THE ROCKS THAT DID NOT FALL: UNDERSTANDING THE NEAR-SOURCE GROUND MOTIONS OF AN ACTIVE NORMAL FAULT

On July 8th, 2021 a M6.0 normal faulting event rocked Little Antelope Valley near the California-Nevada border, the latest prominent earthquake to occur in the central Walker Lane. Earthquake hazard along the eastern Sierra south of Reno is dominated by range-bounding normal faults like the Slinkard Valley fault, which hosted this sequence, motivating our effort to better characterize it. In the 1990s, series of field surveys in this region identified numerous fragile geologic features that were deemed unlikely to remain standing in the event of strong shaking. This included several sites in Meadowcliff Canyon, on the hanging wall of the 2021 event and only 6 km from the rupture surface. Despite this proximity, the fragile geologic features in the canyon remain intact today. In this study, we work to unravel this mystery by combining advanced source characterization techniques with detailed analyses of strong ground motion. High-precision hypocentral locations reveal a clear mainshock fault plane, the southern extent of the mapped Slinkard Valley fault, which strikes north-north-west and dipping down to the east. The mainshock nucleated near the base of this structure, which did not break the surface but trigger aftershocks updip and to the north and south. Application of Bayesian source spectral analyses indicate that the mainshock event had a relatively high-stress drop (~ 20 MPa), and that within the aftershock sequence, there is a clear trend of increasing stress drop with hypocentral depth along the fault plane. Peak ground acceleration and velocity recordings at regional stations agree well with the NGA-West2 suite of active crustal ground motion models and would predict PGA of ~0.3g at the Meadowcliff site, an amplitude deemed likely sufficient to topple fragile geologic features. However, our analyses demonstrate that while the level of ground motion may be high, the pulse duration for this high stress-drop event at the Meadowcliff site is too short to supply the impulse necessary to damage the larger fragile geologic features documented previously. This study provides a unique vantage point from which to interpret rarely observed, strong motions recording from close to an active normal fault.