3-D GROUND PENETRATING RADAR VISUALIZATION OF AN EARTHQUAKE INDUCED COASTAL SCARP

The objectives of three dimensional (3-D) ground penetrating radar (GPR) surveys are to construct images of the subsurface and to utilize this data to better understand the internal stratigraphy and geometry of geomorphic environments. Studies involving the use of GPR along the southwest Pacific Ocean coastline of Washington State, USA show a shingle-like accretionary depositional pattern of beach and upper shoreface reflections interrupted by steeper dipping erosional scarps. These scarps are dated to coseismic subsidence events associated with regional subduction earthquakes during the late Holocene. A 3-D GPR survey was conducted along one of these earthquake induced coastal scarp to better visualize the stratigraphic relationships of a prograding shoreline within a tectonically active coastal environment.

The survey site was located on the Willapa barrier spit, north of the community Long Beach, Washington, USA. Identical length GPR profiles (225 MHz) running parallel to each other along an x-y grid (6 m x 30 m) were collected on a level unpaved road running parallel to the slope of a previously imaged scarp. The GPR transects were spaced 0.25 m apart with a step size of 0.10 m to provide a high amount of horizontal resolution. The traces have a sampling interval of 400 ps and were vertically stacked 8 times. Data was collected with a pulseEKKO 1000 system using an automatic step odometer which proved to be significantly more time-efficient than manual data collection; total survey took 3 hours, including setup. The profiles were processed using pulseEKKO software, compiled and viewed in several 3-D graphics rendering program. A CMP survey was performed at the site resulting in an average near surface velocity of 0.09 m/ns being calculated.

Through the use of the collected 3-D GPR dataset internal stratigraphy of this coastal geomorphic environment can be imaged. The data shows reflections to 140 ns (approximately 7 m) with one facies interpreted as normal coastal progradation. A second facies of steeper dipping reflections from 10 to 30 m is interpreted as the uppermost boundary of an erosional scarp which was formed by massive erosion due to coastal subsidence. The project demonstrates that 3-D GPR can be an effective and time-efficient method of data collection and stratigraphic interpretation of the subsurface.