Published : Friday, October 18, 2019 | 5:04 AM
Geophysicists from Caltech and JPL in Pasadena say the recent Ridgecrest earthquake sequence in July may have triggered movement on the Garlock Fault, on the northern edge of the Mojave Desert, which the scientists say is capable of producing a magnitude 8 earthquake.
The Ridgecrest earthquake sequence consisted of three main shocks, including a magnitude 6.4 foreshock on July 4, a 5.4 followed by a magnitude 7.1 mainshock on July 5, and more than 100,000 aftershocks, according to a USGS summary.
The Garlock Fault, which stretches more than 300 kilometers across Southern California, has been relatively quiet for the past 500 years, but the strain placed on it by July’s earthquake appears to have gotten the fault to start creeping.
Indeed, the fault has slipped two centimeters at the surface since July, the scientists say in a new study that was released Thursday by the journal Science.
“This is surprising, because we’ve never seen the Garlock fault do anything. Here, all of a sudden, it changed its behavior,” Zachary Ross, assistant professor of geophysics at Caltech and the lead author of the study, told the Los Angeles Times. “We don’t know what it means.”
The Ridgecrest sequence rattled most of Southern California, but the strongest shaking occurred about 200 kilometers north of Los Angeles. It caused at least one death and about 25 injuries
Ross said the sequence was “a real test of our modern seismic monitoring system.”
“It ended up being one of the best-documented earthquake sequences in history and sheds light on how these types of events occur,” he added.
Ross and his team looked at new images and data gathered by orbiting radar satellites and ground-based seismic monitors to piece together a picture of an earthquake rupture that is far more complex than found in models of many previous large seismic events. The satellites observed the ruptures that reached the surface and showed images of ground deformation that extends out over 100 kilometers in every direction from the rupture; the seismic meters observed the seismic waves that radiated out from the earthquake.
The Ridgecrest event, the largest earthquake sequence in Southern California in two decades, has also taught scientists that large earthquakes can occur in a more complex fashion than they have assumed in the past.
A Caltech statement said major earthquakes were 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 a maximum possible magnitude that is dictated primarily by the length of the fault. Seismologists began rethinking that model following the magnitude 7.3 earthquake that struck Landers, California, in 1992, which involved the rupture of several different faults.
In the new study, Ross and his team said the Ridgecrest sequence provided another example of how massive earthquakes can be generated by a web-like network of smaller interconnected faults that, when they rupture, trigger one another like falling dominoes.
The sequence involved about 20 previously undiscovered faults crisscrossing in a geometrically complex and geologically young fault zone.
“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. “It is remarkable that we now can resolve this level of detail.”
A Los Angeles Times report Thursday said a large quake on the Garlock fault could cause strong shaking in the San Fernando Valley, Santa Clarita, Lancaster, Palmdale, Ventura, Oxnard, Bakersfield and Kern County, one of the nation’s most productive regions for agriculture and oil.
Several military installations in the area could also get strong shaking, including Edwards Air Force Base, Naval Air Weapons Station China Lake and Fort Irwin National Training Center.
A major quake on the fault could also destabilize the San Andreas fault, which could result in the worst shaking Southern California has felt since 1857, and send destructive tremors through Los Angeles and beyond, the report said.
Another Caltech scientist, Dr. Jennifer Andrews, staff seismologist at the institute, said the interaction between faults, such as that observed during the Ridgecrest sequence, depends on how the faults are oriented relative to each other, and how the stress is building up.
“Certainly any movement on a fault will locally be relieving stress there,” Andres said. “And if there’s another one oriented well next to it, it might be relieving the stress on that one as well.”
The important thing, Andrews said, is to put up enough seismic instruments around any active fault for people to get the best possible warning. She does agree, however, that the Ridgecrest event was a bit more complicated than most other events they have monitored.
“When you look at something like Ridgecrest, none of us would’ve said, ‘put your sensors there.’ Well, we knew it was an active area, but we wouldn’t have known exactly which fault was about to break,” Andrews said. “It wasn’t particularly well mapped at that time. It wasn’t something that we were on high alert for, so we certainly are nowhere near to prediction.”
Asked whether the event and the impact it has had on other faults would affect Pasadena and nearby areas, Andrews said it’s business as usual.
“We’re now basically at our normal likelihood for any fault to move and any earthquake to happen,” Andrews said.
Ross’ paper is titled “Hierarchical interlocked orthogonal faulting in the 2019 Ridgecrest earthquake sequence.”
Co-authors of the study include Zhongwen Zhan, assistant professor of geophysics; Mark Simons, John W. and Herberta M. Miles Professor of Geophysics; Egill Hauksson, research professor of geophysics; Caltech graduate students Benjamín Idini, Zhe Jia, Oliver L. Stephenson, and Minyan Zhong; Caltech postdoctoral scholar Xin Wang; and Eric J. Fielding, Angelyn W. Moore, Zhen Liu, and Jungkyo Jung from JPL.
The research was funded by multiple sources including NASA, USGS, and the National Science Foundation.