ka. Fault‐propagation fold modeling indicates ∼3.2 m of reverse slip on a blind, 50° southwest‐dipping fault can reproduce the observed deformation. Fault scaling relations suggest a M 6.1–7.6 earthquake with a 13 to 73‐km‐long surface rupture and 2.3–3.2 m of dip slip may be responsible for the deformation observed in the paleoseismic trench. An earthquake near this magnitude in Greater Victoria could result in major damage, and our results highlight the importance of augmenting instrumental monitoring networks with remote sensing and field studies to identify and characterize active faults in similarily challenging environments. Plain Language Summary: Faults occurring in the upper plate above a subduction zone are often located near densely populated coastal areas, but their hazard is often underappreciated due to their low deformation rates. In the northern Cascadia forearc on the west coast of North America, high‐resolution topography and geologic mapping show a ∼2.3‐m‐high scarp across a ∼14,000 year‐old land surface 10 km north of downtown Victoria, British Columbia, Canada. This newly identified fault, the XEOLXELEK‐Elk Lake fault (XELF), crosses Saanich Peninsula within Greater Victoria and poses a hazard to the region's ∼400,000 inhabitants. Therefore, determining whether it produced recent large earthquakes is important for updating regional earthquake hazard models and increasing earthquake preparedness. To study the earthquake history of the fault, we used shallow geophysical techniques and excavated a trench across the scarp to examine the sedimentary record of deformation. These combined methodologies determined a single large earthquake, of magnitude 6.1–7.6, likely occurred on the XELF between ∼4,700 and 2,300 years ago. A similar future earthquake on the XELF Lake fault could cause major damage to the Greater Victoria area. Thus, our results can improve future earthquake hazard assessments. Key Points: We document a single Holocene earthquake on a previously unidentified fault in the northern Cascadia forearc near Victoria, CanadaTrenching, shallow geophysics, and fault‐fold modeling show 2.3–3.2 m of dip slip during an M 6.1–7.6 earthquake between 4.7 and 2.3 kaA combination of methodologies can enhance the study of active faults in forearcs, urbanized areas, and previously glaciated terrain [ABSTRACT FROM AUTHOR]

The San Juan fault (SJF), on southern Vancouver Island, Canada, juxtaposes the oceanic Wrangellia and Pacific Rim terranes in the northern Cascadia forearc, and has been suggested to play a role in multiple Mesozoic‐Cenozoic terrane accretion events. However, direct observations of the SJF's kinematics have not been documented and its exact role in accommodating strain arising from terrane accretion is unknown. To test if, how, and when the SJF accommodated accretion‐related strain, we use geologic mapping, kinematic inversion of fault‐plane slickenlines, and dating of marine sediments to constrain the timing and direction of brittle slip of the SJF. P‐ and T‐axes from kinematic inversions indicate predominantly left‐lateral slip. Left‐lateral brittle faulting cross‐cuts ∼51 Ma magmatic intrusions and foliation, providing a maximum age of brittle deformation. The fault zone is non‐conformably overlain by a >300 m‐thick sequence of clastic marine shelf and slope sediments that are not left‐laterally offset. A strontium isotope age of foraminifers helps constrain the depositional age of the sediments to late Eocene–early Oligocene, bracketing left‐lateral slip to the Eocene. Eocene left‐lateral slip is temporally and kinematically consistent with regional southwest‐northeast compression during accretion of the Siletzia ocean island plateau, suggesting brittle slip on the SJF accommodated strain resulting from the accretion of this terrane. This result does not support hypotheses that brittle slip along the SJF directly accommodated earlier accretion of the Pacific Rim terrane to Wrangellia, instead it offsets the older accretionary boundary between these two terranes. Key Points: The San Juan fault is a high‐angle fault that accommodated brittle left‐lateral slipCross‐cutting relations between the fault, metamorphic foliation, and overlying sediments, constrain left lateral slip to the EoceneEocene left‐lateral slip is consistent with strain induced by the accretion of the Siletzia terrane [ABSTRACT FROM AUTHOR]