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Hoodoos may be seismic gurus

Hoodoo prediction Towering chimney-like sedimentary rock spires known as hoodoos may provide an indication of an area's past earthquake activity.

The research by scientists including Dr Rasool Anooshehpoor, from the United States Nuclear Regulatory Commission, may provide scientists with a new tool to test the accuracy of current hazard models.

Hoodoo formations are often found in desert regions, and are common in North America, the Middle East and northern Africa.

They are caused by the uneven weathering of different layers of sedimentary rocks, that leave boulders or thin caps of hard rock perched on softer rock.

By knowing the strengths of different types of sedimentary layers, scientists can determine the amount of stress needed to cause those rocks to fracture.

The United States Geological Survey (USGS) use seismic hazard models to predict the type of ground motion likely to occur in an area during a seismic event. But, according to Anooshehpoor, these models lack long term data.

"Existing hazard maps use models based on scant data going back a hundred years or so," says Anooshehpoor. "But earthquakes have return periods lasting hundreds or thousands of years, so there is nothing to test these hazard models against."

The researchers examined two unfractured hoodoos within a few kilometres of the Garlock fault, which is an active strike-slip fault zone in California's Red Rock Canyon.

Their findings are reported in the Bulletin of the Seismological Society of America.

"Although we can't put a precise age on hoodoos because of their erosion characteristics, we can use them to provide physical limits on the level of ground shaking that could potentially have occurred in the area," says Anooshehpoor.

The researchers developed a three-dimensional model of each hoodoo and determined the most likely place where each spire would fail in an earthquake.

They then tested rock samples similar to the hoodoo pillars to measure their tensile strength and compared their results with previously published data.

USGS records suggest at least one large magnitude earthquake occurred along the fault in the last 550 years, resulting in seven metres of slip, yet the hoodoos are still standing.

This finding is consistent with a median level of ground motion associated with the large quakes in this region, says Anooshehpoor.

"If an earthquake occurred with a higher level of ground motion, the hoodoos would have collapsed," he says.

"Nobody can predict earthquakes, but this will help predict what ground motions are associated with these earthquakes when they happen."

Neat solution

Dr Juan Carlos Afonso from the Department of Earth and Planetary Sciences at Sydney's Macquarie University says it's an exciting development.

"In seismic hazard studies, it's not just difficult to cover the entire planet, it's hard to cover even small active regions near populated areas," says Afonso.

"You need lots of instruments, so it's great if you can rely on nature and natural objects to help you."

He says while the work is still very new and needs to be proven, the physics seems sound.