Utilizing Geophysical Imaging to Investigate the Blackhawk Landslide

The Blackhawk Landslide (BHL) is a prehistoric landslide that transported 2.8x10⁸ m³ of crushed rock approximately 9 km from its origin at Blackhawk Mountain, which is located along the southeastern edge of Lucerne Valley, California. Long-runout landslides such as the BHL can be distinguished from other types of landslides by their exceptionally high volume and runout (suggesting a low coefficient of friction), and tall lateral ridges. A number of conceptual models including Shreve's air layer lubrication hypothesis and Campbell's self-lubrication hypothesis have been proposed in an attempt to describe the mechanisms by which the BHL and other long-runout landslides operate.

We seek to image the interior and exterior of the BHL using direct-current resistivity, gravity, ground penetrating radar (GPR), and light detection and ranging (LiDAR) to examine the mobility characteristics of this long-runout landslide. Specifically, we aim to improve our understanding of the nature of the boundary between the landslide material and underlying surface, and compare the results of our findings with presently available models for the mechanism responsible for the landslide. Furthermore, we will attempt to apply the findings from this case study to other long-runout landslides.

In March of 2019, we carried out initial surveys using resistivity, gravity, and GPR in an attempt to locate the slide plane approximately 1 km from the distal edge of the landslide. GPR profiles perpendicular to the direction of travel reveal horizontal stratification for the upper 6-8 m of the landslide, which provides evidence that it travelled as a nearly nondeforming sheet. Inverse resistivity models indicate a potential interface at 8-12 m depth near the western lateral edge, which is consistent with the depth of the stratification observed in the GPR profiles. Future surveys will be carried out with newly acquired resistivity and GPR equipment that will be able to generate deeper profiles with higher resolution. Additionally, measurements will be made throughout the year to observe the effects of rain, which may illuminate differences in porosity and/or permeability.