After a 500-year slumber, the lesser-known Garlock Fault, near the infamous San Andreas Fault, is showing “unprecedented” movement after a series of earthquakes in July, according to a study published on Friday.

The analysis by geophysicists from the California Institute of Technology (Caltech) and NASA’s Jet Propulsion Laboratory, both in Pasadena, documents a series of ruptures in a web of interconnected faults, with rupturing faults triggering other faults.

The domino-like sequence of ruptures also increased strain on a nearby major fault, according to the study published in the journal Science.

On July 4, the Ridgecrest earthquake rocked the Southland — which was just the foreshock at about 6.4 magnitude. A larger, 7.1 earthquake was the “main event” a day later, according to the United States Geological Survey (USGS).

More than 100,000 aftershocks followed the main quake, according to the USGS.

“This ended up being one of the best-documented earthquake sequences in history,” said Zachary Ross, assistant professor of geophysics at Caltech and lead author of the paper.

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Ross developed an automated computer analysis of seismometer data that detected the high number of aftershocks with precise location information, and the JPL team members analyzed data from international radar satellites to map fault ruptures at Earth’s surface.

“I was surprised to see how much complexity there was and the number of faults that ruptured,” said Eric Fielding, JPL co-author.

The rupture of the Ridgecrest mainshock erupted only a few miles from the major regional Garlock Fault, triggering “shallow creep” and a substantial “earthquake swarm,” according to the study.

The satellite and seismometer data together depict an earthquake sequence that is far more complex than those found in the models of many previous large seismic events, according to JPL.

The San Andreas Fault and the Garlock Fault, north of Los Angeles County.

Major earthquakes are commonly thought to be caused by the rupture of a single long fault, such as the more than 800-mile-long San Andreas Fault, with the maximum possible magnitude dictated primarily by the length of the fault, according to the USGS.

After a large 1992 earthquake in Landers, California ruptured several faults, seismologists began rethinking that model, said JPL officials.

The Ridgecrest earthquake sequence involved about 20 previously undiscovered, smaller faults crisscrossing in a geometrically complex and geologically young fault zone, according to the study.

“We actually see that the magnitude 6.4 quake simultaneously broke faults at right angles to each other, which is surprising because standard models of rock friction view this as unlikely,” Ross said.

The complexity of the event is only clear because of the multiple types of scientific instruments used to study it. Satellites observed the surface ruptures and associated ground deformation extending out over 60 miles in every direction from the rupture, while a dense network of seismometers observed the seismic waves that radiated from the earthquake.

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Together, this data allowed scientists to develop a model of how the faults slipped below the surface and the relationship between the major slipping faults and the significant number of small earthquakes occurring before, between and after the two largest shocks, according to researchers.

The Ridgecrest earthquake ruptures ended just a few miles shy of the Garlock Fault, a major east-west fault running more than 185 miles from the San Andreas Fault to Death Valley, according to the study.

The fault has been relatively quiet for the past 500 years, but the strain placed on the Garlock Fault by July’s earthquake activity triggered it to start slowly moving, a process called fault creep. The fault has slipped 0.8 inches at the surface since July, the scientists said.

“It’s going to force people to think hard about how we quantify seismic hazard and whether our approach to defining faults needs to change,” Ross said. “We can’t just assume that the largest faults dominate the seismic hazard if many smaller faults can link up to create these major quakes.”

The event illustrates how little we still understand about earthquakes, JPL officials said.

“The 2019 Ridgecrest sequence brought to an end the long earthquake silence in California,” the study said.

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