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Deadly San Andreas Fault Longer Than Thought

Matt Kaplan
for National Geographic News
July 31, 2008
 
If the tremor that struck California earlier this week was not enough of a reminder of the region's dangerous side, a new study says the powerful San Andreas Fault extends further south than previously believed.

Tuesday's magnitude 5.4 quake in greater Los Angeles occurred along one of many lesser known fault lines that fan out from the San Andreas like glass fractures.

The new finding, meanwhile, adds 18 miles (30 kilometers) of earthquake potential to the deadly major fault that devastated San Francisco and Loma Prieta in 1906 and 1989, respectively.

A scientist discovered the San Andreas Fault was longer while he was studying boiling pools of mud called mud pots and small, erupting mud volcanoes near the Salton Sea (learn more), a saltwater lake about 165 miles (265 kilometers) southeast of Los Angeles.

Study author David Lynch of the U.S. Geological Survey (USGS) was studying the size, activity, and gas chemistry of the mud pots, which result from a range of geothermal activities and are sometimes associated with fault lines.

"As locals showed me where many of the pots were, I noticed that they tended to line up," Lynch said. "I plotted them on a map and realized they aligned with the San Andreas."

Faulty Assumption

The San Andreas Fault is a boundary between the Pacific and North American tectonic plates.

When the stress between the plates becomes too great, the rocks shift, causing the earth to shake.

Over time, the earthquake movements tend to leave visible traces on the surface, such as furrows and cracks in the ground, making it possible to identify where a fault line actually is. Geologists have suspected that the San Andreas Fault might extend beyond the visible traces in Southern California, but so far there has been no evidence.

Using satellite imagery and physical searches of the land, Lynch and study co-author Ken Hudnut, also of the USGS, identified a cluster of 33 boiling mud features, which, when plotted, formed a clear line extending 18 miles (about 30 kilometers) southeastward from the previously accepted endpoint of the San Andreas Fault.

The scientists say the usual fault line indicators are not visible near the Salton Sea because the San Andreas Fault has not been active in the area for quite some time.

Without regular activity disturbing the ground and creating surface markings, agriculture and erosion have effectively erased it from the landscape, they said.

The study appears in the August issue of the Bulletin of the Seismological Society of America.

Shaken But Not Stirred?

"Clearly something interesting is going on here," said Michael Manga, a geophysicist at the University of California, Berkeley.

Manga, who is not involved with the study, wondered why the mud pots and mud volcanoes are active.

"Documenting their history—when they appeared, how long they last—may be a useful test of this hypothesis that they are an extension of the fault," Manga said.

"Knowing history may also provide insight into the plumbing of the fault and the origin of the fluids and high pressure needed to make mud volcanoes."

Roger Musson, a seismologist at the British Geological Survey, was not so impressed with the finding.

"This study only extends the length of the San Andreas by a few percent of its total length," he said. "Interesting though it is, [this] does not represent a major change [in] our understanding of the seismic hazard of Southern California."
 

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