← Back to Sessions

Seismic Response of Rock Towers at the Trona Pinnacles, California

Session: Probabilistic Seismic Hazard Assessment: Where Do We Go from Here? I

Type: Oral

Date: 4/20/2021

Presentation Time: 10:00 AM Pacific

Description: 

Precariously balanced rocks (PBRs) have been used to constrain the intensity of past earthquake shaking. We analyzed the seismic response of another category of fragile geologic features, rock towers (RTs), using finite-element simulations. RTs differ from PBRs in that RTs are materially connected to bedrock at their bases. We conducted the simulations using cm-resolution photogrammetric shape models and compared the results to paired seismic recordings from RTs and far-field bedrock as well as other field observations at the Trona Pinnacles, a group of RTs in southern California. We also took a semi-analytical approach to define archetypical RT shapes and thereby extend the applicability of computationally intensive simulations to RTs elsewhere. The results of our simulations reproduce the seismic recordings in several important ways when using identical aftershock excitations. In particular, the simulations capture the first vibrational mode and the magnitude of amplification. However, simulations predict an important second vibrational mode that was not observed. We interpret the differences to result from variations in material properties such as density and stiffness. Both the numerical simulations and semi-analytic methods indicate that peak stresses occur near the base of the RT during shaking. We posit that an earthquake-driven, damage-accumulation process takes place on a geologic time scale, in which successive earthquakes progressively damage the base of the RT. Using a rebound hammer, we documented the effect of this stress localization in the field, showing that rebound numbers are systematically higher near the middle of RTs than near the base. Rebound number correlates well with rock strength, and we interpret lower values at the base to reflect the growth of fractures. Ongoing work aims to constrain the probability of toppling of a RT, given shaking intensity, geometric parameters, and bulk tensile strength.

Presenting Author: Joaquin Garcia-Suarez

Student Presenter: No

Authors