A Signal in Giant
Earthquakes That Could Save Lives
The full power of the biggest temblors could be determined
in as little as 10 to 15 seconds after they begin, a new study finds, and long
before it ends.
Rescue workers searched Balaroa village after a
7.5-magnitude earthquake struck Palu, Indonesia, on Oct. 6. 2018
By Robin George Andrews
May 29, 2019
Seismologists have never had a better understanding of
earthquakes. But tragedy after tragedy shows that quakes still surprise and
shock people with their mercurial behavior. Precise predictions of when and
where quakes will occur, and how deadly they may be, are not yet possible. If,
however, researchers could chronicle how quakes grow, they might be able to
better forecast how powerful they will become.
The mightiest quakes are far from instantaneous. They can
last minutes, which makes them less like a single subterranean blast and more
like a series of explosions moving outward. A new study, published on Wednesday
in Science Advances, explains that the outward journey of these explosions
differs depending on the power of the quake.
That means that the final magnitude of a quake could be
determined in as little as 10 to 15 seconds after it begins, and long before it
ends.
A single-digit leap in earthquake magnitude means that 32
times more energy is being released. Many factors determine the hazard level of
a quake, but small increases in magnitude can make the difference between
merely damaging and catastrophic. If final earthquake magnitudes could be
ascertained early on, it would give rapid, more precise warnings to populations
yet to be shaken.
Diego Melgar, an assistant professor of seismology at the
University of Oregon, explained that this connection was not what he and the
paper’s other co-author, Gavin P. Hayes of the United States Geological Survey,
were originally looking for. Instead, they had been gathering data from quake
databases to make the most accurate simulations of the most powerful quakes.
“And along the way we just stumbled upon something
interesting,” Dr. Melgar said: a key moment in time that frames an earthquake’s
future.
The team took a close look at 3,000 earthquakes recorded by
the agency’s seismometers. The data captured by these sensors can show the
energy release of an earthquake over time far from the source. The researchers
also dug through 30 quakes’ worth of GPS station data, where an antenna bolted
to the ground tracks the development of the rupture close to the earthquake.
Building on earlier work, the team described how large
earthquakes evolve. Immediately after they start, they grow chaotically for a
few seconds, a pandemonium that lasts longer for more prolonged quakes. The
rupture then organizes itself — for reasons that are for now unclear — into
something resembling a pulse, a ring-shaped area that moves outward from the
source of the quake over time.
This pulse ring denotes where the rock is breaking or
slipping. A thinner pulse is less likely to keep growing into a bigger event,
whereas a thicker pulse is more likely to do so. The team argues that because
of these differences, the dimensions can be used to determine the quake’s final
magnitude mid-rupture.
Since the 1980s, seismologists have been debating if such a
feat is possible. Some said that final magnitudes could be calculated right at
the quake’s birth, while others suspected that seismologists would have to wait
for the rupture to terminate. Others, like Dr. Melgar and Dr. Hayes, fall
somewhere in between.
Stephen Hicks, a seismologist at Imperial College London who
was not involved with the research, said the data suggested that the
correlation between rupture evolution and final magnitude was not a
coincidence. The way in which big quakes accelerate may be a recurrent feature
among the overarching chaos.
Men-Andrin Meier, a seismologist at the California Institute
of Technology, said that his own research also showed it was possible to
determine final magnitudes mid-quake. But he differs from Dr. Melgar and Dr.
Hayes on how soon into the rupture the final magnitude can be calculated. Their
new paper places it at around 10 seconds, but Dr. Meier says that this depends
on magnitude and can vary wildly.
One limitation of the new model is that it assumes an
average earthquake behavior. In reality, “any individual earthquake still has a
personality,” Dr. Melgar said. The evolution of certain earthquakes may not fit
with expected patterns, making mid-rupture calculations of final magnitudes
more difficult or, in some cases, easier.
Dr. Melgar also acknowledges that powerful quakes,
especially those above magnitude 8.5, are rarer than their weaker counterparts.
More data on big temblors, from simulations or real events, is required to
shore up this model.
“It’s a good speculative idea, we just need to fill it in
before we can have a lot of confidence in it,” said John Vidale, a professor of
seismology at the University of Southern California.
Recent reporting on earthquakes
The Deadliest Quake of 2018 Was Among the Fastest Ever Feb.
5, 2019
The Earth’s Shell Has Cracked, and We’re Drifting on the
Pieces Dec. 18, 2018
In a Land of Quakes, Engineering a Future for a Church Made
of Mud Nov. 3, 2018
For Tsunami Forecasters, Speed Is Everything Jan. 23, 2018
Keine Kommentare:
Kommentar veröffentlichen