Probable surface ruptures along the San Juan fault in the recently-deglaciated northern Cascadia forearc

Abstract

Forearc crustal faults pose significant risks within the Cascadia subduction zone, and a wealth of new lidar data allow for their identification and characterization. In the northern Cascadia forearc, the E-W-striking, terrane-bounding San Juan fault (SJF) displays topographic lineaments that are more conspicuous in lidar imagery than those along other known active faults in the region, but it remains unresolved whether the fault has ruptured in large prehistoric earthquakes and poses a risk to the ~600,000 people living on southern Vancouver Island. We map a relatively continuous and linear set of 2-150 m high, north-facing scarps along the fault trace, but find limited evidence of clear offset of Quaternary sediment that would be diagnostic of neotectonic activity. An alternative hypothesis is that the scarp formed due to differential glacial erosion of the distinct bedrock units juxtaposed by the fault. We argue against this hypothesis using Schmidt hammer measurements, which reveal harder rocks on the lowered (north) side of the scarp than on the raised (south) side, suggesting that south-side-up motion is tectonically driven. In combination with the vertical scarps, curved channel offsets are consistent with dextral-reverse slip, which would invert the sinistral offsets accrued by the fault in its early history. We also use electrical resistivity tomography to investigate one prominent, uphill-facing scarp. Disruption beneath the scarp to otherwise continuous subsurface structure is further evidence of late Quaternary rupture. Though somewhat equivocal, these results strongly motivate future paleoseismic investigations along the SJF.