Doubt cast on the predictive value of earthquake foreshocks

Nepal Earthquake [image credit: BBC]

Back to the drawing board for earthquake forecasting, by the sound of it.

A new study questions previous findings about the value of foreshocks as warning signs that a big earthquake is coming, instead showing them to be indistinguishable from ordinary earthquakes, reports Science Daily.

No one can predict when or where an earthquake will strike, but in 2011 scientists thought they had evidence that tiny underground tremors called foreshocks could provide important clues. If true, it suggested seismologists could one day warn people of impending temblors.

But a new study published in the online June 4 issue of Nature Geoscience by scientists at Stanford University and Bogaziçi University in Turkey has cast doubt on those earlier findings and on the predictive value of foreshocks.

The previous evidence came from a 7.6 magnitude earthquake in 1999 near Izmit, Turkey, that killed more than 17,000 people. A 2011 study in the journal Science found that the deadly quake was preceded by a series of small foreshocks — potential warning signs that a big seismic event was imminent.

“We’ve gone back to the Izmit earthquake and applied new techniques looking at seismic data that weren’t available in 2011,” said lead author William Ellsworth, a professor (research) of geophysics at Stanford School of Earth, Energy & Environmental Sciences. “We found that the foreshocks were just like other small earthquakes. There was nothing diagnostic in their occurrence that would suggest that a major earthquake was about to happen.”

“We’d all like to find a scientifically valid way to warn the public before an earthquake begins,” said co-author Fatih Bulut, an assistant professor of geodesy at Bogaziçi University’s Kandilli Observatory and Earthquake Research Institute. “Unfortunately, our study doesn’t lead to new optimism about the science of earthquake prediction.”

How do earthquakes begin?

Scientists including Ellsworth have proposed two ideas of how major earthquakes form, one of which — if scientists can detect them — could warn of a larger quake.

“About half of all major earthquakes are preceded by smaller foreshocks,” Ellsworth said. “But foreshocks only have predictive value if they can be distinguished from ordinary earthquakes.”

One idea, known as the cascade model, suggests that foreshocks are ordinary earthquakes that travel along a fault, one quake triggering another one nearby. A series of smaller cascading quakes could randomly trigger a major earthquake, but could just as easily peter out. In this model, a series of small earthquakes wouldn’t necessarily predict a major quake.

“It’s a bit like dominos,” Bulut said. “If you put dominos on a table at random and knock one over, it might trigger a second or third one to fall down, but the chain may stop. Sometimes you hit that magic one that causes the whole row to fall.”

Another theory suggests that foreshocks are not ordinary seismic events but distinct signals of a pending earthquake driven by slow slip of the fault. In this model, foreshocks repeatedly rupture the same part of the fault, causing it to slowly slip and eventually trigger a large earthquake.

In the slow-slip model, repeating foreshocks emanating from the same location could be early warnings that a big quake is coming. The question had been whether scientists could detect a slow slip when it is happening and distinguish it from any other series of small earthquakes.

Continued here.

via Tallbloke’s Talkshop

June 5, 2018 at 04:54PM

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