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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‐ andT‐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.

 

The loss of life and economic consequences caused by several recent earthquakes demonstrate the importance of developing seismically safe building codes. The quantification of seismic hazard, which describes the likelihood of earthquake‐induced ground shaking at a site for a specific time period, is a key component of a building code, as it helps ensure that structures are designed to withstand the ground shaking caused by a potential earthquake. Geologic or geomorphic data represent important inputs to the most common seismic hazard model (probabilistic seismic hazard analyses, or PSHAs), as they can characterize the magnitudes, locations, and types of earthquakes that occur over long intervals (thousands of years). However, several recent earthquakes and a growing body of work challenge many of our previous assumptions about the characteristics of active faults and their rupture behavior, and these complexities can be challenging to accurately represent in PSHA. Here, we discuss several of the outstanding challenges surrounding geologic and geomorphic data sets frequently used in PSHA. The topics we discuss include how to utilize paleoseismic records in fault slip rate estimates, understanding and modeling earthquake recurrence and fault complexity, the development and use of fault‐scaling relationships, and characterizing enigmatic faults using topography. Making headway in these areas will likely require advancements in our understanding of the fundamental science behind processes such as fault triggering, complex rupture, earthquake clustering, and fault scaling. Progress in these topics will be important if we wish to accurately capture earthquake behavior in a variety of settings using PSHA in the future.

 

ABSTRACT New paleoseismic trenching indicates late Quaternary oblique right-lateral slip on the Leech River fault, southern Vancouver Island, Canada, and constrains permanent forearc deformation in northern Cascadia. A south-to-north reduction in northward Global Navigation Satellite System velocities and seismicity across the Olympic Mountains, Strait of Juan de Fuca (JDF), and the southern Strait of Georgia, has been used as evidence for permanent north–south crustal shortening via thrust faulting between a northward migrating southern forearc and rigid northern backstop in southwestern Canada. However, previous paleoseismic studies indicating late Quaternary oblique right-lateral slip on west-northwest-striking forearc faults north of the Olympic Mountains and in the southern Strait of Georgia are more consistent with forearc deformation models that invoke oroclinal bending and(or) westward extrusion of the Olympic Mountains. To help evaluate strain further north across the Strait of JDF, we present the results from two new paleoseismic trenches excavated across the Leech River fault. In the easternmost Good Hope trench, we document a vertical fault zone and a broad anticline deforming glacial till. Comparison of till clast orientations in faulted and undeformed glacial till shows evidence for postdeposition faulted till clast rotation, indicating strike-slip shear. The orientation of opening mode fissuring during surface rupture is consistent with right-lateral slip and the published regional SHmax directions. Vertical separation and the formation of scarp-derived colluvium along one fault also indicate a dip-slip component. Radiocarbon charcoal dating within offset glacial till and scarp-derived colluvium suggest a single surface rupturing earthquake at 9.4±3.4  ka. The oblique right-lateral slip sense inferred in the Good Hope trench is consistent with slip kinematics observed on other regional west-northwest-striking faults and indicates that these structures do not accommodate significant north–south shortening via thrust faulting.