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A layer of fine-grained talc and other minerals makes some faults much weaker.
Click on image for full size
Credit: Zina Deretsky, NSF

It’s Not Your Fault – A Typical Fault, Geologically Speaking, That Is
News story originally written on December 16, 2009

Some faults look strong and like they wouldn’t cause an earthquake. But it turns out that they can slip and slide like weak faults causing earthquakes. Scientists have been looking at one of these faults in a new way to figure out why.

The faults that are thought to be very stable are called low-angle normal faults. They dip into the ground at an angle less than the angle of a typical roof.

However, they are not always stable. They do slip.

Scientists who are watching actual low-angle faults have found that they move. Scientists wanted to know how this happens.

An unusual low-angle normal fault on the Italian Island of Elba allowed scientists figure out how this happens. The fault sits exposed on the island’s beach so it is easy to examine the rocks from the fault. Often, scientists only get to work with small bits of rock from faults that are collected by drilling underground. But in this case they had an entire beach of rock to examine.

They found that the rock was very weak in one direction. The rock was made up of small wafers that slid almost like a deck of cards when pushed. Small patches of talc and clay minerals allowed the fault to slide.

Last modified January 11, 2010 by Lisa Gardiner.

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