The Deadliest Quake of 2018 Was Among the Fastest Ever
Last September, an earthquake triggered a deadly tsunami in Indonesia. Scientists now have clocked the speed of rupture at a blistering 9,600 miles per hour.
A bridge destroyed by the earthquake and tsunami that struck Palu, in Indonesia, on Sept. 28. The seismic rupture unzipped 80 miles of crust in 30 seconds. Credit Beawiharta Beawiharta/Reuters
By Marcia Bjornerud
Feb. 5, 2019
On Sept. 28, a powerful earthquake struck the Indonesian island of Sulawesi, triggering a tsunami that devastated the provincial capital, Palu. The two events together killed more than 2,200 people in the region.
Although Indonesia is one of the most seismically active countries in the world, the Palu tsunami came as a surprise to geophysicists. A tsunami occurs when a quake on the seabed abruptly pushes water upward, producing a dangerously tall wave. Usually the culprit is a megathrust earthquake, as one tectonic plate slides, or subducts, beneath another. The tsunami that hit Sumatra in 2004, causing 230,000 deaths, was generated by a megathrust quake.
In contrast, the tsunami in September was caused by what is known as a strike-slip quake. These occur at seismic faults where two tectonic plates are sliding past each other. The ground motion in such quakes is mostly horizontal — in Sulawesi, the rocks on either side of the fault lurched past each other by more than 10 feet — and rarely produce tsunamis. But the Palu earthquake caused an underwater landslide, which produced a small tsunami that grew as it swept up a narrowing bay.
And, it turns out, something even more unusual was at work, according to two papers published in the journal Nature Geoscience on Monday. As the fault ruptured, the leading edge of the rupture tore through the crust much faster than usual, perhaps magnifying the shaking that led to the underwater landslide. Such behavior has been predicted in theory, but had not been conclusively documented in nature.
Historically, seismological theories have been hard to test against actual observations. However, thanks to a growing stream of detailed data from seismometer arrays and high-resolution satellite imagery, scientists are increasingly able to compare their models to the behavior of individual earthquakes in real time.
A view of damage to the Tondo district in Palu, a few days after the tsunami. Credit Antara
Foto/Reuters
Earthquakes are still earthquakes: vibrations caused when elastic energy stored in rocks is suddenly released. But their idiosyncrasies — the personalities of their seismic faults — are starting to show. The two recent studies offered a case in point.
An earthquake starts underground, on a tiny stretch of a fault line, when the forward pressure of one tectonic plate exceeds the force of friction holding it back. Suddenly, a slip occurs; a rip appears in the crust and quickly propagates away from its origin, much like a run in a pair of hosiery.
As the rip hurtles along, it releases elastic energy in the form of seismic waves, which ripple outward at different speeds. Pressure waves, or P-waves, are the fastest; shear waves, or S-waves, are slower but cause more ground-shaking; and last are the leisurely but devastating Rayleigh waves, which cause the ground to roll like swells on the sea.
Typically, and according to basic geophysical theory, a rupture travels no faster than the slowest seismic waves. But the new research indicated that the rupture from the Palu earthquake outran even its own S-waves, making it one of the first documented “super-shear” earthquakes.
In one paper, Anne Soquet of the Université Grenoble Alpes, in France, and three co-authors examined optical and radar imagery from Japan’s Advanced Land Observing Satellite 2, which showed millimeter-scale displacements of the Earth’s surface before and after the Sulawesi quake. The data revealed that the rupture began north of Palu, on a previously unknown fault segment, and in 30 seconds traveled at least 80 miles southward.
On average, the rupture unzipped the crust at a rate of 2.7 miles per second, or 9,600 miles per hour, almost 25 percent faster than is typical, and among the fastest ever recorded for rocks at shallow depths. This blistering pace was made possible by an unusually flat, smooth stretch of the fault south of Palu, Dr. Soquet’s team wrote. Their conclusion was consistent with theoretical models suggesting that only geometrically simple faults could transmit such ruptures.
