On April 18, 1906, record dealer Peter Bacigalupi’s day in San Francisco began like no other.
“I was awakened from a deep slumber by a terrible tremor that seemed like a jerking broncho [sic]’ he wrote of the historic earthquake, which appeared to toss 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 in which “the solid earth took on the motions of a raging ocean, while smokestacks crashed onto our roof, while plaster and ornaments littered the floors “.
Bacugalupi and Cameron survived to tell their stories of California’s deadliest natural disaster. An estimated 3,000 people in the San Francisco Bay Area did not.
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Although definitive measurements were not available at the time, the 7.8 magnitude disaster is believed to have been a specific type of earthquake known as supershear.
In a supershard, the fault—in the case of 1906, the San Andreas—fractures faster than seismic shear waves can travel through rock. The result is an accumulation of energy that smashes through rocks like sound waves back up against a speeding fighter jet erupting in a sonic boom.
Supershards were considered relatively rare, with fewer than a dozen such events confirmed and six others debated since 1906.
However, new research from UCLA shows that these types of violent earthquakes are more common than previously thought, especially along mature strike faults like the San Andreas.
Using advanced imaging technology, a research team led by UCLA geophysicist Lingsen Meng studied all 86 earthquakes measuring 6.7 or greater along strike faults between January 1, 2000 and January 2, 2020. After analyzing each event, the team came to concluded that 14% of them were actually supershards – a significant leap considering that supershards were previously thought to account for less than 6% of all earthquakes.
The results were published last month in the journal Nature Geoscience.
“They’re really applying these imaging methods extensively to study many, many large earthquakes, most of which have not been studied with these advanced imaging methods,” said seismologist Eric Dunham, a supershard expert at Stanford University who was not involved research.
In the absence of tools that could analyze faulting on continents and oceanic crust equally effectively, “we were just guessing” which events qualify as supershear, Dunham said. “This paper shows that they may not be as rare as we thought.”
Previously, seismologists suggested that these types of earthquakes occurred more frequently on continents than in submarine faults, since most confirmed supershears were recorded on land.
But using a technique called back-projection, which analyzes delays between seismic waves to determine how fast they’re moving, the team realized that supershards are just as common in the ocean as they are on dry ground — they’ve historically been much more difficult to monitor.
Their analysis found that in addition to five previously confirmed supershear earthquakes documented in their dataset, an additional seven also met the supershear criteria.
“I’m a bit surprised we found so many,” Meng said of the newly identified supershards, all of which occurred along underwater fault lines beyond the reach of most land-based monitors.
Supershards are more likely to occur along long, mature faults like the San Andreas, where years of activity have eroded away many of the twists and bumps that could slow an earthquake’s energy.
Just as it’s easier to pick up speed on a long, straight runway than a winding road, a rupture along a long, straight fault will accelerate faster than a knotted one, Meng said.
The strength of a supershard is determined by the speed at which it ruptures. When sound waves accumulate in front of a jet traveling faster than the speed of sound, they eventually merge into a single wave that a person on the ground hears as an explosion or sonic boom.
And just as a sonic boom is louder than the roar of a typical engine, a super shard shakes harder.
“The same amount of energy released by the bug is released in less time. So that always gives you a stronger shake,” Meng said.
Current building codes are already designed to account for the possibility of a supershear earthquake, said Elizabeth Cochran, a seismologist with the US Geological Survey in Pasadena. But the violence of a supershear is more likely to cause the secondary crises that cause so much devastation in a large earthquake, such as fires and landslides.
“That’s worrying,” she said. “When you have a super shear fracture, you can expect stronger shaking intensities, which can then translate into greater damage opportunities.”
Only about 2% of the 28,000 buildings destroyed in the 1906 earthquake collapsed from the tremors. The vast majority were destroyed by raging fires after the tremors ripped through gas and water mains. The magnitude of an earthquake is important, as is what happens after the shaking has stopped.