Ridgecrest earthquake packed the power of 45 nuclear bombs, but its impact was muted
By RONG-GONG LIN II
JUL 13, 2019 | 10:00 AM
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."
Direction matters
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.
Shaking was severe or violent along the 300-mile stretch of the San Andreas fault that ruptured during the great 1906 earthquake. Eureka, along the fault, got worse shaking than Sacramento, even though the capital is closer to the epicenter west of San Francisco. (USGS)
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.
By RONG-GONG LIN II
JUL 13, 2019 | 10:00 AM
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."
Direction matters
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.
Shaking was severe or violent along the 300-mile stretch of the San Andreas fault that ruptured during the great 1906 earthquake. Eureka, along the fault, got worse shaking than Sacramento, even though the capital is closer to the epicenter west of San Francisco. (USGS)
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.
Source: https://www.latimes.com/local/lanow/la-me-ridgecrest-earthquake-power-energy-20190713-story.html
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