A periodic San Andreas Fault earthquake may still be imminent despite the lull

Earthquakes are notoriously unpredictable, but one section of the San Andreas Fault is an unusual case. Decades ago, researchers found that earthquakes near Parkfield in central California occurred about every 22 years.

In the last few decades, this periodicity seems to have been broken and the area has seemed calmer. But three researchers say we shouldn't get complacent, and an earthquake might happen.

An aerial view of a section of the San Andreas Fault. Image credits: Doc Searles.

The Earth's crust is divided into large parts called tectonic plates. These plates are constantly moving, causing some parts of the plates to bend and break. Geological faults are fractures in the Earth's crust where significant displacement has occurred between rock masses due to tectonic forces. These faults are the main sites of earthquakes in many parts of the world, and the San Andreas is one of the most famous.

The San Andreas Fault extends about 1,200 kilometers (about 745 miles) across California, demarcating the tectonic boundary between the Pacific Plate and the North American Plate. This fault is known for its ability to generate significant seismic activity.

In a certain part, near Parkfield, California, the San Andreas Fault behaves in an unusual way. North of Parkfield, the two plates move against each other at a constant rate. But south of Parkfield, the fault is closed. Therefore, every year, both sides are under more pressure. Researchers can (roughly) calculate when this stress should break, resulting in an earthquake with a magnitude greater than 6.0.

There is always some degree of approximation in this type of calculation, but in this case, the researchers estimated a periodicity of 22 years. For more than a century, this has seemed appropriate. After seismic events recorded in 1857, 1881, 1901, 1922, 1934 and 1966, it didn't take exactly 22 years, but it certainly fits the theory. However, the next major earthquake occurred in 2004, and there has been no earthquake since then. So is the patrol broken?

The earthquake may still be coming

Normally, before an earthquake occurs in a region, low-frequency seismic waves weaken, while high-frequency waves increase. But there seems to be no sign of that at the moment, so what's going on?

Earthquakes along the San Andreas Fault are concentrated around stress. When the sides of a fault move, they create stress, and when enough pressure builds up, it explodes into an earthquake. Researchers say the 2004 quake was delayed because other nearby quakes dissipated some energy.

A team from the National Institute of Geophysics and Volcanology, the Berkeley Seismological Laboratory, and the University of California, Berkeley, analyzed signs of stress buildup and release around the fault, which can be designed to more accurately predict earthquakes.

The team found that recent seismic behavior (things like the behavior of seismic waves, microtremors, and wave attenuation) differs from the 2004 earthquake, suggesting a potential change in fault behavior in the long term. In particular, they looked at seismic attenuation, which reflects changes in the stress state of the fault. It is worth noting that the attenuation behavior showed some anomalies compared to the period preceding the main shock in 2004. This may be an indication of critical stress levels.

Earthquakes are still unpredictable

While there are indications that stress levels are high and that the fault may be approaching a critical state, it is not clear that Line 22 is still in place or that an earthquake may be imminent. However, the research certainly underscores the need for increased awareness and preparedness. This period of apparent calm certainly does not mean that we should become complacent.

Given what we know regarding seismic activity in the area, continued monitoring and further studies are important to further improve our understanding of seismic hazards at Parkfield and along the San Andreas Fault.

Parkfield also isn't the only seismic area around San Andreas.

To the south, the Los Angeles Basin and the area around San Francisco are also very active areas. Both regions have their own unique seismic patterns, which require customized monitoring and preparation strategies. The complexity of the San Andreas Fault, coupled with its vast area and the different behaviors exhibited across different sectors, demonstrate the critical need for a multifaceted approach to seismic risk management.

Journal reference: Luca Malagini et al., Seismic attenuation and stress on the San Andreas Fault at Parkfield: Are we decisive yet?, Frontiers in Earth Sciences (2024). doi: 10.3389/feart.2024.1349425

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