Meters of Synchronous Holocene Slip on two Strands of a Fault in the Western Puget Sound Lowland, Washington

Stratigraphy and tree stumps of Price Lake and adjacent wetlands suggest that two strands of the Saddle Mountain fault zone ruptured in the same century or two 1000-1300 cal yr B.P., perhaps as a prelude to the tectonic paroxysm that included the Seattle earthquake of 1020-1050 cal yr B.P. Initially dated in the 1970s, times of fault rupture near Price Lake are now closely limited by radiocarbon analyses of delicate plant remains that probably formed just before and just after the faulting. Rupture on the East Saddle Mountain strand produced a west-facing scarp that impounded Price Lake on the east. Rupture on the West Saddle Mountain strand made an additional west-facing scarp that projects through the lake. Cones and needles from the submerged soil between the two scarps gave concordant ages between 1080 and 1290 cal yr B.P. (2 sigma; 2 samples). A western redcedar cone collected from the submerged forest soil west of the West scarp yielded a similar age range of 1050-1270 cal yr B.P. (2 sigma). These statistically indistinguishable ages imply that the East and West strands produced their most recent surface ruptures within the same century or two. Such similarity in scarp age is further shown by crossdating of the annual rings of drowned, growth-position Douglas-fir, and by lake-surface level and modest decay shared by the tops of all the growth-position stumps in the current lake. Had the West strand ruptured earlier than the East strand by more than a few hundred years, stumps west of the West scarp would have died before and therefore rotted more than stumps to its east. If instead the West strand ruptured after the East strand by more than a few hundred years, the tops of stumps west of the West strand would lie below present lake level. Dip slip 1000-1300 cal yr B.P. likely exceeded 4 m on the East strand and 2 m on the West strand. The 4-m estimate is based on heights of stumps and buried soils between the scarps. There, the tallest stumps in growth position extend 3 m below the lake surface, and corresponding flooded soils extend 1 m above the lake surface. The 2-m estimate for the West strand is based on contrasting depths to a flooded soil. The two estimates agree with trench excavations made in the 1970's across the East and West scarps, which revealed 3.5 m and 1.8 m of reverse slip, respectively. SW-plunging striations on the hanging wall of the East strand indicate left-lateral movement. Down-to-the-west slip with a left-lateral component might be explained by underthrusting of the Olympic Mountains in combination with northward forearc migration.