Part of the San Andreas Fault may be preparing for an earthquake

A section of the San Andreas Fault, where earthquakes regularly occur, may give off a distinct signal before shuddering to life, new research suggests. The signal indicates the opening and closing of cracks under the surface.

This part of the fault line, known as the Parkfield in central California, shakes regularly about every 22 years. It last ruptured in 2004, so another earthquake may be imminent. However, the signal is not currently occurring in the rift section, and the section is not behaving exactly as it did the last time it ruptured, according to a study published March 22 in the journal Frontiers in Earth Science.

The differences may mean the next quake won't happen right away, or they may mean the epicenter will be different from the epicenter of the 2004 quake, which was southeast of the small town of Parkfield. There will be no way to know until the next earthquake actually occurs, said Luca Malanini, the study's lead author and director of research at the National Institute of Geophysics and Volcanology in Italy.

“We're waiting,” Malanini told Live Science.

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The San Andreas Fault marks the boundary between the Pacific and North American tectonic plates. South of Parkfield, the fault is closed, meaning the two plates are not moving against each other. North of Parkfield, the San Andreas Fault moves freely, with plates creeping against each other at a constant rate of 1.4 inches (3.6 cm) per year. Parkfield is a transition zone between these two systems. When this area of ​​the fault comes to life, it produces an earthquake with a magnitude of about 6. Because of the remote location, these quakes rarely threaten human life or property, although quakes that occur on one fault can affect stresses on other faults nearby, Malagnini said. .

But researchers are monitoring Parkfield closely in hopes of finding activity that will help them predict when the next earthquake will strike. Being able to detect reliable triggers for earthquakes – pressure on rocks, for example, or changes in subsurface permeability – would help scientists warn people of impending earthquakes, potentially saving lives. Parkfield, with its frequent earthquakes, may be a good place to look for these clues to extrapolate which parts of the fault are more dangerous. But so far, this goal has remained elusive.

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In the new research, Malanini and his colleagues measured seismic wave attenuation, or how sound waves lose energy as they move through the Earth's crust. Malagnini said the attenuation is related to the permeability of the rocks. In the period of stress that precedes an earthquake, cracks open and close in the strained rock around the fault. The new study found that before the 2004 Parkfield earthquake, attenuation of low-frequency waves increased in the six weeks before the earthquake, while attenuation of high-frequency waves decreased.

This is the result of pressure on the rocks as the Pacific plate in the west moves against the North American plate in the east, Malagnini said. As pressure increases, long cracks ranging in size from several hundred feet to one mile (1.5 km) open in the subsurface. These long cracks absorb some of the stress on the surrounding rock, so shorter cracks in the rock are closed. This decrease in short cracks and increase in long cracks explains the bifurcation in energy loss for different seismic waves, Malagnini said.

Malagnini said there were signs at the moment that Parkfield was entering the final phase of a calm period. The timing is right, for one thing: Parkfield has “skipped” earthquakes before, but those quakes that weren't detected in the 22-year cycle occurred when nearby, unrelated quakes changed pressures in the area. There were no such earthquakes this time. Another possible hint is that the variance in attenuation measurements has fallen very low since 2021. A similar decline in this measurement occurred in 2003 before the 2004 Parkfield earthquake.

However, Malagnini said, there is still no evidence of bifurcation in the attenuation measurement that preceded the 2004 quake. He added that he suspects the next quake will hit Parkfield this year, but the epicenter may not be in the same place as it was in 2004. , which means that these measurements will look different.

Malanini won't try to predict the next quake until today, but he hopes that after it happens, he and his team will be able to tease out signals to look for in the future.

“I will wait for the next earthquake,” Malagnini said. “And then we'll look back.”

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