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 happen approximately every 22 years.

In the past few decades, that periodicity seems to have been broken and the area appears much calmer. But a trio of researchers say we shouldn’t get complacent, and an earthquake may still happen.

The Earth’s crust is split into large chunks called tectonic plates. These plates are constantly moving around, which causes some parts of the plates to bend and break. Geologic faults are fractures in the Earth’s crust where significant displacement has occurred between rock masses due to tectonic forces. These faults are the primary sites for earthquakes in many parts of the world — and San Andreas is one of the most notorious.

The San Andreas Fault stretches approximately 1,200 kilometers (around 745 miles) through California, delineating the tectonic boundary between the Pacific Plate and the North American Plate. This fault is infamous for its potential to generate significant seismic activity.

In one particular part, near Parkfield, California, the San Andreas fault behaves in an unusual way. North of Parkfield, the two plates move up against another at a constant rate. But south of Parkfield, the fault is locked. So, every year, the two sides gather more and more pressure. Researchers can (approximately) calculate when this pressure should break, resulting in an earthquake of a magnitude over 6.0.

There’s always a bit of approximation with this type of calculation, but in this case, researchers estimated a periodicity of 22 years. For over a century, this seemed to fit. Following recorded seismic events in 1857, 1881, 1901, 1922, 1934, and 1966 — not exactly 22 years, but definitely fitting the theory. However, the next big earthquake came in 2004, and there hasn’t been one since. So, is the periodicity off?

An earthquake may still be coming

Usually, before an earthquake in the area, low-frequency seismic waves attenuate, while high-frequency waves increase. But there seems to be no sign of either right now, so what’s happening?

Earthquakes along the San Andreas fault are focused around stress. When the sides of the fault move, they create stress, and when enough stress accumulates, it ruptures into an earthquake. The 2004 earthquake was delayed, the researchers say, because other earthquakes nearby dissipated some of the energy.

The team from Istituto Nazionale di Geofisica e Vulcanologia, the Berkeley Seismological Laboratory, and University of California, Berkeley, analyzed signs of stress accumulation and release around the fault, which can be modeled to predict earthquakes more accurately.

The team found that the recent seismic behavior (things like seismic wave behavior, microtremors, wave attenuation) differ from the 2004 earthquake — suggesting a possible change in the fault’s long-term behavior. In particular, they looked at seismic attenuation, which reflects changes in the stress state of the fault. Notably, the attenuation behavior has shown some anomalies compared to the period before the 2004 mainshock. This could be indicative of critical stress levels.

Earthquakes are still unpredictable

While there are indications that stress levels are high and the fault may be nearing a critical state, it’s not clear that the 22-ish is still in place or that an earthquake may be impending. However, the research certainly underscores the need for heightened awareness and preparedness. The period of apparent calm definitely doesn’t mean we should get complacent.

Given what we know regarding the seismic activity in the region, continuous monitoring and further studies are important to keep improving our understanding of earthquake risks at Parkfield and along the San Andreas Fault.

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

Further south, the Los Angeles basin and the area around San Francisco are also highly active regions. Both areas have their own unique patterns of seismicity, which require tailored monitoring and preparation strategies. The complexity of the San Andreas Fault, along with its vast expanse and the different behaviors exhibited across various segments, demonstrates the critical need for a multifaceted approach to earthquake risk management.

Journal Reference: Luca Malagnini et al, Seismic attenuation and stress on the San Andreas Fault at Parkfield: are we critical yet?, Frontiers in Earth Science (2024). DOI: 10.3389/feart.2024.1349425