FOLDING AND RUPTURE OF AN UPLIFTED HOLOCENE MARINE PLATFORM IN THE SEATTLE FAULT ZONE, WASHINGTON, REVEALED BY AIRBORNE LASER SWATH MAPPING

The Seattle Fault zone (SFZ) is a 5 to 7 km-wide, east-west trending zone of south-dipping thrust faults, north-dipping backthrusts, and folds. Puget Sound marine terraces record uplift of a wave-cut platform about A.D. 900 (Bucknam et al., 1992) during one, or probably several, earthquakes in the SFZ (Nelson et al., 2000). Airborne Laser Swath Mapping (ALSM) surveys of Kitsap County and part of King County document spatial patterns of terrace uplift, as measured from shoreline angles at the landward edge of the uplifted platform. The shoreline angles are mapped using profiles and slope images interactively generated from ALSM "bald Earth" DEMs with a 1.8-m grid spacing and decimeter-level vertical accuracy. On the east side of Puget Sound, terraces (up to 8 m above MHHW) from Williams Point to north of Alki Point define a 5-km-wide, north-vergent anticline with planar limbs. The fold amplitude is 6 m, and the limbs dip more steeply north (0.25°) than south (0.10°). The hinge, located at Alki Point, is above the SFZ Frontal thrust suggesting slip on it during the ~ A.D. 900 earthquakes produced the fold. Three 1 to 2 m-high south-side-up terrace offsets on the south limb suggest lesser slip occurred on more southerly thrusts during, or after, the Frontal thrust events. The offsets align with the projection of three thrusts mapped offshore in seismic-reflection profiles. Western Puget Sound terraces also define a broad anticline extending 13 km west from southern Bainbridge Island to Dyes Inlet, but the structure is more complex with significant along-strike heterogeneity over short distances. The anticline hinge is locally modified by backthrusts producing a tightened, south-vergent fold with an amplitude as large as 8 m. The hinge in the Bainbridge Island terrace coincides with the Toe Jam Hill backthrust scarp in the island interior. On the south limb a terrace around Waterman Point, southwest of Bainbridge Island, is ruptured by two backthrusts expressed as 3-m-high, south-facing scarps. Terrace rupture indicates that slip occurred on the backthrusts during the ~ A.D. 900 earthquakes, consistent with dated surface-faulting events along the Toe Jam Hill fault scarp. Farther south, localized terrace uplifts as large as 3 m suggest slip may have also occurred on two unnamed thrusts within the SFZ during, or after, the ~ A.D. 900 events.