San Andreas May Be Getting Ready for an Earthquake, Say Seismologists

A region of the infamous San Andreas Fault could produce an earthquake any day now, scientists suspect.

The San Andreas Fault, which is the border section between two huge tectonic plates beneath Earth’s surface, stretches for more than 800 miles through California. The plates are relatively static, meaning they can build up large pressures over time. And when they move, they can produce big earthquakes.

A new paper published in the journal Frontiers in Earth Science by researchers from University of California in Berkeley, the U.S. Geological Survey and the National Institute of Geophysics and Volcanology in Rome, Italy, focused on a part of the fault in Parkfield, California.

The region produces earthquakes in a rough pattern of every 22 years, more or less. While earthquakes are incredibly difficult to predict, the pattern is invaluable for semiologists studying the fault line.

The last quake in Parkfield occurred in 2004—around 14 years late—meaning another should be imminent. So scientists want to find out whether there could be a better way to forecast the quakes.

Scientists found that before the last earthquake in the region, there was a lack of increasing energy in low-frequency seismic waves, while there was a decrease in high-frequency waves. That is known as attenuation and occurred over a course of six weeks before the last earthquake.

“We cannot find signs about the Parkfield asperity having reached its critical state yet,” the authors wrote in the study.

That does not mean that an earthquake is not building up, as the fault line could be behaving differently this time.

For example, researchers note an area of pressure in areas of the fault that could eventually build up to an earthquake. It is likely however, that this would cause an earthquake with a more displaced epicenter.

“We note that between 2001 and the end of 2003 the variance of the attenuation parameter went up significantly, before dropping at the end of 2003, in coincidence with the occurrence of the San Simeon earthquake,” the authors wrote. “A somehow similar increase in variance started in 2011 and lasted roughly until the beginning of 2020. Finally, in mid-2021 the variance of the attenuation parameter on the Pacific side of the [San Andreas Fault] went down to very small values and never recovered its level ever since.”

The findings could also help researchers predict quakes in future. Luca Malagnini of the National Institute of Geophysics and Volcanology in Italy told Live Science that an earthquake could occur this year, but it is not possible to say for certain.

“I’ll be waiting for the next earthquake,” Malagnini said. “And then we’ll look back.”

Luckily, the Parkfield area is as densely populated as other regions along the fault line, so the quakes are not as destructive. There have long been concerns that a huge earthquake, dubbed “the big one,” along the San Andreas Fault is overdue because multiple segments appear to be significantly stressed.

The U.S. Geological Survey previously predicted that a magnitude 6.7 earthquake will occur along the fault line in the next 30 years.

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