In the second study, a team led by Han Bao of the University of California, Los Angeles, assembled a second-by-second timeline of the rupture from surface radar imagery and the dense network of seismic stations around the Indian Ocean. This team, too, observed that the rupture outran its S-waves. Much as a motorboat or supersonic jet outruns its wake, the rupture generated an expanding, V-shaped pattern of disruption, known as a Mach cone, behind it.
The team was able to calculate precisely when the different kinds of seismic waves arrived at different monitoring stations. It found that the rupture from the Palu earthquake unfolded in distinct phases, slowing at about 10 seconds and 25 seconds after the initial slip, perhaps because of bends in the fault or variations in rock friction.
Palu residents wait for word of missing family and friends. Credit Aaron Favila/Associated Press
Even with those impediments, the rupture traveled at super-shear velocity, and it did so right from the start. That was surprising: Current models suggest that a rupture must travel some minimum distance before hitting super-shear speeds, just as a sprinter needs a few yards to reach top speed.
Maybe, Dr. Bao and his co-authors wrote, the initial stretch of the fault zone was made up of heavily fractured and damaged rock. The rupture could have cruised right through it, without expending energy breaking up pristine rock.
Together, the two new papers offer the strongest evidence yet that seismic ruptures can reach super-shear speeds. In 2013, an aftershock of an earthquake that occurred 380 miles below the Sea of Okhotsk was inferred to have done so, but the speed and length of a rupture are hard to measure precisely at such depth.
The findings give a geophysicist much to ponder. Was the super-shear behavior intrinsic to this fault, or was it prompted by something specific in how the quake began? Are certain kinds of rock, or older and more damaged faults, more likely to produce super-shear quakes?Ultimately, how unique was this event?
The implications are humanitarian as well as scientific. Strike-slip faults can be found around the world, including in many densely inhabited areas: the San Andreas fault in California; the Anatolian fault system in Turkey; the Dead Sea fault in the Middle East; and the Enriquillo fault in Haiti. Humankind’s seismic neighbors will be around for a long time, we’d do well to get to know them.
Correction: Feb. 5, 2019
A headline on an earlier version of this article overstated the speed of the Palu earthquake in 2018. It was among the fastest ever recorded, not the fastest.
NYT, Science
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Deadly earthquake traveled at 'supersonic' speeds—why that matters
A powerful temblor in Indonesia offered a detailed look at
supershear, a phenomenon that can create the geologic version of a sonic boom.
By Maya Wei-Haas
PUBLISHED February 4, 2019
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.
Earthquakes are unpredictable and can strike with enough
force to bring buildings down. Find out what causes earthquakes, why they're so deadly, and
what's being done to help buildings sustain their hits.
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.
It'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.
Earthquake
superhighway
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 struck by the sharpness of the rupture
at the surface south of the city of Palu and by the great amount of
displacement in this area,” study coauthor Anne Socquet, of the Université
Grenoble Alpes in France, writes in an email.
This analysis suggests that the land largely shifted
horizontally, and that the change was massive: The ground offset by 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; buildings seemingly
cut in two.
“This is definitely huge for a [magnitude] 7.5 earthquake,”
Socquet says. “And this is likely 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, an 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 even 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 almost as fast as another type of earthquake
wave known as a pressure wave. These commonly zoom by 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.
Ampuero 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
always the case in the real world, where zones of fractures around the break
can slow the speeds of a quake's associated waves through 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 weirdly slow for supershear, but at
least its waves and rupture would have moved at the right relative speeds.
However, the scientists won't know for sure 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 zags 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 temblor 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 it barrel through bends higher up,
Ampuero notes. Did smaller foreshocks supercharge the big quake? This could
have sent it galloping out of the gates. But this early speed could also have
to do with the roughness of the fault, which could stick the sides together
like the rough sides of sandpaper 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 here could likely happen on other faults,
especially major plate-boundary faults,” says 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.”
Maya Wei-Haas is a science staff writer for National
Geographic.
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