The Rocks That Did Not Fall: A Multidisciplinary Analysis of Near-Source Ground Motions From an Active Normal Fault

On 8 July 2021 a M6.0 normal faulting earthquake rocked the community of Walker and the surrounding region near the California-Nevada border. In the 1990s, field surveys of nearby Meadowcliff Canyon identified numerous precarious rocks deemed likely to topple in the event of strong shaking. Despite their proximity (-6 km) to the 2021 earthquake, the precarious rocks still remain standing. In this work, we combine advanced source and ground motion characterization techniques to help unravel this mystery. High-precision hypocentral locations reveal a clear north/south-striking, east-dipping rupture plane along the southern extension of the Slinkard Valley fault. The mainshock nucleated near the base of the fault, triggering thousands of aftershocks. Bayesian source spectral analyses indicate that the mainshock had a moderately-high stress drop (similar to 17 MPa), and that aftershocks with deeper hypocenters have higher stress drops. Peak Ground Acceleration (PGA) recordings at regional stations agree well with existing ground motion models, predicting PGA of similar to 0.3 g in Meadowcliff Canyon, a level sufficient to topple precarious rocks based on PGA-derived stability criteria. We demonstrate that despite these large ground accelerations, the pulse duration in Meadowcliff Canyon is too short to supply the impulse necessary to damage these features, observations which support the application of dynamic toppling models that account for the joint effects of pulse amplitude and duration when assessing rock fragility. This study provides a unique vantage point from which to interpret rarely-observed strong-motion recordings from close to an active normal fault, one of many that dominate hazard along the eastern Sierra.