When the magnitude 7.1 earthquake ruptured the earth in the Mojave Desert, it packed the energy of 45 nuclear bombs of the type that fell on Hiroshima.
But a variety of factors lessened the potency and impact of what was the most powerful Southern California earthquake in nearly two decades.
The massive temblor, it’s important to note, ruptured on a fault whose northwest-southeast direction pushed the worst shaking away from populated areas.
The area Ridgecrest sits in is riddled with faults — in the Eastern California Shear Zone — that have produced some of the state’s biggest quakes in the modern record, like the magnitude 7.5 Owens Valley earthquake of 1872 and the magnitude 7.3 Landers earthquake in 1992.
But this particular fault packed its biggest punch either toward the Sequoia National Forest to the northwest or largely uninhabited expanses of the Mojave Desert. The most populated area that got the worst shaking was Naval Air Weapons Station China Lake, which was right on top of the fault rupture and saw damage to its elementary school.
Northern Los Angeles County would have experienced more shaking had the quake occurred on a fault with a different tilt. For example, a rupture on the nearby Garlock fault, one of California’s faster-moving faults that runs on a northeast-southwest alignment, would be capable of directing heavier shaking to areas like Bakersfield and Ventura County.
“If this earthquake had been on the Garlock fault, then, yeah, Bakersfield, the cities in the Mojave Desert, would have been impacted more strongly, and L.A. would have felt stronger shaking,” Caltech seismologist Egill Hauksson said. “You generate energy, it piles up and it heads toward the west and south.”
There’s a direction to earthquakes that becomes especially pronounced when they are large — say, a magnitude 6.5 or greater. An earthquake will begin at a particular point under the Earth’s surface, then move along a fault. In the case of the July 5 earthquake, 30 miles of the fault moved — with the earthquake moving in two directions at a speed of perhaps 1½ to 2 miles per second about 10 miles northwest of the epicenter and 20 miles to the southeast.
You feel worse shaking if you’re in the direction of the path of the earthquake, much like hearing the high pitch of a fire engine racing toward you. “If the earthquake is coming at you, your ground motions are going to be stronger … the [shaking] waves are all packed together quite closely,” said U.S. Geological Survey seismologist Elizabeth Cochran.
But if the quake is not headed in your direction, the shaking waves spread out, “and you don’t get them all hitting you at once,” Cochran said.
The worst shaking intensities recorded in the July 5 quake were violent — level 9 on the Modified Mercalli Intensity Scale, capable of causing buildings to collapse — but they occurred in a relatively small area outside of towns. Relatively speaking, Ridgecrest and Trona got a glancing blow, receiving very strong (level 7) shaking and strong (level 6) shaking, respectively.
Direction has mattered in other historic California earthquakes. The great 1906 earthquake for instance had an epicenter just west of San Francisco. It then ruptured in two directions — to the northwest and southeast.
The worst shaking all occurred along the 300-mile rupture length. As a result, Eureka, more than 200 miles northwest of San Francisco, felt stronger shaking than Sacramento, just 75 miles northeast but nowhere near the path of the San Andreas fault.
Direction will be a key factor in the level of damage in future big quakes in California, such as something on the scale of the epic magnitude 7.8 temblor that hit the southern San Andreas fault in 1857. A quake heading from Monterey County toward Southern California would lessen the destruction in Los Angeles by sending a lot of shaking energy into the sparsely populated Mojave Desert, Hauksson said.
But an earthquake that comes from near the Mexican border, around the Salton Sea through Palm Springs into San Bernardino, would be much worse for L.A.
In that direction, shaking waves would go from the Coachella Valley into the San Bernardino Basin, which traps seismic energy and will reverberate as it sends that intense shaking west, through the San Gabriel Valley and into the Los Angeles Basin, Hauksson said.
A less ‘sharp’ earthquake
There are other ways that Southern California caught a break with the July 5 quake. It was also less sharp than the 1994 Northridge earthquake despite being many times larger.
The difference is like “if you hit the gas pedal versus if you hit it lightly,” Hauksson said. Both approaches will still get you to, say, 60 mph — like earthquakes of the same magnitude — but how quickly they move can determine how bad the shaking can be.
The Northridge quake was like a driver flooring the accelerator pedal of a sports car. The Searles Valley quake, by contrast, felt relatively more like a driver easing onto a freeway on-ramp.
“The ground motions are slightly under average for an earthquake of this size,” said USGS research geophysicist Morgan Page.
A skinny fault
The Searles Valley quake was also on a vertical fault that most Californians are familiar with — something that might look like a curtain dividing two blocks of earth vertically underneath our feet. There’s only so much surface area closest to the vertical fault line.
“There are few people who can be on top of a line,” Hauksson said.
The Northridge quake, by contrast, occurred on a dreaded horizontal fault — mostly parallel to the Earth’s surface, rather than perpendicular.
“And that leads to a wider area of shaking,” Hauksson said. “If you put a slanting surface at depth, you can put a lot of people on that” — all directly on top of the fault that’s moving.
What matters most is how close you are to the fault, Cochran said. In the Northridge quake, “you have a lot of [ground surface] area that’s closest to that fault.”
The threat of Mojave Desert faults
Earthquake faults in the Mojave Desert are something that concerns seismologists, despite their lower profile compared with the better-known San Andreas, San Jacinto and Hayward faults.
The Garlock fault runs along the northern boundary of the Mojave Desert, and USGS simulations have shown it could produce earthquakes up to magnitude 7.7.
A worst-case scenario would be a magnitude 7.7 earthquake that begins on the eastern end of the Garlock fault in eastern San Bernardino County and unlocks the fault to the southwest, bringing severe shaking to towns such as California City and Tehachapi; Edwards Air Force Base and Lancaster would see very strong shaking. Even Santa Clarita and the San Fernando Valley would see strong shaking, with much of the L.A. Basin and the San Gabriel Valley seeing moderate shaking — worse than what L.A. encountered last week.
California is seismically active because it sits on the boundary of two giant tectonic plates, the Pacific and North American. Earthquakes happen as the southwestern side of California slides up northwestward toward Alaska, compared to its northeastern half.
Though much of that motion results in earthquakes along the San Andreas fault, the seismic stresses are also relieved on other fault strands, including those in the Eastern California Shear Zone.
For all the dodged bullets, many in the quake zone are still suffering, with major damage to some homes, businesses and roads.
Karen Byrd, 39, said Trona was still reeling hard from the quake – most restaurants were still closed, as is the Family Dollar market and post office.
But water and electricity are back up, and though her home had been filled with debris — from broken glass jars to porcelain dolls to a priceless dish set — the family can at least continue living there.
“Thank the Lord we don’t have major structural damage,” she said.
Dan Tolbert, 62, spends time with his dogs as he and his wife, Ronnie, 60, prepare to bed down for the night on a pair of mattresses in front of their earthquake-damaged home in Trona on July 10. Their night was interrupted when a scorpion crawled on their mattresses and they ended up spending the night in their truck. “If we keep feeling tremors tomorrow we’ll be out here again,” Ronnie said.(Genaro Molina / Los Angeles Times)
Karen Byrd, 39, collects photo frames knocked off the wall at her home in Trona, Calif.(Irfan Khan / Los Angeles Times)
Ronnie Tolbert, left, delivers food to Robert VanHorn, 81, almost a week after a 7.1 earthquake near Trona.(Genaro Molina / Los Angeles Times)
Benny Eldridge, 76, looks at a quake-damaged room in his Trona home, which he helped build with his father-in-law in 1961. The house has been red-tagged.(Genaro Molina / Los Angeles Times)
Benny Eldridge, 76, and his wife, Anna Sue, 75, sit in front of their damaged home in Trona.(Genaro Molina / Los Angeles Times)
Joyce Harrison Moore, 72, looks out from her damaged home almost a week after a pair of earthquakes battered Trona. “This town will either die or get back on its feet,” Moore said.(Genaro Molina / Los Angeles Times)
Ronnie Tolbert stands beside her damaged fireplace.(Genaro Molina / Los Angeles Times)
Valerie Helton, 60, facing the camera, receives a hug of support from Ronnie Tolbert. Helton and her daughter Jessica Sizemore Helton, left, have refused to leave their home since last week’s quakes. “This is all I have,” said Sizemore Helton.(Genaro Molina / Los Angeles Times)
Ralph “Zeb” Haleman, 67, carries cases of water home Sunday in Trona, Calif., where residents were still without water and electricity was spotty after last week’s quakes.(Irfan Khan / Los Angeles Times)
Kay Byrd, 64, gives herself an insulin shot. Byrd and her family are camping outside in Trona, Calif., wary of returning home after major earthquakes.(Irfan Khan / Los Angeles Times)
Brooke Thompson, 8, plays on the sleeping bag that her family slept in after a pair of major earthquakes drove them out of their home in Trona, Calif.(Irfan Khan / Los Angeles Times)
The Byrd family has breakfast next to where they spent the night under a salt cedar tree, afraid to return to their Trona, Calif., home of 21 years after major earthquakes.(Irfan Khan / Los Angeles Times)
Ronnie Tolbert, left, and her husband, Danny, sleep on mattresses in the front yard of their Trona home, which was damaged in a 7.1 magnitude earthquake.(Robert Gauthier / Los Angeles Times)
The Horta family sleeps in the back of their pickup truck in a fire station parking lot in Trona as the sun rises hours after being forced from their home by a magnitude 7.1 earthquake.(Robert Gauthier / Los Angeles Times)
