EVIDENCE FOR ONE OR TWO LATE HOLOCENE EARTHQUAKES ON THE UTSALADY POINT FAULT, NORTHERN PUGET LOWLAND, WASHINGTON

Trenches across the Utsalady Point fault in the northern Puget Lowland of Washington reveal evidence of at least one and probably two late Holocene earthquakes. The "Teeka" and "Duffers" trenches were located along a 1.4-km-long, 1- to 4-m-high, northwest-trending, southwest-facing, topographic scarp recognized by Airborne Laser Swath Mapping. Glaciomarine drift stratigraphy reveals evidence of vertical (115 to 130 cm) and left-lateral (200 to 220 cm) fault offsets in Teeka trench. Radiocarbon ages from a buried soil A horizon and overlying slope colluvium along with historical constraints suggest that this faulting occurred 100 to 400 cal yr B.P. (A.D. 1550 to 1850). In Duffers trench, vertical separation of 370 cm is accommodated by faulting (~210 cm) and folding (~160 cm); significant lateral slip is also likely but can not be constrained. Stratigraphic relationships and radiocarbon ages from buried soil, colluvium, and hanging-wall fissure sediment suggest the deformation at Duffers occurred in two earthquakes, between 100 to 500 and 1100 to 2200 cal yr B.P., but deformation in a single earthquake is also possible. For the two-earthquake hypothesis, deformation at Teeka trench in the first event involved folding but not faulting.

Regional relationships suggest the earthquake(s) had M > ~6.7 and that offshore rupture of uplift may have produced tsunamis. Based on this investigation and related recent studies, the maximum recurrence for large ground-rupturing crustal-fault earthquakes in the Puget Lowland is about 400 to 600 years and is probably much less. Left-lateral faulting on the Utsalady Point fault is not consistent with regional deformation models based on GPS data, highlighting the need for a denser GPS network in this complex, tectonically active region.