Survey of Damaged Tufa Pinnacles in Trona Following the 2019 M7.1 Ridgecrest Earthquake

Christine A. Goulet, Xiaofeng Meng, Andrea Donnellan, & Gregory A. Lyzenga

Published August 15, 2019, SCEC Contribution #9867, 2019 SCEC Annual Meeting Poster #243

The determination of ground motion upper limits is a key knowledge gap for the design of critical infrastructure such as power plants or even dams. Precariously balanced rocks (PBRs) are a type of fragile geologic features (FGFs) that can be toppled by strong earthquake shaking and they can provide constraints on the quantification of such extreme ground motions. The presence of intact PBRs is evidence that no strong ground accelerations have occurred for as long as the PBR has been in its state, which can be several thousand years under desertic climates. The observation of damaged/not damaged PBRs following earthquakes is critical in calibrating and validating PBR ground motion assessment methodologies developed from lab experiments and numerical modeling.

The 2019 M7.1 Ridgecrest earthquake was the largest event to occur in the state of California in 20 years. The M7.1 occurred on July 5, 2019, a day after a significant M6.4 in the same area. On July 12, as part of the SCEC reconnaissance field work, we visited the Trona Pinnacles where hundreds of fragile tufa pinnacles reside. The pinnacles are located within 5 km of the mapped M7.1 fault trace. The pinnacles were not included in the SCEC PBR database, but they have been heavily photographed, both by researchers and tourists, and images are readily available to provide the pre-damage state of the pinnacles.

We observed evidence of fresh damage on several of the pinnacles, characterized by the white color of the damaged carbonate material composing the tufa, the angularity of the released rocks and fragments, and several observations of displaced sand and soil where rocks and boulders have bounced during their fall or at the location of their landing. We confirmed damage at two pinnacles relative to pre-event pictures provided by Jim Brune and identified several more potentially damages spires. We documented our observations with geo-located pictures taken from several angles, and captured most the pinnacles reasonably accessible by car or by foot. A subset of damaged pinnacles have been imaged using structure from motion with the use of small uninhabited aerial systems (sUASs) and volumes of the observed toppled rocks have been estimated. We have collected a very exciting dataset to work from and will present results of our analyses to date. In addition, our database of pictures and 3D images can serve as the reference of pre-event state before the next earthquake.

Key Words
precariously balanced rocks, fragile geologic features, rock damage, extreme ground motion, damage imaging

Citation
Goulet, C. A., Meng, X., Donnellan, A., & Lyzenga, G. A. (2019, 08). Survey of Damaged Tufa Pinnacles in Trona Following the 2019 M7.1 Ridgecrest Earthquake. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Ridgecrest Earthquakes