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).
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Image courtesy of L. Grant Ludwig
New Earthquake Information Unearthed by San Andreas Fault Studies
News story originally written on January 21, 2010
Scientists have taken a new look at part of the San Andreas Fault on the Carrizo Plain in Southern California. What they learned might change our understanding of faults and earthquakes.
The Carrizo Plain is 100 miles north of Los Angeles. This is the site of an enormous 1857 earthquake that rattled Southern California.
To better understand the fault, geologists took measurements of the land surface with LiDAR (Light Detection and Ranging). These measurements produced a view of the Earth's surface with at least 10 times more detail than ever seen before. This new detailed view of the Earth’s surface allowed scientists to see how much the fault moves.
To study earthquakes of the past, they looked at stream channels that cut across the fault. Movement along the fault over time has caused these channels to be offset so that the downstream part no longer matches up with the upstream part. Using the detailed overhead views made with LiDAR measurements to study the channels, scientists were able to figure out when earthquakes offset the channels in the past.
The LiDAR data was combined with data collected from trenches dug across the fault, radiocarbon-dating sediment samples, and historic weather data for the Carrizo Plain to get a detailed picture of what the San Andreas Fault has been doing for the past few hundred years.
They found that the fault slipped differently with different earthquakes in the past. If faults slipped the same way each time then scientists would have a better chance of forecasting earthquakes, but with the fault slipping differently each time, forecasting would be difficult.
"Our results show that we don't understand the San Andreas Fault as well as we thought we did," said Lisa Grant Ludwig, one of the project scientists. "We therefore don't know the chances of earthquakes as well as we thought."
Since the 1857 earthquake, approximately five meters of strain, or potential slip, has been building up on the San Andreas Fault in this area, ready to be released in a future earthquake. So, there is potential for a big earthquake in this part of California.
"The recent earthquake in Haiti is a reminder that a destructive earthquake can strike without warning," Lisa Grant Ludwig said. "One thing that hasn't changed is the importance of preparedness and earthquake resistant infrastructure in seismically active areas around the globe."
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