At 10:33 a.m. local time on 4 July 2019, a magnitude 6.4 earthquake struck California’s Mojave Desert near Ridgecrest (population 28,000). The next day, a magnitude 7.1 earthquake—roughly 11 times more powerful—occurred at 8:19 p.m. on a different fault in the same area.
“This was a unique set of events,” said Abhijit Ghosh, a seismologist at the University of California, Riverside. Usually, there’s one large earthquake followed by smaller aftershocks, he said. “This time we had a large, damaging earthquake immediately followed by an even larger, damaging earthquake.”
Ground shaking was felt across Southern California. The Did You Feel It? website from the U.S. Geological Survey (USGS) recorded over 40,000 reports of shaking for each of the large earthquakes.
While the Ridgecrest area was still ringing with aftershocks—over 3,500 were detected within a week and a half—researchers working in Southern California and beyond rushed to the epicenters of the earthquakes.
Over the next days, weeks, and months, personnel associated with academic institutions, the USGS, the California Geological Survey, the U.S. Navy, and other organizations would collect a wide range of seismological, geological, and geodetic data. “It was an all-hands-on-deck effort,” said Elizabeth Cochran, a seismologist at the USGS in Pasadena, Calif.
The measurements and observations, which are currently being analyzed, will shed light on earthquake-triggering mechanisms, the structure of seismic faults, and how surface rupture affects buried infrastructure such as gas pipes and sewer lines, scientists anticipate.
A Hurried Trip
Ghosh and five students arrived in Ridgecrest on the morning of 6 July with five seismometers stuffed in Ghosh’s white Chevy Suburban. Their hurried trip was guided by Omori’s law, which states that the number of aftershocks decays exponentially with time. “If you miss those first few days, you’re missing the lion’s share of the data,” said Ghosh.
Over the next 2 weeks, Ghosh and his students installed 25 seismic stations around the Mojave Desert. Data from the stations, which will remain in place for 6 months, will shed light on the complex fault structure near the boundary between the Pacific and North American plates, said Ghosh.
This area, part of the Eastern California Shear Zone, is well known for having scads of crosscutting faults. It’s fiendishly complicated compared with models that focus on one isolated fault, researchers agree.
“The traditional way of simulating ruptures essentially involves far simpler structures,” said Yehuda Ben-Zion, a geophysicist at the University of Southern California in Los Angeles. Because earthquakes like those in the Ridgecrest sequence require models that go beyond what the community is currently using, said Ben-Zion, “it’s an impetus to move forward.”
The Southern California Earthquake Center (SCEC), a National Science Foundation– and USGS-funded organization led by Ben-Zion, seized the opportunity to learn from the Ridgecrest earthquakes. By 12 July, SCEC researchers had installed over 460 seismometers around Ridgecrest. The instrumentation, collected by Ben-Zion, was contributed by Sandia National Laboratories, USGS, the Incorporated Research Institutions for Seismology Portable Array Seismic Studies of the Continental Lithosphere Instrument Center, and other groups.
Scientists from the Pasadena and Moffett Field offices of the USGS also rushed to Ridgecrest soon after the initial large earthquake. “They wanted to get on the ground quickly and see if there was surface rupture,” said the USGS’s Cochran, who helped coordinate the response. And they found it. “Just as daylight was fading, they found the break across the highway.”
Over the coming days and weeks, USGS scientists installed over 200 seismometers, mostly along the fault that produced the magnitude 7.1 temblor. The goal, said Cochran, was to watch how a fault evolves through time after an earthquake.
By blanketing the landscape with seismometers, researchers hoped to carefully study this earthquake sequence, which included unusually large earthquakes. “It terminated a hiatus of large earthquakes in Southern California that’s lasted for almost 20 years,” said Ben-Zion.
For starters, scientists want to better understand the structure of the complicated Eastern California Shear Zone and how earthquakes on one fault potentially trigger ground movement on other faults. The two largest earthquakes of the Ridgecrest sequence occurred close in space and time but on different faults.
“Did the magnitude 6.4 earthquake somehow trigger the magnitude 7.1 earthquake?” asked Ghosh. “If it was indeed triggered, what’s the mechanism?”
Other faults in the area that didn’t rupture should also be watched, researchers agree.
One is the roughly 250-kilometer-long Garlock fault that skirts the large Southern California city of Bakersfield. There’s already evidence that the Garlock fault is more active than it was before the Ridgecrest sequence, said Ghosh. That’s potentially bad news given the size of the Garlock fault.
“It’s a larger fault, meaning that it can produce larger earthquakes,” said Ghosh.
Cochran and her USGS colleagues also analyzed how quickly seismic waves from aftershocks traveled through the ground. Using measurements of increased velocities over time after the magnitude 7.1 earthquake, Cochran and her collaborators estimated that the subsurface landscape was knitting itself back together. There are a lot of ideas to explain this “healing,” said Cochran, such as cracks closing because of confining pressure and mineral deposition essentially cementing cracks back together. But the details remain elusive. “We don’t actually know what the physical mechanism is,” said Cochran.
Cracked Pavement and Broken Pipes
Scientists traveled to Ridgecrest armed with more than just seismometers: They also came ready to collect geological and geodetic data. Scott Brandenberg, a geotechnical engineer at the University of California, Los Angeles, arrived in Ridgecrest around 3:00 p.m. local time on 5 July. (He experienced the magnitude 7.1 earthquake that evening from the parking lot of Ridgecrest’s Super 8 hotel.)
