INITIAL GEOPHYSICAL SURVEY FINDINGS ON THE INTERIOR STRUCTURE OF THE BLACKHAWK LANDSLIDE

The Blackhawk Landslide (BHL) is a prehistoric, long-runout landslide that descended from Blackhawk Mountain located on the southeastern side of Lucerne Valley, California. The BHL exhibits several characteristic traits of long-runout landslides, including tall lateral ridges and a 9 km runout that transported 2.8x10⁸ m³ of crushed rock. We seek to examine the nature of the slide plane and the mobility characteristics of the BHL by imaging the interior and exterior of the slide using direct-current resistivity, gravity, ground penetrating radar (GPR), and light detection and ranging (LiDAR) techniques. We will compare the results of our findings with presently available conceptual models that attempt to describe the mechanisms by which long-runout landslides operate, including Shreve’s air layer lubrication hypothesis and Campbell’s self-lubrication hypothesis. Furthermore, we will attempt to apply the findings from this case study to other long-runout landslides.

Initial surveys conducted in March, October, and November of 2019 utilized resistivity, gravity, GPR, and LiDAR in various locations along the BHL, with an emphasis on the toe, where the internal structure is likely to be least complex, and lateral edges. Resistivity models indicate a potential interface at 8-12 m depth near the western lateral edge, which is consistent with horizontal stratification for the upper 6-8 m observed in GPR profiles and provides evidence that the slide may have travelled as a nearly nondeforming sheet. Gravity measurements exhibit an increasing trend in the southeast direction, which may indicate a region of higher density beyond the eastern extent of the BHL. LiDAR surveys span 0.3 km² of the toe of the slide and reveal that local lateral ridges vary between 13.6 and 24.5 m in height. Future surveys will utilize resistivity and GPR equipment capable of generating deeper profiles with higher resolution. We will also explore the effect of rainfall on imaging the slide plane using resistivity measurements.