After five earthquakes along the San Andreas fault, this dry riverbed has been offset 10 meters. The lower image shows the location of the fault (white line) and the location of the dry riverbed (blue).
Click on image for full size
Image courtesy of L. Grant Ludwig

New Earthquake Information Unearthed by San Andreas Fault Studies
News story originally written on January 21, 2010

Recent studies of stream channel offsets along the San Andreas Fault reveal new information about fault behavior--changing our understanding of the potential for damaging earthquakes.

The studies were conducted at the Carrizo Plain, 100 miles north of Los Angeles and site of the original "Big One"--the Fort Tejon quake of 1857--by scientists at Arizona State University (ASU) and the University of California at Irvine (UCI).

Applying a systems science approach, the teams report results of a pair of studies in the journal Science Express on January 21st. The results incorporate the most comprehensive analysis of this part of the San Andreas fault system to date.

"These research results challenge the widely accepted characteristic earthquake model and could transform our understanding of fault behavior," said David Fountain, program director in the National Science Foundation (NSF)'s Division of Earth Sciences, which funded the research.

"The results show a substantially reduced estimate of time between large earthquakes on the south-central San Andreas Fault, which implies more frequent smaller earthquakes than previously believed. This in turn has significant implications for earthquake hazards in southern California."

In one of the studies, ASU geologists Ramon Arrowsmith and Olaf Zielke employed topographic measurements from LiDAR (Light Detection and Ranging), which provided a view of the Earth's surface at a resolution at least 10 times higher than previously available, enabling the scientists to "see" and measure fault movement, or offset.

To study older earthquakes, researchers turned to offset landforms such as stream channels, which cross the fault at a high angle.

The scientists' detailed overhead views of Carrizo Plain stream channels measured the offset features linked to large earthquakes in this section of the southern San Andreas Fault.

"This virtual approach is not a substitute for going out and looking at the features on the ground," says Zielke. "But it is a powerful approach that is repeatable by other scientists."

A team led by UCI's Lisa Grant Ludwig, Sinan Akciz and Gabriela Noriega determined the age of offset features in Carrizo Plain dry stream channels. They studied how much the fault had slipped during previous earthquakes. The distance a fault "slips," or moves, determines its offset.

By digging trenches across the fault, radiocarbon-dating sediment samples, and studying historic weather data for the Carrizo Plain channels, and combining them with LiDAR data, the researchers found something new.

Rather than seeing the same slip repeat in characteristic ways, they found that the slip varied from earthquake to earthquake.

"When we combine our offset measurements with estimates of the ages of these offset features, and the ages of prior earthquakes, we find that the earthquake offset from event to event in the Carrizo Plain is not constant, as is current thinking," Arrowsmith said.

"The idea of slips repeating in characteristic ways along the San Andreas Fault is very appealing, because if you can figure that out, you are on your way to forecasting earthquakes with some reasonable confidence," added Ludwig.

"Our results show that we don't understand the San Andreas fault as well as we thought we did," she said.  "We therefore don't know the chances of earthquakes as well as we thought."

Before these studies, the magnitude 7.8 Fort Tejon earthquake of 1857 (the most recent earthquake along the southern San Andreas Fault) was thought to have caused a nine-to-ten meter slip along the Carrizo Plain.

But the data the teams acquired show that it was actually half as much, and that slip in some of the prior earthquakes may have been even less.

The researchers also found that none of the past five large earthquakes in the Carrizo Plain dating back 500 years produced slip anywhere near nine meters. The maximum slip seen was about five to six meters, which includes the slip caused by the Fort Tejon quake.

This result changes how we think the San Andreas Fault behaves, the researchers say. It probably is not as segmented in its release of accumulated stress as was thought.

This makes forecasting future earthquakes harder because geologists cannot rely on the assumption of constant behavior for each section.

It could mean that earthquakes are more common along the San Andreas, but that some of those events may be smaller than previously expected.

Since the 1857 quake, an approximate five meters of strain, or potential slip, has been building up on the San Andreas fault in the Carrizo Plain, ready to be released in a future earthquake.

In the last five earthquakes, the most slip that has been released was five to six meters in the big 1857 quake. This finding points to the potential of a large temblor along the southern San Andreas fault.

"Our collaboration has produced important information about how the San Andreas Fault works," said Arrowsmith. "I am optimistic that these results, which change how we think about how faults work, are moving us to a better understanding of the complexity of the earthquake process."

"The recent earthquake in Haiti is a reminder that a destructive earthquake can strike without warning," Ludwig said. "One thing that hasn't changed is the importance of preparedness and earthquake resistant infrastructure in seismically active areas around the globe."

Text above is courtesy of the National Science Foundation

Last modified February 26, 2010 by Lisa Gardiner.

You might also be interested in:

Cool It! Game

Check out our online store - minerals, fossils, books, activities, jewelry, and household items!...more

What Is an Earthquake?

The expression "on solid ground" is often used to describe something as stable. But sometimes the solid ground underfoot is not stable. It moves as Earth's tectonic plates move. Sometimes it moves gradually....more

Haiti Earthquake January 2010

A major earthquake causing widespread devestation and extensive loss of life struck the nation of Haiti on January 12, 2010. The earthquake had a magnitude of 7.0. Haiti is on the island of Hispaniola...more

Triggers of Volcanic Eruptions in Oregon's Mount Hood Investigated

A new study has found that a mixing of two different types of magma is the key to the historic eruptions of Mount Hood, Oregon's tallest mountain, and that eruptions often happen in a relatively short...more

Oldest Earth Mantle Reservoir Discovered

Researchers have found a primitive Earth mantle reservoir on Baffin Island in the Canadian Arctic. Geologist Matthew Jackson and his colleagues from a multi-institution collaboration report the finding--the...more

It’s Not Your Fault – A Typical Fault, Geologically Speaking, That Is

Some geologic faults that appear strong and stable, slip and slide like weak faults. Now an international team of researchers has laboratory evidence showing why some faults that 'should not' slip are...more

Extended Period of Lower Solar Activity Linked to Changes in Sun's Conveyor Belt

A new analysis of the unusually long solar cycle that ended in 2008 suggests that one reason for the long cycle could be a stretching of the sun's conveyor belt, a current of plasma that circulates between...more

Growth Spurt in Tree Rings Prompts Questions About Climate Change

Anyone who has ever cut down a tree is familiar with the rings radiating out from the center of a tree trunk marking the tree's age. Careful study of tree rings can offer much more: a rich record of history...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA