HR: 16:45h
AN: G24A-06    [Abstracts]
TI: Constraining the Kinematics of the A.D. 900 Seattle Fault Earthquake With Geomechanical Modeling and LIDAR Surface Elevation Data
AU: * Muller, J R
EM: jmuller@core2.gsfc.nasa.gov
AF: NASA Goddard Space Flight Center, Planetary Geodynamics Lab Mail Code 698, Greenbelt, MD 20771 United States
AU: Harding, D J
EM: David.J.Harding@nasa.gov
AF: NASA Goddard Space Flight Center, Planetary Geodynamics Lab Mail Code 698, Greenbelt, MD 20771 United States
AB: Within the Puget Sound region of Washington State, several lines of geological evidence suggest that the largest upper-crustal earthquake within the past 2500 years occurred on the Seattle fault system in A.D. 900. Constraining the rupture characteristics of this event is of singular importance in evaluating the upper-bound seismic hazard and tsunami threat posed by upper-crustal (non-subduction) earthquakes to the Puget Lowland region. It is only possible to model the fault geometry, slip distribution, and moment magnitude of this earthquake with a data set of the surface elevation changes caused by this event. Due to the historic age of this earthquake, we use elevations of an uplifted marine terrace, digitally extracted from LIDAR images, as a novel source of coseismic surface deformation data for this event. Ideal for this forested region, LIDAR images, acquired via airborne laser swath mapping (ALSM), offer a drastic improvement over earlier topographic mapping techniques due to its improved resolution and its ability to measure the ground surface beneath dense vegetative cover. The LIDAR images reveal a single uplifted terrace, dated to 1000 cal yr B.P. near Restoration Point, that 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 nine meters 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, reverse fault scarps, the regional uplift pattern is a doubly-plunging antiform with steepened north limb, consistent with its location directly above the hanging wall of the frontal thrust of the south-dipping Seattle fault zone. Preliminary forward modeling results show that the earthquake magnitude is greater than M7. The greatest slip of the A.D. 900 event was most likely confined to the frontal thrust of the Seattle fault system and was centered beneath Puget Sound betweem Restoration Point and Alki Point and localized in the upper ten kilometers of the crust. Slip along the Toe Jam Hill and Waterman backthrusts merely modify this larger uplift pattern and would not have been sufficient to generate the observed regional distribution of terrace uplift.
DE: 1242 Seismic deformations (7205)
DE: 7209 Earthquake dynamics and mechanics
DE: 7221 Paleoseismology
DE: 7223 Seismic hazard assessment and prediction
DE: 8040 Remote sensing
SC: Geodesy [G]
MN: 2005 Joint Assembly