BALANCED ROCK ORIENTATIONS ARE CONTROLLED BY FRACTURE PATTERNS

Analysis of aerial photos reveals a strong bedrock fracture pattern that influenced the development of balanced rocks between the San Jacinto and Elsinore faults in southern California. Precariously balanced rocks can put an upper limit on the amount of ground motion that has occurred in seismically active areas. This research focuses on a 200 km-long band of balanced rocks between the San Jacinto and Elsinore faults. The band and the majority of these rocks are oriented with their long-axes sub-parallel to the strikes of the faults (about 130° from north). Rocks oriented in this way will tend to topple in a fault-perpendicular direction, when exposed to high enough levels of shaking. Current ground motion models predict that the highest levels of ground shaking on strike-slip faults will be perpendicular to the faults. It is curious, therefore, that favorably oriented rocks such as these have not already been toppled. In order to better understand this peculiarity, the focus of my summer SCEC internship was to study the fractures around five different balanced rocks at three sites. If these fractures are randomly oriented and determine the long-axis orientation of the balanced rocks, then seismic ground motions must have been different than predicted by current models. My results, however, show a fault-parallel fracture pattern in the vicinity of the balanced rocks. Based on this result, I suggest the orientation of the balanced rocks reflects their manner of formation, rather than anomalous ground motions during earthquakes.