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article imageA new look at the creeping segment of San Andreas fault

By Karen Graham     Jan 13, 2015 in Science
One of the most famous earthquake fault lines in the world is the San Andreas fault, running 800-miles in a north-south direction in the state of California. It has been called a "sleeping giant" with good reason. Now we are learning more about the giant.
Few people realize it, but the San Andreas fault is divided into three segments. The northern segment of the fault runs from Hollister north through the Point Reyes National Seashore, before moving northwest into the Pacific Ocean. The central section goes from Hollister to Parkfield, and it is this part of the fault line that "creeps." The southern segment of the fault runs from Parkfield on down to the Mexican border.
Map of California showing San Andreas fault and its segments.
Map of California showing San Andreas fault and its segments.
What is of interest to scientists is the middle section of the fault, the one that is creeping. It has long been thought that the middle segment of the fault line didn't produce earthquakes of any magnitude. Geologists assumed that because the rocks on either side of the fault just slipped past each other, there was no pressure being built up.
In the middle section, the slipping rocks continually are releasing any pressure before the strain on the fault line can build up resulting in a large earthquake. Instead, there are almost continuous microquakes, literally thousands of them every year. Scientists studying this creeping in 2013 felt like more information was needed to understand if the creeping of this section had any relationship to larger earthquakes in other parts of the fault.
In a new study published on Dec. 20, 2014, in the journal Geophysical Research Letters, the researchers were able to create a 3D computer model, using satellite and GPS observations to make probability estimates on where the fault line was locked and where it was slipping.
Aerial view of a portion of the San Andreas fault.
Aerial view of a portion of the San Andreas fault.
The very slight deformations shown on satellite imaging gave the scientists a better picture of the creeping segment of the San Andreas fault and showed them very clear images of where the fault was creeping, and where it was locked. This is especially important because we now have a clearer picture of how the three segments of the fault are connected.
Lead study author Romain Jolivet, a geophysicist at the University of Cambridge in the United Kingdom, explained that instead of sharp boundaries separating the creeping sections from the north and south locked sections, there was a "patchwork of stuck and sliding spots." The researchers identified at least three rather large locked patches in these sections, capable of producing a magnitude 6.0 earthquake.
Two of the spots were in the north, near San Juan Bautista, and one was in the south, below the town of Parkfield. The locked spots are called asperities, and are storing up about 0.4 inches (1 centimeter) of pressure yearly. "They're big enough to significantly affect the state of stress along the 1906 and 1857 rupture sections," Jolivet told Live Science, referencing the two deadly earthquakes that previously had hit California.
The Fort Tejon earthquake of 1857 was one of the greatest earthquakes ever recorded in the U.S.  (7....
The Fort Tejon earthquake of 1857 was one of the greatest earthquakes ever recorded in the U.S., (7.9 magnitude) and left an amazing surface rupture scar over 350 kilometers in length along the San Andreas fault.
Jolivet surmised the patches could correspond to the magnitude-7.9 Fort Tejon earthquake in 1857. "The 1857 earthquake was preceded by foreshocks that clearly happened in the region of the creeping section," he said. Fort Tejon was an army post in 1857. Luckily, because the region was so sparsely populated, only two people were killed.
Jolivet pointed out the 1906 San Francisco earthquake stopped in the transition zone. He said this is why the study is so important in understanding how earthquakes start and stop along the San Andreas fault. Because big earthquakes in the past have started or stopped along the transition zone, researchers have recently been thinking of the possibility of a big earthquake rupturing the entire length of the San Andreas fault.
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