ACTIVE FAULTING AT THE NORTHEAST MARGIN OF THE GREATER PUGET LOWLAND: A TRENCHING AND WETLAND CORING STUDY OF THE KENDALL FAULT SCARP, WHATCOM COUNTY, NORTHWEST WASHINGTON

LiDAR mapping in North Fork Nooksack River valley reveals a prominent, 4 km long, south-side up fault scarp that occurs along trend and within 500 m of the Boulder Creek fault. Thrust faulting of Late Pleistocene glacial outwash over Holocene soils produced the east-west trending fault scarp. Each time the 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 in trench and wetland stratigraphy demonstrates that Puget Sound seismic hazard assessments must now consider surface rupturing events and associated ground motion potential in the Bellingham area. Consequently, the USGS is incorporating this earthquake history in the 2007 update of the National Earthquake Hazard maps. Furthermore, the northern limit of north-south compression in western Washington, previously considered to be the Devils Mountain fault zone to the south, should now be expanded northward to include active faulting in the Nooksack River valley.