Sometimes a rock can tell a story by just standing there. Southern California's Red Rock Canyon is home to many tall, slender, often top-heavy—and therefore exceptionally fragile—rock formations called hoodoos. Yet, some of the formations persist despite large earthquakes that have shaken the nearby Garlock fault within the last millennia, a sign that ground motions from those quakes weren't strong enough to shatter the sandstone spires. By measuring the formations (image) and then assessing the typical strength of the rocks from which they are made, researchers could estimate the upper limits of ground motions and accelerations experienced by the hoodoos during recent temblors. The largest quake to have occurred in the area—with an estimated magnitude of 7.4—probably occurred sometime around 550 years ago, when the sides of the Garlock fault slipped about 7 meters past each other. Because the hoodoos survived that temblor despite being just a few kilometers from the fault, the side-to-side accelerations experienced by the hoodoo during the quake must have measured no more than 0.36 g, or a little more than one-third that of Earth's gravity, the researchers report online today in the Bulletin of the Seismological Society of America. Even though the fragile formations erode rather quickly on a geological timescale, hoodoos and other similarly unstable formations can provide insight into the ground motions during past quakes. That, in turn, helps scientists evaluate seismic hazard for a region, even when only a limited amount of data from modern instruments is available, the researchers contend.
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