Slight shifts in the magnetic field preceded the California earthquakes

The 6.0 magnitude South Napa earthquake in California in August 2014 ruptured the ground in some places, including this vineyard. Credit: USGS, public domain

Researchers studying medium and large earthquakes in California have detected detectable changes in the local magnetic field that occur 2-3 days before the earthquake. In a study now published in the Journal of Geophysical Research: Solid Earth, William Heflin and his team found the magnetic field shift signal to be faint but statistically significant, and seismologists hope their method can be improved to eventually help predict earthquakes.

“It’s a modest signal,” said Dan Schneider, director of QuakeFinder, earthquake research at Stellar Solutions, a systems engineering services company. “We don’t claim that this signal is there before every earthquake, but it’s very interesting,” said Schneider, one of the study’s authors.

The idea that the magnetic field might change before earthquakes has been around for a while, but it has always been controversial. The USGS states that, “Despite decades of work, there is no convincing evidence for the electromagnetic precursors of earthquakes.”

In collaboration with the Google Accelerated Science team, the scientists made use of magnetic field data from an array of magnetometers at 125 sensor stations along major faults in California. They collected data from 2005 to 2019, in which 19 earthquakes with a magnitude of 4.5 or more occurred on the faults.

Their multi-station analysis took into account other types of processes that may affect magnetometers but are unrelated to earthquakes, such as rush hour traffic. Distinguishing between this type of noise and potential earthquake-related signals is the longest barrier to interpreting this data, Schneider said. After training their algorithms on half of the data set, the researchers identified a signal indicating changes in the magnetic field between 72 and 24 hours before the earthquakes.

In the future, Schneider said, he would like to further refine the models to remove more ambient noise from the magnetometers. In this study, for example, calculating the mean effect of solar activity greatly improved the results. In the ongoing work, the team will use the remote station’s data to eliminate noise from solar activity.

The work suggests that “there may be regular detectable changes in the magnetic field which, with further study and isolation, could indeed support the construction of a prediction system in the future,” Schneider said.

Can earthquake rates of magnitude 4 be used to predict large earthquake events? More information: William D. Heavlin et al, Case-control study on a decade of California geomagnetometers reveals modest signal 24-72 hours before earthquakes, Journal of Geophysical Research: Solid Earth (2022). doi: 10.1029/2022JB024109

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