Mapping and Statistical Analysis of Precariously Balanced Rocks at Centennial Bluff, CA, Using UAV-Based 3D Semantic Mapping Method

Zhiang Chen, Devin McPhillips, Katherine M. Scharer, & Zachary E. Ross

Submitted September 7, 2025, SCEC Contribution #14738, 2025 SCEC Annual Meeting Poster #TBD

The spatial distribution and statistical characteristics of precariously balanced rocks (PBRs) provide important insights into the intensity and variability of past ground motions and are thus a natural archive for paleoseismology. In this study, we aim to map the spatial distribution of PBRs and quantify their statistical fragility characteristics at Centennial Bluff, California, a granitic outcrop near the Nevada border where numerous PBRs survived strong shaking from the nearby 2021 M6.0 Antelope Valley earthquake. This site, however, presents a challenging mapping environment due to its rugged rocky terrain with abundant exposed boulders and outcrops. To address this challenge, we use a UAV and machine learning to identify and characterize fragile rock features across the landscape. To identify features, we extend a 3D segmentation workflow by integrating a 2D rock classification neural network, which detects all rock surfaces (freestanding rocks and bedrock) while excluding non-rock objects segmented by the Segment Anything Model, such as bushes, shadows, and background terrain. The filtered rock masks guide our 3D segmentation algorithm, enabling automated separation of rock bodies from surrounding terrain in UAV-SfM point clouds.

We have validated the 3D segmentation results against independently acquired field photographs of PBRs. We then apply a series of post-processing filters that focus on height-to-width ratio (H/W) as a proxy for fragility. The H/W distribution reveals a pronounced bimodal pattern. We interpret the lower H/W mode as the product of a past large earthquake that reduced rock slenderness, and the higher H/W mode as reflecting more recently produced PBRs that have not yet been reshaped by strong shaking. These findings have important implications for PBR production and transport modeling. Ongoing work will integrate detailed field measurements to refine these interpretations and further constrain the local ground motions at Centennial Bluff.

Key Words
Fragile geological features, precariously balanced, paleoseismology rocks,

Citation
Chen, Z., McPhillips, D., Scharer, K. M., & Ross, Z. E. (2025, 09). Mapping and Statistical Analysis of Precariously Balanced Rocks at Centennial Bluff, CA, Using UAV-Based 3D Semantic Mapping Method. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology