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 and cross-cutting the region's nothing patchwork or crop fields. Minutes after the shaking began, locals were caught unaware at a wall of water that crashed onshore with devastating results.
As the sun set that evening, thousands were missing. Within days, the click of corpses permeated the air. The 7.5-magnitude event was 201
In the efforts to understand how this fatal series of events clicks into place, much attention has been 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 to be unusual and incredibly fixed by 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 more than 16 minutes. Such a fast rupture causes earthquake waves to pile up as a Mach front, similar to the pressure wave from a plane traveling at supersonic speed. It is 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 the supershear speed, 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.
handful of supershear earthquakes, and even fewer with this level of detail, ”says seismologist Jean-Paul Ampuero of the University of the French Cote d'Azur, a coauthor of one of the studies.
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 people in a better way.”
Unzipping the Earth
During an earthquake, the entire length of a fracture doesn't break all at once. Rather, the planet is at a rate known as the rupture speed
Stephen Hicks, a seismologist at the University of Southampton, explains the phenomenon at a colorful flier sitting on a table at the American Geophysical Union Fall Meeting in Washington, DC He makes a tiny tear on one side, and says: "It'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 floor in two.
That is the ground breaking that fast, which is pretty amazing. 19659016] Stephen Hicks,
University of Southampton
It's this speed that caught geologists' attention with the Indonesia event. To get a closer look, Ampuero and his colleagues harnessed the power of the growing global network of seismic stations, which detects 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 like 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. "Every ear is one seismometer on the ground."
This revealed that the temblor broke so much that the rupture speed overtook a type of radiating waves known as shear waves, thus the term "supershear." Quake cracked southward through some 93 miles of Earth's surface.
"That is the ground breaking, which is pretty amazing," says Hicks, who wasn't involved in the research.
A second team took a closer look at changes to the surface after the temblor ripped through, using data and imaging from satellites before and after the event
“We were immediately taken by the sharpness of the surface at the surface south of the city or Palu and by the great amount of displacement in this area, ”coauthor Anne Socquet, or the University of Grenoble Alpes in France, writes in an email.
This analysis suggests that the country largely shifted horizontally, and that the chang e was massive: The ground offset at 16.4 feet at its maximum point south of Palu City. The shift was so large, it was easily seen in images of the region post-quake. Roads were offset;
"This is definitely huge for a [earthquake]," Socquet says. “And this is probably enhanced by the fact that this earthquake was supershear.” It didn't happen just at the surface, either, but also as deep as roughly three miles underground.
In the southern stretches of the fault, important feature behind this rapid speed and the deep shift is what Socquet calls its "maturity." Tectonics have tested this break time and time again, continually shoving the blocks of Earth side by side and carving the fault into a fairly continuous, smooth, straight break — features previously associated with other examples of super-fast ruptures.
Anatomy of supershear
Yet within this category of rare events, the Palu quake may stand apart. Most supershear earthquakes actually travel even faster than the one in Palu, cruising along as fast as another type of earthquake wave known as a pressure wave. These commonly zoom at around 11,200 miles an hour. But Ampuero and his colleagues found that while the Indonesia quake was fast enough to be supershear, it didn't hit this top speed.
"It's extremely rare to see events in this intermediate range," he says. and his colleagues believe the discrepancy is due to the fact that earthquake models, including the one used in this work, commonly assume that the rocks surrounding a fault are one intact unit. But that's not the case in the real world, where zones of fractures around the break can slow down the quakes associated with the surface.
If true for Sulawesi, this would mean the quake's pressure waves could have moved about as fast as its rupture speed, as is expected for supershear ruptures. The quake was still slow for supershear, but at least its waves would have moved at the right relative speeds. However, the scientists do not know that this was the case without more study in the region.
That's not the only thing unusual about the event. September's earthquake also seemed largely undeterred by two major bends in the fault. Zigs and saws along the rupturing fault usually slow earthquakes, like cars on a winding road, but not this one. And unlike most supershear breaks, which need a little warmup, the Palu seemed to hit its galloping pace early on.
"This earthquake is like a Lamborghini," Bohon says. "It goes from zero to 60 in no time."
This behavior raises even more questions. Could the fault be straighter at depth? This would have helped make the barrel through bends higher up, Ampuero notes. Did smaller foreshocks supercharge the big quake? This could have sent it out of the gates. But this early speed could also have the roughness of the fault, which could stick the sides together as the rough sides of the paper and cause the ground to break with extra oomph.
More to come?
These unusual features make this earthquake all the more valuable, since they can help researchers better understand both where and how super-fast quakes can happen. The scientists who reviewed the work all stressed the significance of this information for future modeling and hazard assessments not just in Indonesia, but around the globe.
"What happened most likely on other faults, especially major plate boundary faults, "Eric Dunham, a geophysicist at Stanford University."
"This type of fault is the same one we can find in California, Northern Turkey, Northern Aegean, the Dead Sea fault zone, Central Asia," says earthquake geologist Sotiris Valkaniotis , who was not involved in the new studies. "The detections from this earthquake apply worldwide."