HR: 0800h
AN: S51C-1024    [Abstracts]
TI: Mw = 7.2-7.4 Estimated for AD900 Seattle Fault Earthquake by Geomechanical Modeling the Uplift of a LIDAR-mapped Marine Terrace
AU: * Muller, J R
EM: jmuller@core2.gsfc.nasa.gov
AF: NASA Goddard Space Fligh Center, Mail Code 698, Greenbelt, MD 20771 United States
AU: Harding, D J
EM: david.j.harding@nasa.gov
AF: NASA Goddard Space Fligh Center, Mail Code 698, Greenbelt, MD 20771 United States
AB: Inverse modeling of slip on the Seattle fault system, constrained by elevations of an uplifted marine terrace mapped on LIDAR images, suggests the moment magnitude of the A.D. 900 Seattle earthquake was 7.2 to 7.4. The terrace elevations that constrain the slip inversion are extracted from images generated from LIDAR surveys of the Puget Sound collected in 2000. The images reveal a single uplifted terrace, dated to 1000 cal yr B.P. near Restoration Point, which is morphologically continuous along the southern shoreline of Bainbridge Island and is visible at comparable elevations within a 25 km by 12 km region encompassing coastlines of West Seattle, Bremerton, East Bremerton, Port Orchard, and Waterman Point. Considering sea level changes since A.D. 900, the maximum uplift magnitudes of shoreline inner edges approach 9 m and are located at the southernmost coastline of Bainbridge Island and the northern tip of Waterman Point, while tilt magnitudes are modest - approaching 0.1 degrees. Although the terrace is locally offset and tilted near the Toe Jam Hill and Waterman north-dipping backthrusts, the regional uplift pattern is a doubly-plunging antiform with a steepened north limb, consistent with its location directly above the hanging wall of the frontal thrust of the south-dipping Seattle fault zone. We used a two-step process for modeling the slip distribution and earthquake magnitude. First, we use a non-linear inversion for uniform slip on a planar dislocation to determine that a rupture length of 30 km and width of 18 km best matched the mapped region of uplift. Second, constrained by this fault area, we created more complex fault geometries that match published interpretations of the subsurface geometry of the Seattle fault. For each of several different fault geometry interpretations, we then use a linear inversion code to solve for distributed slip on the fault surfaces. Moment magnitudes of 7.2 to 7.4 are calculated directly from the different slip solutions. In general, the greatest slip of the A.D. 900 event was confined to the frontal thrust of the Seattle fault system and was centered beneath Puget Sound between Restoration Point and Alki Point. Although multiple slip events are recorded in local paleoseismological trenches across the backthrust fault scarps (Nelson et al., 2003) and multiple terraces may be present locally adjacent to the backthrusts, we infer that the regional terrace uplift was associated entirely with the A.D. 900 event because (a) multiple terraces are not observed regionally in the LIDAR images, and (b) it was the largest and latest event observed in the trenches and the marsh excavation by Sherrod et al. (2000) at Restoration Point.
DE: 1209 Tectonic deformation (6924)
DE: 7221 Paleoseismology (8036)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8020 Mechanics, theory, and modeling
DE: 8040 Remote sensing
SC: Seismology [S]
MN: Fall Meeting 2005