Science in Action

One of the most important questions a geologist tries to answer about any active fault is how fast the fault moves.  A fault's motion is not constant.  Most faults do not move for tens, hundreds, or even thousands of years.  Then they may suddenly break with an abrupt motion that we call an "earthquake."

Some time ago, geologists realized that if they could establish the date when Wallace Creek cut its modern channel, they could figure out the rate at which the fault has been moving since then: it would be the offset of the present channel -- 420 feet (130 m) -- divided by the number of years that have passed.

They discovered that one way to establish when the creek cut its new channel would be to figure out the age of the youngest material in the older channel.  This could be done by using a method called carbon-14 dating on charcoal samples that were deposited in the bed of the older channel.

After digging a series of trenches, geologists found that the youngest material in the older channel was about 3800 years old.  The modern channel of Wallace Creek must have been cut just after the 3800-year-old material was deposited.  Since Wallace Creek has been offset 420 feet (130 m) since then, the fault must be slipping at an average rate of about 1.3 inches (34 mm) per year.

Slip Rate on the San Andreas fault at Wallace Creek: