This project will be a two-pronged investigation based on the combination of data obtained from remote sensing of the seafloor and both onshore and offshore core data.
Marine Geophysical Data:
Regional mapping of the northern slope and basin, including the Puerto Rico Trench, was conducted aboard the R.V. Ewing beginning in 1996 during the EW96-05 project initiated by Grindlay, Mann, and Dolan. The R.V. Ewing was equipped with DMS 2000 single-channel seismic (SCS) system, Hydrosweep multibeam bathymetric system, and the HMR1 sidescan sonar system (cruise report, 1996). Thirty-six, NNE-SSW ship tracks provide ~ 5,600 km of seismic reflection data for the northern insular slope of Puerto Rico, including the Puerto Rico Trench. The geophysical data will be used to establish both the geometry of the headscarp located at - 66.64° W 18.93° N and map the extent and thickness of the debris flow in order to better evaluate the volume of slope-forming material involved in the failure event.
Geologic Data:
Initial thicknesses of the carbonate strata pre-slope failure(s) is taken from core sample CPR-4, drilled by Kewanee in 1960 on the north coast of Puerto Rico in combination with seismic profiles of adjacent unfailed slope.
In the early 1960's, J.R. Connolly and Maurice Ewing collected samples of shelf, basin, and trench sediment. A significant amount of sediment within the trench is Pleistocene in age (1.8 Ma), suggesting the transport of shallow water marine sediments into deeper waters. Evidence of powerful turbidity flows is discovered in at least two cores where Connolly and Ewing observed 20-50 cm deposits of fine sand having traveled 100 km from a shallow water origin. The identification of slump material in cores downslope from the scarp, used in conjunction with the seismic profile lines, will contribute to a more accurate calculation of the magnitude and runout extent of the slope failure that created the amphitheater-shaped scarp.
Profile Generation and Data Calculations:
The combined geophysical data will be imported into FLEDERMAUS v. 4.3 for 3-dimensional spatial viewing (x-representing longitude, y-representing latitude, and z-representing depth in meters). Data will also be imported into ArcView GIS v. 3.2 and 3-D Spatial Analyst for profile generation.
Profiles of the headscarp height (m) from crown to base, depth measurements (m), and headscarp width and length (km) will be generated to constrain the geometry of the scarp located at -66.64° W 18.93° N. Because the adjacent intact slope-forming material serves as a good representative of what the slope/depth gradient was prior to the slope failure event, scarp slope/depth gradients will be compared to that of neighboring, unfailed slope strata. Profiles that extend across the trench floor downslope from the scarp will be created to map the extent of the debris flow.
The volume of slope-forming material involved in the failure will be based on the geometry of the headscarp and the extent and thickness of the debris flow. The results of the various profiles for the large scarp will be applied to the McAdoo wedge geometry equation for disintegrative landslides { Vslide = (k)(As)(h cos ð) } (McAdoo et al., 2000).
Scarp area, scarp slope angle, runout, and slope gradient adjacent to the scarp will be calculated using an average of individual cells. The average estimate of the potential volume (V) that could fail will be calculated by multiplying the slide area (As), potential headscarp height (h), average scarp slope gradient (cos ð), and a shape coefficient (k) using the McAdoo et al. (2000) wedge geometry equation. Seismic profile lines covering the debris runout indicate that the mass-wasting event was disintegrative, tapering off to smaller amounts toward the slide base. That morphology lends itself a shape coefficient of k=0.5.