SCEC Award Number 19113 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Impact of Contact and Interface Modeling on Precarious Rock Fragilities
Investigator(s)
Name Organization
Christine Wittich University of Nebraska-Lincoln
Other Participants 1 Graduate Research Assistant, TBD
SCEC Priorities 5b, 5c, 4b SCEC Groups Geology, SAFS, EEII
Report Due Date 03/15/2020 Date Report Submitted 03/14/2020
Project Abstract
 Current state-of-the-art methods for predicting overturning of a precarious rock include detailed surveying of the rock’s geometry followed by numerical simulations and ultimately fragility analysis, in which the probability of overturning is related to a measure of earthquake intensity. However, there are significant sources of uncertainty at each analysis stage of the precarious rock, which impact the resulting probabilities of overturning to unknown extents. For example, recent field surveys of precarious rocks have highlighted the potential for complex interface conditions that are not readily captured by traditional surveying techniques. Therefore, a precarious rock may appear that is in uniform contact with a rock pedestal; however, the base of the rock may have eroded into a configuration where it is in contact at only a few discrete points on the pedestal. As a result, the overall objective of this project was to quantify and assess the impact of interface geometric variations on the probabilistic overturning predictions of precarious rocks. This was conducted through a numerical case study on a representative precariously balanced rock, that was previously surveyed in Jacumba, CA. Results indicated that the probability of overturning can be underestimated by as much as 20% due to inaccurate interface measurements. Given the substantial differences observed in the probabilistic overturning for this case study, further investigation is warranted to study the effect of the interface shape on the rate of overturning in a more holistic sense.
Intellectual Merit This project aligns with the objectives and priorities of the Earthquake Geology disciplinary committee, which aims in part to foster research in outstanding seismic hazard issues and in the earthquake history of southern California. To this end, the analysis of precarious rocks and fragile geologic features has been identified as a particular strategy to evaluate ground motion hazard and inform seismic hazard methodologies. While precarious rocks are recognized as a means to evaluate hazard, it is also understood that existing analysis techniques carry potentially significant uncertainty and the development of analysis techniques is a noted research priority of this particular disciplinary committee. This project aimed, in part, to address this research priority through the analysis and quantification of uncertainty associated with the interface geometry of precarious rocks and the impact that this may have on subsequent fragility analyses. This project may also have an indirect impact on the ongoing research objectives and priorities of the San Andreas Fault System (SAFS) interdisciplinary working group. While this working group aims to develop projects that investigate the Cajon Pass Earthquake Gate Area, it is also a research priority to incorporate precariously balanced rock studies to aid in understanding the paleoseismology and related ground motion history. While this project did not incorporate precariously balanced rocks in the vicinity of the Cajon Pass Earthquake Gate Area, this project highlights the importance of interface geometry for future applications.
Broader Impacts This project further quantified and generated knowledge regarding the overturning of precariously balanced rocks. Given the application of these rocks to constraining rare seismic hazard, this project can ultimately impact the construction of more earthquake resistant critical facilities - a net benefit to society. In addition, this project supported a Ph.D. student at the PI's institution, which currently includes only a small number of students and faculty studying earthquake engineering and earthquake science. A second student, an undergraduate researcher and underrepresented minority in STEM (as defined by NSF), was also able to join this project as a McNair Scholar.
Exemplary Figure Fig. 3 – Fragility analyses of: a) Model 1, and b) Model 2, as well as c) cross-sections at different probability levels.