When an earthquake strikes, the instinct of many Californians is to ask: Which fault ruptured — the Newport-Inglewood, the Hayward, the mighty San Andreas?
But scientists are increasingly saying it’s not that simple.
New research shows that the Ridgecrest earthquakes that began in July ruptured at least two dozen faults. It’s the latest evidence of how small faults can join together to produce a large earthquake, and how those quakes can cover a wider area than expected.
The findings are important in helping understand how earthquakes can grow in the seconds after a fault ruptures, when two blocks of earth move away from each other. In areas blanketed by a crisscross pattern of faults, an earthquake on a smaller fault can destabilize bigger ones, beginning a process that leads to a much stronger earthquake.
In the case of Ridgecrest, some follow-up earthquakes came seconds after; the largest one came some 34 hours later.
It has only been in recent decades that earthquake scientists have understood how smaller faults in California join together to create a more powerful earthquake.
After the 1992 Landers earthquake, scientists were astonished to find that the magnitude 7.3 temblor in the Mojave Desert had ruptured on five separate faults.
As the years have gone by, more evidence has accumulated that earthquakes can and do happen on multiple faults — such as the magnitude 7.1 Hector Mine earthquake, about 20 miles east of the Landers quake, and the magnitude 7.2 quake on Easter Sunday 2010 near Mexicali.
Detailed observations, outlined recently in the journal Science by experts at Caltech and NASA’s Jet Propulsion Laboratory, underscore how scientists’ understanding of earthquake fault ruptures has evolved.
Researchers discovered the Fourth of July Ridgecrest temblor was actually three distinct earthquakes — magnitudes 6.1, 6.2 and 6.2 — on a trio of faults. Added together, they produced enough energy to create a magnitude 6.4 temblor, said Zachary Ross, Caltech assistant professor of geophysics, the lead author of the paper. The first two quakes ruptured at right angles to each other, forming the shape of the capital letter L: the first to the northwest and southeast, the second to the southwest. The third quake also ruptured to the southwest. The faults ruptured over 12 seconds.
“That’s something we haven’t seen before, and it’s detailed on such a small scale,” said Caltech seismologist Egill Hauksson, a coauthor and expert on Southern California seismology.
The second large quake, on July 5, was actually made up of four smaller events that ruptured over 22 seconds, producing a magnitude 7.1 event, the most powerful in California in the last 20 years. In that same event, at least 20 smaller faults that intersected the main faults also ruptured, according to the study, making the zone of land atop moving faults wider than might be expected.