LATE HOLOCENE EARTHQUAKES ON THE WATERMAN POINT REVERSE FAULT, ANOTHER ALSM-DISCOVERED FAULT SCARP IN THE SEATTLE FAULT ZONE, PUGET LOWLAND, WASHINGTON

The Seattle fault zone—a 70-km-long, east-west belt of young reverse faults—is one of several fault zones in the Puget Lowland that accommodates 4-7 mm/yr of north-south shortening resulting from northward migration of forearc blocks along the Cascadia convergent margin. Like crustal faults in similar densely populated basins, the faults may pose a greater earthquake hazard than do much longer but more distant plate-boundary faults. Airborne Laser Swath Mapping (ALSM, also known as LiDAR mapping) of the Puget Lowland reveals previously unknown scarps beneath the forest canopy along some young reverse faults. One such fault is marked by a 1- to 5-m-high, south-facing, scarp that trends east from Waterman Point, about 4 km northeast of Bremerton and 17 km west of Seattle in the Seattle fault zone. Three trenches across the scarp display hanging walls of Oligocene sandstone and mudstone thrust over forest A horizons developed on drift, but differ in the number of earthquakes identified. One trench shows late Holocene slip of 5.5 m along a single exposed fault. In a second trench, two fault splays bounding folded drift and soil could be the result of one or two earthquakes. In the third trench, a second earthquake is recorded by a second buried forest A horizon developed on slope and hanging-wall-collapse colluvium produced during the first earthquake. Although 14C ages from the second buried A horizon in the third trench are indistinguishable from those from the A horizons buried during the first earthquake, the two buried A horizons in the third trench show that the large net fault slips (5.5 m and 4.0 m) in the first and second trenches are the result of two earthquakes. The youngest of 16 14C ages on charcoal fragments from the older buried A horizons suggests an AD 970-1160 age for the first earthquake, which is probably the AD 900-930 earthquake that raised marine terraces and triggered a tsunami in Puget Sound. The second earthquake apparently post-dates the first by less than a few hundred years. If the first earthquake on the Waterman Point fault correlates with the youngest earthquake on a similar, previously studied en echelon scarp 3 km to the northeast, the record of earthquakes on faults of the Seattle fault zone in the past 2500 years increases to 4 or 5.