Using LIDAR Surface Deformation Mapping to Constrain Earthquake Magnitudes on the Seattle Fault in Washington State, USA

Abstract

Airborne laser swath mapping (ALSM) conducted by the Puget Sound LIDAR Consortium reveals uplift, folding, and faulting associated with earthquakes on the Seattle fault zone (SFZ) in Washington State, USA. The SFZ is a 5-to 7-km-wide, east-west trending zone of south-dipping thrust faults, north-dipping backthrust faults, and folds that runs through the Seattle urban area. Marine terraces along the shoreline of the Puget Sound, visible in ALSM images, record uplift of a wave-cut platform about A.D. 900 during a large earthquake on the SFZ. Bald Earth digital elevation models (DEMs) with a 1.8-meter grid spacing document spatial patterns of terrace uplift, as measured from the elevation of shoreline angles at the landward edge of the uplifted platform. Shoreline angles were identified using profiles and slope images interactively generated from the DEMs. The bald Earth DEMs were produced by applying a virtual deforestation, despike filtering algorithm to point clouds of laser data acquired by the TerraPoint ALTMS instrument, a small-footprint, discrete-return system that records up to four returns per laser pulse. A data density of ~1 pulse per square meter was acquired using 50% sidelap between adjacent swaths. The elevations of the uplifted terraces provide a clear picture of earthquake deformation. On the east side of Puget Sound, marine terraces define a 5-km-wide, north-vergent geologic anticline with planar limbs. The fold amplitude is 6 meters, and the limbs dip more steeply north (0.25deg) than south (0.10deg). The fold hinge is located above the frontal thrust fault of the SFZ. Near Bainbridge Island, the anticline hinge is locally modified by backthrust faults producing a tightened, south-vergent fold with amplitude as large as 8 m. Terrace deformation patterns suggest that slip on the frontal thrust during the A.D. 900 earthquake produced the anticlinal folding and that lesser slip occurred at that time, or more recently, on structurally higher thrust faults and backthrust faults within the SFZ. Inverse modeling of slip on the Seattle fault, constrained by elevations of the uplifted marine terraces mapped in the ALSM images, provides a well-constrained estimate of the magnitude of the A.D. 900 Seattle fault earthquake. For each of several different Seattle fault subsurface geometry interpretations, we use a linear inversion algorithm to solve for distributed slip on the fault surfaces. We calculate moment magnitudes of 7.2 to 7.4 directly from the different slip solutions. This study represents the first known use of LIDAR-based coseismic deformation mapping in solving for earthquake source parameters.