PALEOSEISMOLOGY OF THE BOULDER CREEK FAULT, KENDALL, WA

Trenching of 2 sites ~1 km apart along the E-trending Kendall scarp, ~35 km NE of Bellingham, WA, documents Holocene reverse faulting and folding. The scarp is ≥4 km long, 2-4 m high and cuts Sumas stade (~12-10 ka) outwash and till. The Kendall scarp may have reactivated the Eocene Boulder Creek normal fault.

In the eastern trench, the oldest unit is till with ~4m of vertical relief on the top surface. Overlying the till is a sequence of 3 pebbly to cobbly sands, each with a soil above it. The coarse sands occur directly below the scarp and are interpreted as colluvium. At the N end of the trench, organic silts and clays overlying till are interpreted as wetland deposits. The wetland units grade up into forest soil and are abruptly overlain by diatom rich mud. No faults are observed in the trench. We infer 2 or 3 earthquakes from the stratigraphy; during each event, hangingwall folding and uplift shed colluvium onto the footwall burying the soil. The oldest earthquake also disrupted drainage and created the wetland. Between earthquakes, the wetland dried up and a forest soil formed. In the second event, colluvium buried the new surface and the wetland subsided again. A third possible event is interpreted from the upper buried soil/colluvium pair and correlation with wetland cores further N. Assuming 3 earthquakes to create the 4 m scarp, no original surface relief, and 45° fault dip, each earthquake produced ~1.9 m thrust slip.

In the western trench, a ~2 m high scarp is developed in outwash gravels. Imbricate thrusts dip ~20°S. One earthquake is interpreted from faulting of gravel over soil with a vertical component of >1m; the fault tip is buried by colluvium. A possible second event disrupted younger colluvial units. At a roadcut exposure ~200 m W of the trench, gravels are folded but not faulted at the surface.

These data, combined with data from Barnett et al. (this session), indicate the Kendall scarp reflects a complex underlying structure. Imbricate thrusts dip from <20 to >45° in outwash, and locally bound highly folded and overturned beds. Till appears to have deformed by folding at shallow depth. The earthquake history and magnitude of uplift, however, are consistent along strike, indicating 3 Holocene events with ~1-1.3 m vertical offset each. Although the length and depth of the fault are unknown, the fault slip data imply events with M 6.8-7.2.