JUMPING ROCKS AS AN INDICATOR OF GROUND MOTION DURING THE 4 JULY 2019 M6.4 RIDGECREST EARTHQUAKE

Fragile geologic features, including rocks or rock formations displaced or toppled during earthquakes, provide insight into spatial extent and intensity of strong ground shaking. This study focuses on ‘jumping rocks’ or individual rock clasts displaced and disturbed during the July 4, 2019 M6.4 Ridgecrest, California earthquake. The goal of this study is to use these data as a primary input for models of near-fault strong ground motion. In the fall of 2019, we collected measurements of jumping rocks at 170 sites in orthogonal transects along the southern 8 km portion of rupture. Jumping rocks were identified primarily based on the presence of empty soil sockets on the nearby ground surface. We developed a rubric and GIS routine to measure, locate, and describe jumping rock measurements in the field. At each measurement site, we recorded clast dimensions, jump displacement type, magnitude and azimuth, substrate type, slope gradient and aspect, and location relative to the surface rupture trace. Preliminary results suggest that the prevalence of jumping rocks strongly depends on substrate type, rock size and density, fault complexity, local topography, and proximity to the rupture. These results suggest ground motion clearly attenuates on a meter scale moving away from the fault. Additional work will focus on accessing and surveying the northern extent of the rupture to strengthen our data set.