Brandenberg had come as part of Geotechnical Extreme Events Reconnaissance (GEER), a volunteer organization of geotechnical engineers, engineering geologists, and Earth scientists that, since the 1980s, has been conducting geotechnical engineering reconnaissance after disasters. The point of GEER is to provide coordination, said Brandenberg, because there’s the risk of having “a whole bunch of disjointed efforts” after an earthquake.
Using cameras, tape measures, and GPS, Brandenberg and his colleagues mapped roughly 2 kilometers of surface rupture manifested as fissures in the ground, scarps, and cracked pavement. This on-the-ground fieldwork provided an in-depth view of the aftereffects of ground shaking but covered just roughly 3% of the total surface rupture: Together, the magnitude 6.4 and 7.1 earthquakes produced about 70 kilometers of rupture. “Our ground-based mapping efforts focused on very detailed measurements over a short length of the fault rupture,” said Brandenberg.
The opportunity to analyze any amount of surface rupture is rare, said Brandenberg, because even the largest earthquakes don’t always produce it. “Ridgecrest was the first Southern California earthquake since Hector Mine in 1999 that ruptured the ground surface.”
Surface rupture is of interest because it’s liable to affect underground infrastructure like water pipes, gas pipes, electric utilities, and sewer lines, said Brandenberg. And indeed, broken water pipes, power outages, and fires were all reported following the Ridgecrest earthquakes.
“There’s just so much in the ground,” Brandenberg said.
Collecting these observations early on was critical because surface rupture data are “perishable,” said Brandenberg. People walk and drive over these surface features, and aftershocks alter them, he said. “Just during the time that we were there, the surface rupture features had started changing.”
A significant fraction of the fault zone researchers wanted to study fell within the Naval Air Weapons Station China Lake. Larger than the state of Rhode Island, the secure facility required coordination with the Navy. Military personnel were very cooperative, Brandenberg said, and regularly escorted scientists around the facility.
On 19 July, just 2 weeks after the magnitude 7.1 earthquake, Brandenberg and his colleagues published a summary of their reconnaissance work.
Going to the Sky
Other research groups opted for a bird’s-eye view of how the earthquakes changed the landscape. Mike Oskin, a geologist at the University of California, Davis, is part of a team that, starting in late July, flew a small aircraft to collect lidar observations. These remote sensing data, which cover 600 square kilometers of the Mojave Desert near Ridgecrest, trace features as small as a few centimeters. They’ll reveal surface features such as cracking and scarps potentially missed—or simply not surveyed—by fieldwork, said Oskin, and provide an important permanent record of transient features.
Oskin and his colleagues collected lidar observations that traced all roughly 70 kilometers of surface rupture. The point clouds generated from these measurements will accordingly be enormous, Oskin said. “It’s going to be trillions of points.”
The researchers also collected aerial observations of fragile geological features known as the Trona Pinnacles. These calcium carbonate spires, which are up to 40 meters high, formed thousands of years ago in hot springs that dotted the area. Pieces of these spires toppled during the Ridgecrest earthquakes, said Oskin, and there’s interest in using these geological features as paleoseismometers.
Another view from the sky is being provided by Andrea Donnellan, a geophysicist at NASA’s Jet Propulsion Laboratory in Pasadena, and her collaborators. Donnellan and her colleagues are using drones to repeatedly survey two approximately 500- by 500-meter regions centered on surface rupture.
The quadcopter drones that Donnellan and her collaborators fly are equipped with 21-megapixel cameras that capture optical images. Since 9 July, the researchers have collected thousands of images with a spatial resolution of 2 centimeters.
There’s a big advantage to repeated looks at the same landscape after earthquakes, said Donnellan, because ground deformation can continue for years: After the El Mayor–Cucapah earthquake in Baja California in 2010, surface deformation persisted for 7 years.
These images captured by Donnellan and her colleagues complement lidar observations, which, although generally covering a wider area, are only one snapshot in time.
Sharing the Science
Researchers are already looking forward to sharing what they’ve learned about these earthquakes. Seismological Research Letters, a publication of the Seismological Society of America, plans to publish a series of papers focused on the Ridgecrest sequence.
“We were aware that a huge amount of data was being collected for the Ridgecrest sequence and that many seismologists need access to the data to conduct in-depth research to better understand the earthquake sequence and its implications,” said Allison Bent, a seismologist with Natural Resources Canada in Ottawa and the editor in chief of Seismological Research Letters. “Papers will be collated and published in a single print issue of Seismological Research Letters, but they will be published online as soon as possible after acceptance.”
Several conferences have also included results about the earthquakes and discussions of the mobilization effort. The Geological Society of America’s Annual Meeting, held in September in Phoenix, featured a special session about the Ridgecrest earthquakes. Researchers gathered to discuss earthquake early warning, the age of the Eastern California Shear Zone, and the types of slip that occurred during the Ridgecrest sequence, among other results. Also in September, the 2019 Southern California Earthquake Center Annual Meeting included two workshops, a plenary session and over 65 posters about the Ridgecrest earthquakes.
This week, AGU’s Fall Meeting will feature more than 100 sessions on the Ridgecrest earthquakes.
—Katherine Kornei (@katherinekornei), Freelance Science Journalist
23 December 2019: This article was updated to reflect that the 2019 Ridgecrest earthquake was the first earthquake in Southern California since the Hector Mine event in 1999 to rupture the ground surface, not the first earthquake in all of California to do so.