The San Andreas Fault is sleepy near Los Angeles. Researchers have an idea why

It has been about three centuries since the last great earthquake on the southern San Andreas Fault, the most treacherous seismic hazard in California. For decades researchers have puzzled over why it has been so long. The average interval of large earthquakes along that portion of the fault has been 180 years over the past 1,000 years.

While seismologists agree that Southern California is due for the Big One, a group of researchers published a paper Wednesday in the journal Nature that offers a reason for the period of seismic silence along the southern San Andreas, the tension-wracked meeting point of the North American and Pacific tectonic plates.

The theory hinges on the idea that while the friction of tectonic plates is the primary driving force behind earthquakes, there are other factors, including the weight of large bodies of water. Building on prior research, the scientists drew a link between the occurrence of large earthquakes and the filling of a lake that has grown and ebbed across the centuries.

“We are not trying to predict any earthquake that is going to happen in the future, but we might be able to say why we haven’t had one in the last 300 years,” said Ryley G. Hill, a doctoral candidate in the department of geological sciences at San Diego State University and the Scripps Institution of Oceanography in La Jolla, California.

Hill and his co-authors found that major earthquakes along the southern San Andreas fault tended to happen when a large body of water, Lake Cahuilla, was filling or was full with water from the Colorado River in what are now the Coachella and Imperial valleys.

The lake has drained over the past three centuries and all that remains is the vestigial Salton Sea. The authors of the paper believe that the process of the ancient lake’s emptying and disappearance stabilized the fault to a certain degree.

Lucy Jones, a seismologist and the chief scientist at the Dr. Lucy Jones Center for Science and Society, called the explanation a “plausible reason for the current long interval since the last earthquake.”

But the research, she said, “does not make me want to say don’t worry about the next one.”

Jones was not involved in the study.

Seismologists say that one of the consequences of the three-century interval since the last big earthquake, defined as having a magnitude of 7 or greater, is that more tension has built up as the two grinding tectonic plates have moved in opposite directions.

“It’s accumulated a large amount of energy analogous to a taut rubber band,” said Belle Philibosian, a research geologist with the U.S. Geological Survey in the San Francisco Bay Area. “We should be anticipating that the rubber band could snap at any time.”

The southern San Andreas, part of a network of faults that run across California, has the most potential for destruction because of the large number of people who live in the area — 10 million people in Los Angeles County alone. The Northridge earthquake in Los Angeles in 1994, with a 6.7 magnitude, killed more than 70 people and led to $20 billion in damage. It was caused by one of the many faults that make up the broader San Andreas system, but not by the main San Andreas Fault that is the longest in California and is capable of a much more powerful event.

The research published in Nature, which builds on a paper on which Philibosian was a writer in 2011, raises questions about plans to rehabilitate parts of the Salton Sea, which was formed when an irrigation canal burst in the early 1900s. Today it is highly polluted and shrinking, relying on runoff from nearby farm irrigation for water. As the sea dries out, toxic dust is left behind and blown into the air, posing a hazard for nearby residents.

Impounding more water in the Salton Sea could tamp down the dust. Recent ideas have included importing desalinated seawater — which a panel rejected last year — and paying farmers to divert their Colorado River allocations. But a major change in the water level could also trigger seismic activity, according to Philibosian.

“This earthquake will happen eventually — probably sooner rather than later — no matter what we do,” she said.

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