Figure 1. (Left) Location of the Trona Pinnacles relative to the southeastern end of the  2019 M7.1 surface traces. (Right) Numbering and locations of a subset of spires within the Trona Pinnacles National Monument.

Understanding the upper limits of ground motions is key to designing critical infrastructures (such as power plants and dams) that have long lifespans and need to withstand extreme ground shaking from large earthquakes. But unfortunately the short history of seismic instrumentation (a few decades) is only a fraction of the recurrence interval of large earthquakes (hundreds to thousands of years). However, we can rely on Fragile Geological Features (FGFs) to constrain the upper limits of ground motions in the absence of instruments (Brune 1996, Anderson et al. 2011 and 2014, Grant et al. 2015). Precariously balanced rocks (PBRs), a type of FGF, have been used in the past to provide estimates of ground motions a region has experienced (Hanks et al. 2013). PBRs toppled by strong earthquake shaking can provide upper limits of ground motions, while intact PBRs are evidence that no strong ground accelerations have occurred for as long as the PBR has been in its state (which can be several thousands of years in arid climates). The observation of damaged versus undamaged PBRs following earthquakes is critical in calibrating and validating PBR ground motion assessment methodologies developed from lab experiments and numerical modeling (e.g. Anooshehpoor et al. 2004 and 2013).

Figure 2. A precariously balanced tufa piece detached from Spire 01. The white surfaces and the very fresh traces of impact in the sand point at damage from the Ridgecrest sequence. Dr. Meng holding a 1.82 m (6 ft) measuring stick next to fallen rock in (D).

The M7.1 Ridgecrest earthquake was the largest event to occur in the state of California in 20 years. The mainshock occurred on July 5, 2019, one day after a M6.4 earthquake in the same area. As part of the SCEC post-event reconnaissance field work, we planned to visit locations of known PBRs to inspect them for damage. However, the only known PBRs were located in areas where shaking was very weak (below 0.2g from instrumental and ShakeMap data). We opted to turn our attention to the Trona Pinnacles National Monument, where over 500 tufa spires sat within 5 km of the mapped M7.1 fault trace (Figure 1). Tufa spires are more fragile than typical PBRs, so they are more prone to degradation over time and have a shorter survival span, which need to be considered when evaluating ground motion levels. The tufa spires presented the advantage of having been extensively photographed by researchers and tourists alike, enabling a visual comparison of their post Ridgecrest and pre-shaking states. As soon as we entered the Trona Pinnacles National Monument, we observed dramatic damage of several features—all of which appeared to be very fresh. Several rock fragments, ranging from a few to several feet in diameter, lay on roads and trails, with undisturbed evidence of their detachment origin and landing in a very recent past. This was a very rare opportunity to document fresh damage to spires, and to provide critical information for refining ground motion assessment techniques that use FGFs.

Figure 2 shows a picture that Jim Brune took in 2001 of “Spire 01” and what we observed on July 12, 2020. The damage to this tufa spire was obvious and quite spectacular. We found several depressions, spaced by 1.5-2m, between Spire 01 and the final landing spot of the rock. The depressions were extremely fresh, caused by the fallen rock bouncing in the soil. We also documented the rolling path taken by the rock before it finally stopped. In addition, we found many spires in the identical state as in pictures from 2001. Since both damaged and undamaged FGFs help to constrain the range of ground shaking, we documented the current state of spires with geo-located pictures taken from many different angles and measured the fallen rocks’ paths and sizes. A subset of damaged pinnacles were also surveyed using small unmanned aerial vehicles (sUAVs) to determine 3D structure and volumes of the observed spires and toppled rocks using a processing method called structure from motion (SfM) (Figure 3).

We are now cataloguing all the field pictures and gathering all available online images of pre-event conditions of these FGFs. The end product will be a database (similar to the SCEC PBR database) that includes numbered IDs to identify the spires (e.g. as shown in Figure 1) and associated documentation of dated and geotagged pictures, as well as 3D imagery and in situ strength properties for a subset of spires. These FGF properties will support different types of modeling to be performed using computational methods, such as those based on finite element modeling analyses. The database can then be used by researchers to refine ground motion estimation methodologies, and provides a reference of dated pre-event state before the next earthquake occurs.

Figure 3. 3D image of a portion of the Trona Pinnacles from structure from motion (SfM) method, view is from the northwest to the southeast. The left-most feature is “Spire 01”, shown in close-up in Figure 2.

About the Authors

	Christine Goulet the SCEC Executive Director for Applied Science at the University of Southern California. She serves as the science lead and technical integrator for large-scale collaborative projects in earthquake hazard and risk. Her research interests are in the field of geotechnical earthquake engineering and applied seismology in the context of performance-based design.
	Xiaofeng Meng is a SCEC postdoctoral researcher at the University of Southern California. His research involves evaluating the contributions from different variability components on ground motions through the removal of ergodic assumption. He uses the CyberShake platform, which provides a controlled testing environment with large ground motion datasets.
	Andrea Donnellan is a principal research scientist at NASA's Jet Propulsion Laboratory. She studies earthquakes and crustal deformation by integrating geodetic imaging data and computational infrastructure with modeling and analysis tools. Her current research focus is on analysis and modeling of geodetic observations in California using UAVSAR, GPS, and Stereo Photogrammetry.

Acknowledgements

This research was supported by the Southern California Earthquake Center. SCEC is funded by NSF Cooperative Agreement EAR-1600087 and USGS Cooperative Agreement G17AC00047. Additional support to SCEC was provided by NASA Grant 80NSSC19K0739 and the Pacific Gas and Electric Company. Portions of this research were carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). We thank our SCEC and NASA colleagues for collaborating in this research.

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