Deadly earthquake traveled at 'supersonic' speeds—why that matters
When the earthquake struck on September 28, 2018, Indonesia's Sulawesi island flowed like water. Currents of mud swallowed anything in their paths, sweeping away entire sections of the city of Palu and crosscutting the region's neat patchwork of crop fields. Minutes after the shaking began, locals were caught unaware by a wall of water that crashed onshore with devastating results.
As the sun set that evening, thousands were missing. Within days, the smell of corpses permeated the air. The 7.5-magnitude event was 2018's deadliest quake, killing more than 2,000 people.
In the efforts to understand how this fatal series of events clicked into place, much attention has focused on the surprise tsunami. But a pair of new studies, published February 4 in Nature Geoscience, tackles another remarkable aspect: The earthquake itself was likely an unusual and incredibly fast breed of temblor known as supershear.
The Palu quake cracked through the earth at nearly 9,200 miles an hour—fast enough to get from LA to New York City in a mere 16 minutes. Such a fast rupture causes earthquake waves to pile up in what's known as a Mach front, similar to the pressure wave from a plane traveling at supersonic speed. This concentrated cone of waves can amplify the quake's destructive power.
“It's like a sonic boom in an earthquake,” says Wendy Bohon, an earthquake geologist at the Incorporated Research Institutions for Seismology (IRIS).
While it's not yet possible to say for sure if the supershear speed intensified the Indonesia quake's landslides, liquefaction, or tsunami, the pair of new studies does offer a rare look at this little-understood and potentially deadly phenomenon.
“We have observed only a handful of supershear earthquakes, and even fewer with this level of detail,” says seismologist Jean-Paul Ampuero of the Université Côte d'Azur in France, a coauthor of one of the studies.
“This is going to tell us something fundamental about the way the Earth works,” says Bohon, who was not involved in either study. “And it has the potential to actually save lives and help us inform people in a better way.”
Unzipping the Earth
During an earthquake, the entire length of a fracture doesn't break all at once. Rather, it unzips the planet's surface at a rate known as the rupture speed.
Stephen Hicks, a seismologist at the University of Southampton, explains the phenomenon by grabbing a colorful flier sitting on a table at the American Geophysical Union Fall Meeting in Washington, D.C. He makes a tiny tear on one side, and says: “Imagine that's your nucleation,” or the start of a rupture on a fault. The rupture speed is how fast that point moves through time, he says, and with a sharp jerk, he rips the flier in two.
t's this speed that caught geologists' attention with the Indonesia event. To take a closer look, Ampuero and his colleagues harnessed the power of the growing global network of seismic stations, which detect the echoes of earthquakes from hundreds of miles away. From that network, they collected data from 51 locations across Australia.
By studying the arrival of earthquake waves at each station, the team recreated the racing rupture. It's similar to how your brain figures out where a sound is coming from, Ampuero explains. If someone is talking to you from the right, the noise arrives at your right ear a fraction of a second before your left. Your brain then uses that delay to locate the speaker.
“What we're doing is the same, [but] instead of using only two ears we're using hundreds of ears,” he says. “Each ear is one seismometer on the ground.”
This revealed that the temblor broke so fast that the rupture speed overtook a type of radiating waves known as shear waves, thus the term “supershear.” Over roughly 36 seconds, the quake cracked southward through some 93 miles of Earth's surface.
“That is the ground breaking that fast, which is pretty amazing,” marvels Hicks, who wasn't involved in the research.