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Earthquake and Fault Zones: How Can Some Rocks Stay Precariously Balanced?

Aug 04, 2015 10:13 PM EDT

What exactly does it mean when boulders are stacked higgledy-piggledy and undisturbed in a state full of fault zones? Researchers led by Lisa Grant Ludwig, a PhD in geology with geophysics and professor of public health at the University of California, Irvine, recently published their findings on just that question regarding 36 cases of long-stacked rocks in the western San Bernardino Mountains in the journal Seismological Research Letters.

The scientists say that the precariously balanced rocks remain in place because fault interaction has weakened earthquake ground shaking near them, according to a release.

They point out that while most earthquake planning scenarios factor in ruptures along one fault, their findings suggest that there could be various ruptures-such as one that might have begun on the San Andreas Fault but then jumped to the San Jacinto Fault, near the Cajon Pass, the release said.

"These faults influence each other, and it looks like sometimes they have probably ruptured together in the past," Grant Ludwig said in the release. "We can't say so for sure, but that's what our data point toward, and it's an important possibility that we should think about in doing our earthquake planning."

Considering that the Cajon Pass includes Interstate-15, the scientists think that the state should consider the possibility of broader disruptions in the area, said a release.

The 36 perilously arranged boulders are only 4 to 7 miles from the San Andreas and San Jacinto faults. Being at least 10,000 years old, the rocks should have experienced ground shaking from a maximum of 100 earthquakes over that time, the researchers noted in the release.

The scientists studied the rocks' geometry and performed tilt analyses, determining the force required to make the rock fall by gravity-to gauge their fragility. Then they compared that fragility with the expected ground acceleration in three earthquake scenarios created by the U.S. Geological Survey's ShakeMapprogram, all of which had magnitudes of more than 7.3, according to the release.

They say that according to those scenarios, the rocks should have toppled long ago. Recognizing that a rupture pattern between the two faults could have preserved the stacked rocks, Grant Ludwig says: "The San Jacinto Fault has been very seismically active; it's produced a lot of earthquakes during [recorded history]. And the southern San Andreas Fault has not; it's been pretty quiet since 1857," she said. "This brings up the question of whether we might have an earthquake on the San Jacinto that triggers one on the southern San Andreas, or vice versa."

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