An earthquake's impact can be predicted - but only after it hits
Joseph Ax, Reuters
U.S. Geological
Survey technician look over cracks along State Route 178 after an earthquake
near Ridgecrest, California, U.S.
July 6, 2019. Gene Blevins,
Reuters
Over the next week, Southern California has only a
27 percent chance of experiencing a third earthquake greater than magnitude 6,
but a 96 percent chance of going through a tremor of magnitude 5 or higher.
Those precise probabilities were generated by
scientists at the United States Geological Survey (USGS), using models based on
longstanding principles of seismic behavior and decades of data on aftershocks
from earthquakes.
But the same predictive power does not extend to
forecasting when and where earthquakes will strike in the first place, experts
acknowledge.
"Even if it's a theoretical possibility, it
may be a practical impossibility," said Andrew Michael, a California-based
geophysicist at the USGS.
A powerful 7.1 magnitude quake shook the remote
town of Ridgecrest in the Mojave Desert on Friday evening, a day and a half
after a 6.4 magnitude temblor was recorded in the same region.
On average, a quake is followed by an even stronger
tremor only about 5 percent of the time, though it happens more frequently in
areas with significant geothermal activity, like the Mojave, according to
Michael.
The USGS first began offering public aftershock
forecasts in the 1980s, Michael said.
The models rely on basic laws governing earthquake
behavior that has been known for the better part of a century, experts said.
The typical shallow earthquake creates a series of
aftershocks that diminish exponentially, with each successive day bringing half
as many tremors. In addition, the frequency of earthquakes drops as the
magnitude increases – a region will have 10 times as many magnitude 6 quakes as
magnitude 7, Thomas Heaton, a seismologist at the California Institute of
Technology, said in an email.
"There is now a catalog that stores the
locations, sizes and times of millions of past earthquakes," he said.
"It is quite straightforward to characterize the statistical behavior of
these events."
The current model used by USGS predicts the number
and size of aftershocks based on the largest earthquake, or mainshock. But
seismologists believe aftershocks behave more like the spread of a disease in
an epidemic, Michael said. Just as each sick patient can infect others, so too
can each aftershock, in turn, create its own subsequent aftershocks.
Michael said the USGS is perhaps a year away from
adopting a newer model that incorporates that theory in making public
forecasts. Eventually, he said, the hope is that officials will be able to
predict how much shaking a particular town might experience in the aftermath of
a quake, so people can be better prepared.
But predicting a big quake in the first place
remains out of reach. The best seismologists have been able to do is to use
historical data to predict how likely it is that an earthquake of a certain
magnitude will hit a particular region over a period of time.
That forecast is crucial for establishing safety
standards in building codes, for instance. But it will not help warn a city's
residents that a quake is imminent.
"The problem is the earth is a complex
thing," said Christopher Scholz, a professor emeritus at Columbia
University's Lamont-Doherty Earth Observatory. Scholz noted, for instance, that
the 1999 Hector Mine quake in the Mojave Desert was thought to have been
triggered by a 1992 quake. That seven-year gap is but a fleeting moment in
geological terms.
Federal and state officials hope a new system that
alerts residents when shaking is detected might give people a few seconds to
seek shelter, say, or stand in a doorway.
"I work on the physics of the rupture process
and I have become convinced that the physics is in a class of phenomena that
are called 'chaotic,'" Heaton said. "If this is true, then we will probably
never predict earthquakes."
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