Kathy Vander Housen, 76 hugs her friend Claire Barker, 76, after Barker told her that she had found her two cats. Vander Housen’s mobile home in Trona had been yellow-tagged by county inspectors, but she did not want to leave without the cats, which had been hiding since the earthquake(Irfan Khan / Los Angeles Times)
Buckled asphalt courses through a parking lot near Trona Rd. in Argus.(Robert Gauthier / Los Angeles Times)
Sammy Chute cuddles Gerard as her family in Trona prepares to evacuate to Ridgecrest, abandoning their home that was knocked off its foundation during a 7.1 earthquake.(Robert Gauthier / Los Angeles Times)
Charles Ware, 68, in his Trona front yard the morning after a 7.1 magnitude earthquake severely damaged his home. Ware said he invested all he had into this house two years ago, doesn’t have earthquake insurance and is afraid he may not be able to rebuild. He was on the phone with his brother in San Diego when the quake hit. “I got to ride it out with my brother,” he said.(Robert Gauthier / Los Angeles Times)
A customer rummages for a six-pack of beer at a damaged Shell food mart in Trona the day after a 7.1 earthquake.(Robert Gauthier / Los Angeles Times)
Hundreds of residents of Ridgecrest, Calif., and surrounding communities attend a town hall meeting at Kerr McGee Community Center about the response to recent major earthquakes.(Irfan Khan / Los Angeles Times)
Christian Fellowship of Trona congregants pray after holding a quick meeting on how to help other community members.(Irfan Khan / Los Angeles Times)
Kern County firemen tackle a fire on Saturday morning at Town and Country Mobile Home Park in Ridgcrest.(Irfan Khan / Los Angeles Times)
Jamie L. Acevedo sits outside her damaged Trona home, waiting to evacuate to Ridgecrest the morning after a 7.1 magnitude earthquake knocked her home off its foundation.(Robert Gauthier / Los Angeles Times)
Gas station owner Roger Sandoval faces the possibility of having to shut his Trona business after a 7.1 earthquake apparently damaged the supply tanks near the pumps.(Robert Gauthier / Los Angeles Times)
Chavela Padilla, left, an emergency response team volunteer, walks with Ronnie Tolbert amid quake-toppled items in Tobert’s Trona home. The damage occurred in a 7.1 temblor hours earlier.(Robert Gauthier / Los Angeles Times)
Chavela Padilla, a Trona emergency response team volunteer, enters her car after checking on a neighbor as her two young boys, Joey, 8, right, and Jimmy, 5, sleep in the back seat at close to 3 a.m. The boys were too scared to be home after experiencing a 7.1 earthquake hours earlier.(Robert Gauthier / Los Angeles Times)
Chavela Padilla, right, an emergency response team member walks with Ronnie Tolbert amid quake-toppled items in Tolbert’s Trona home.(Robert Gauthier / Los Angeles Times)
Brothers Joey, 8, right, and Jimmy Raya, 5, sleep in the back seat of their mother’s car in the parking lot of San Bernardino County Fire Station 57 in Trona after their home was damaged in a 7.1 magnitude earthquake hours earlier.(Robert Gauthier / Los Angeles Times)
Chavela Padilla, a Trona emergcency response team volunteer, assists her neighbor Alicia Marines, 72, who was injured while trying to escape her home during a 7.1 magnitude earthquake. Marines was evacuated to the local fire station. James Raya, Padilla’s husband and also a volunteer, looks on.(Robert Gauthier / Los Angeles Times)
Chavela Padilla, a Trona emergency response team volunteer, stands in the bloody footprints left by homeowner Alicia Marines, 72, who was injured during a 7.1 magnitude earthquake. Padilla volunteered to check on Marines’ residence and collect some fresh clothes.(Robert Gauthier / Los Angeles Times)
Ronnie Tolbert sorts through toppled belongings in her Trona home, damaged in a 7.1 magnitude earthquake hours earlier.(Robert Gauthier / Los Angeles Times)
The aftermath of Friday’s earthquake at a Ridgecrest liquor store.
(Robyn Beck / AFP/Getty Images)
Work on Route 178 between Trona and Ridgecrest.(Etienne Laurent / EPA-EFE/REX )
Workers fill large holes left in Highway 178 between Trona and Ridgecrest by Friday night’s 7.1 earthquake.(Etienne Laurent / EPA-EFE/REX )
Highway workers repair roadway near Ridgecrest on Saturday morning.(Robyn Beck / AFP/Getty Images)
Police and emergency services respond to a fire at a building on Highway 178.(Etienne Laurent / EPA-EFE/REX )
Firefighters respond to a fire at a building on Highway 178 after Friday night’s earthquake near Ridgecrest.(Etienne Laurent / EPA-EFE/REX )
In Ridgecrest, Davia Speed and Peyton Speed, holding 1-month-old Lillian, get into their car after Friday night’s 7.1 earthquake.(Irfan Khan / Los Angeles Times)
A fire burns behind Casa Corona restaurant in Ridgecrest after Friday’s earthquake.(Jessica Weston / The Daily Independent/Associated Press)