Earthquakes are super strong, and they are much less rare than previously thought

On April 18, 1906, San Francisco phonograph dealer Peter Pacigalupi’s day began like any other.

“I was awakened from sleep by my voice by a terrible trembling, which acted in the same manner as the dreaded trachea [sic]He wrote of the historic earthquake that seemed to throw his bed “up and down in all four directions at once.”

Across town, missionary Donaldina Cameron awoke at 5:12 a.m. to a surreal scene where “the solid ground took on the motions of the furious ocean as chimneys crashed on our roof, while stucco and ornaments splattered on the floors.”

Bacogalupi and Cameron survived to tell their stories of California’s deadliest natural disaster. An estimated 3,000 people in the San Francisco Bay Area have not.

Although definitive measurements were not available at the time, the magnitude 7.8 earthquake is believed to be a specific type of earthquake known as a superquake.
In a supersonic earthquake, the fault—in the case of the 1906 San Andreas—bursts faster than seismic shear waves can travel through the rock. The result is a buildup of energy that blasts through rock the way sound waves build up against a speeding fighter plane in a sonic blast.

People watch smoke rise from fires after a violent earthquake struck San Francisco on April 18, 1906.

(News agency)

Super-earthquakes were thought to be relatively rare, with fewer than a dozen such events confirmed and six more discussed since 1906.

But new research from UCLA has found that this type of violent earthquake is more common than previously thought, especially along mature slip faults like the San Andreas.

Using advanced imaging technology, a research team led by UCLA geophysicist Lingsen Meng examined all 86 earthquakes of magnitude 6.7 or greater along slip faults between January 1, 2000, and February 1, 2020. After analyzing each event, the team concluded That 14% of them were actually superquakes—a huge jump, given that superquakes were previously thought to account for less than 6% of all earthquakes.

The results were published last month in the journal Nature Geoscience.

said seismologist Eric Dunham, a super-seismic expert at Stanford University who was not involved in the research.

In the absence of tools that can effectively analyze rift rupture in both continents and oceanic crust, Dunham said, “we’ve been kind of guessing” at which events count as paranormal hearing. “This paper shows that they may not be as rare as we thought.”

Twisted pavement and curbstones from the April 18, 1906 earthquake in San Francisco, California.

The San Francisco earthquake caused sidewalks and kerbstones to bend at the corner of 18th Street and Lexington Avenue.

(News agency)

Previously, seismologists suspected that this type of earthquake occurred more often on continents than undersea faults, which is where most confirmed earthquakes have been recorded on land.

But using a technique called backproject, which analyzes the delays between seismic waves to determine how fast they travel, the team realized that super-quakes are just as common in the ocean as they are on dry land — they’ve been very difficult to monitor historically.

Their analysis found that in addition to five previously confirmed superearthquakes documented in their dataset, seven others also met the criteria for superhearing.

“I’m a little surprised we found this many,” Meng said of the newly identified super-earthquakes, which all occurred along underwater fault lines beyond the reach of most terrestrial monitors.

Super-earthquakes are most likely to occur along long, mature faults such as the San Andreas, where many years of activity has deflected much of the fluctuations and shocks that would slow earthquake energy.

In the same way that it is easier to pick up speed on a long, straight runway than on a winding road, Meng said, a rip will accelerate faster along a long, straight fault than on a chronic fault.

The force of the super earthquake comes from the speed of that rupture. When sound waves accumulate in front of an aircraft moving faster than the speed of sound, they eventually merge into a single wave that a person on the ground hears as a sonic boom or boom.

And just as the boom is louder than the normal roar of an engine, so an audible earthquake vibrates with greater force.

“The same amount of energy from a bug is released in a shorter period of time. This always gives you a stronger vibration,” Meng said.

Existing building codes are already designed to accommodate the possibility of a hearing-force earthquake, said Elizabeth Cochran, a seismologist with the USGS in Pasadena. But SVs are more likely to cause secondary crises that cause a lot of destruction in a major earthquake, like fires and landslides.

“It’s a worry,” she said. “If you have a super-hearing tear, then you can expect a stronger vibration intensity, which can then translate to greater potential for damage.”

Only about 2% of the 28,000 buildings lost in the 1906 earthquake collapsed from the tremors. The vast majority were destroyed by raging fires after earthquakes caused gas and water pipes to explode. The strength of the earthquake is important, as well as what happens after the shaking stops.

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