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3D Mapping and Dynamic Analysis of Precariously Balanced Rocks for Fragility Modeling

We investigate the 3D geometry and dynamic behavior of precariously balanced rocks (PBRs) for fragility modeling. To accurately map and analyze fields of PBRs, we use a combination of advanced technologies, including unpiloted aerial vehicle (UAV)-based Structure-from-Motion, UAV lidar, and iPhone lidar. We developed a method combining machine learning and deterministic segmentation for precise 3D extraction of PBR geometry. The machine learning component employs a deep neural network to detect PBRs in 2D imagery, facilitating their localization in point clouds. Deterministic segmentation utilizes a region-growing algorithm to separate PBRs from their pedestals, guided by user-provided basal contact curves at their interface. This cascading approach enables high-precision geometry extraction. Using the extracted 3D geometries, we perform PBR overturning analysis with the Virtual Shake Robot v2.0 (VSR2). VSR2 is built on the PyBullet physics engine and the Robot Operating System framework, validated against physical PBR overturning experiments conducted on a large-scale shake table. Our work demonstrates cutting-edge tools for PBR mapping and fragility modeling, providing critical insights into local ground motion constraints inferred from fragile geological features. Ultimately, we aim to offer an automated solution for using PBRs to refine seismic hazard curves and enhance our understanding of regional seismic risks. We apply the toolkit to the PBR fields in the Sierra Nevada, east of Fresno, California, relatively far from known fault sources. The PBRs in this region are glacial erratics, deposited by the Tioga (26–19 ka) and Tahoe (60 ka) glaciations. This recent glaciation has stripped the landscape of typical evidence used to identify active faults, and the existence of the PBRs will hopefully help constrain seismic hazard in the area.

Session: Earthquake Shaking and the Geologic Record: Triggered Phenomena and Preserved Fragile Geologic Features - I

Type: Oral

Room: Holiday Ballroom 1

Date: 4/17/2025

Presentation Time: 02:45 PM (local time)

Presenting Author: Ramon Arrowsmith

Student Presenter: No

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