USING AIRBORNE LIDAR FOR FORENSIC STRUCTURAL GEOLOGY— TWO ROCKFALL EXAMPLES FROM YOSEMITE NATIONAL PARK, CALIFORNIA

In order to understand the triggers of two fatal rockfalls in Yosemite National Park—the 1996 Happy Isles event and the 1999 Terbush event—we used a bare earth LiDAR digital elevation model (DEM) to evaluate the possibility that some joints have orientations capable of delivering seepage from tourist facilities near the top of Glacier Point to the rockfall initiation sites. We postulated the existence of hypothetical planes passing through the inferred locations of known water sources and rockfall initiation sites, calculated their dip vectors, and identified DEM cells with dip vectors diverging from the hypothetical discontinuities by ≤ 10°. The angle α between the hypothetical and observed dip directions was calculated from the vector dot product cos α = h•a, where h and a are the direction cosine vectors of the hypothetical and actual dip vectors for each DEM cell. Although the probability of any single DEM cell having an orientation falling within our α ≤ 10° criterion by chance is about p = 1.5%, the probability of several adjacent cells forming a patch discernable on the LiDAR DEM becomes vanishingly small as the patch size grows. For example, we estimate the probability of 10 adjacent cells satisfying our criterion is on the order of p = 10^–18. Previous publications suggest the existence of a diffuse ENE dipping joint set, known as the J2 set, with a mean orientation incapable of delivering groundwater to either of the initiation sites. Our analysis, however, shows two distinct but overlapping joint sets, one dipping NE and the other ENE, instead of a single J2 set. The mean dip vector of the NE dipping set is in excellent agreement with the orientation necessary to convey seepage from an overflowing water storage tank to the Terbush rockfall initiation point. The ENE dipping set alone, which corresponds to the previously identified J2 set, does not have a mean orientation conducive to the delivery of seepage to either rockfall source area. The resultant of the NE and ENE set dip vectors, however, is virtually indistinguishable from the orientation necessary to deliver groundwater from the inferred location of a leach field to the Happy Isles rockfall initiation site. Thus, seepage could have traveled from the leach field to the Happy Isles initiation site by following a combination of NE and ENE dipping joints.