Active faulting at the northeast margin of the greater Puget lowland : a paleoseismic and magnetic-anomaly study of the Kendall fault scarp, Whatcom County, northwest Washington

LiDAR mapping reveals a prominent, 4 km long, south-side up fault scarp that occurs along trend and within 500 m of the Boulder Creek fault in the North Fork Nooksack River valley. Thrust faulting of Late Pleistocene glacial outwash over Holocene soils produced the east-west trending fault scarp. Each time the fault scarp rose, streams flowing south into the Nooksack River ponded along it to form a wetland. Trench and wetland stratigraphy demonstrates a minimum of three late Holocene earthquakes produced the fault scarp. The earliest earthquake generated a fold scarp, followed by two surface-rupturing earthquakes to produce a combined vertical offset of at least two meters. Based on Mazama ash found above the lowest buried wetland soil, the folding event occurred shortly before ~7700 yr BP. Dated organic material from wetland and trench buried soils indicates ages of ~3000 yr BP and ~900 yr BP for the two surface-rupture events. The record of late Holocene earthquakes that produced the Kendall fault scarp demonstrates that Puget Sound seismic hazard assessments must now consider surface rupturing events and associated ground motion potential in the Bellingham area. Furthermore, the northern limit of north-south compression in western Washington, previously considered to be the Devils Mountain fault zone ~60 km to the south, should now be expanded northward to include active faulting in the Nooksack River valley. The active faulting may be along an active strand of the previously mapped Boulder Creek normal fault. The opposing sense of displacement, north-side up on the Boulder Creek fault and south-side up on the Kendall fault scarp, may reflect recent reactivation of the Boulder Creek fault as a north-verging thrust fault. Both the fault and fault scarp straddle a significant aeromagnetic anomaly; we test whether they are structurally related by analyzing aeromagnetic and magnetic ground survey data. Analyses of aeromagnetic data, however, do not provide direct evidence of the recent reverse motion recorded in the trenches but may suggest the location of the Boulder Creek fault hidden beneath a thick glacial outwash cover.