PALEOSEISMIC TRENCHING EXPOSES MULTIPLE LATE QUATERNARY RUPTURES ON THE BEAUFORT RANGE FAULT, NORTHERN CASCADIA FOREARC, BRITISH COLUMBIA, CANADA

Active faults above subduction zones accommodate a portion of plate-boundary deformation and pose significant seismic hazard for nearby communities and infrastructure. Many crustal faults in the northern forearc of the Cascadia subduction zone formed as a result of Eocene terrane accretion and several, including the Beaufort Range fault on central Vancouver Island, BC, Canada, may be actively accommodating deformation. Our geomorphic surveys and mapping of the Beaufort Range fault show systematic vertical and right-lateral offset of post-glacial (< 13 ka) geomorphic piercing points, indicating that the fault has been active in the late Quaternary. Displacement-length scaling relationships suggest that an earthquake rupturing the ~35 km of mapped scarps would be ~Mw 6.9. Characterizing the timing, displacement, and kinematics of individual earthquake ruptures will better constrain their recurrence and magnitude, crucial for assessing seismic hazard and understanding how plate boundary strain is accommodated on crustal faults. Here we show results of paleoseismic trenching completed in summer 2019 on the southern Beaufort Range fault. A fault-perpendicular trench exposed deposits related to five surface-rupturing earthquakes, and preliminary radiocarbon dates from 13 samples collected from colluvial wedges and sediments ponded against the uphill-facing scarp suggest that the five surface-rupturing events have occurred over the past ~12 kyr. The rupture event ages do not appear to cluster temporally, during glacial retreat or other time periods. Colluvial wedge thicknesses (~0.5 m) translate to ~1 m of vertical offset per event. Average right-lateral cumulative offset of channels at this site is 2.5 m, as a result of at least five events. Thus, displacement per event may exceed 1 m. The most recent colluvial wedge is very thin and discontinuous and the scarp free face is well preserved, indicating that it is geomorphically young. Preliminary radiocarbon dates suggest that the most recent rupture occurred in the past ~200 yrs. Right-lateral oblique slip on the Beaufort Range fault accommodates some of the trench-parallel component of Juan de Fuca-North America plate-boundary deformation, and poses significant hazard to Port Alberni, BC, nearby hydroelectric dams, and other communities in the region.