LOS ANGELES-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.
Importantly, the massive temblor 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 the installation's 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 and is 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 1 1/2 to 2 miles per second about 10 miles northwest of the epicenter and 20 miles to southeast.
You feel worse shaking if you're in the direction of the path of the earthquake, much like hearing a 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 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, according to Hauksson.
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.
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."
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 earthquakes ranging from magnitude 6.9 to 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 the Eastern California Shear Zone.