Doubt cast on the predictive value of earthquake foreshocks

Posted: June 5, 2018 by oldbrew in Earthquakes, predictions, research

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.

Comments
  1. oldbrew says:

    Scientists Find Dozens of Hidden Earthquakes Buried Under Antarctica’s Ice
    By Rafi Letzter, Staff Writer | June 5, 2018

    Hundreds of hidden earthquakes may be buried beneath Antarctica’s ice, new research suggests.

    To prove that, nearly a decade ago, a team of scientists spent months flying around East Antarctica. Their results have finally been published, and they contradict an older established geologic hypothesis.

    Researchers used to believe that the weight of the massive ice sheets on the continent pin the crust beneath them in place, preventing it from moving. Even now, the U.S. Geological Survey’s (USGS) website reports that the continent has uncommonly few quakes compared to other continents, though the USGS acknowledges that the handful of seismic sensors in the region might not be enough to pick up all the quakes. But the new results, published yesterday (June 4) in the journal Nature Geoscience, suggest that East Antarctica has just as many earthquakes as other, physically similar parts of the planet.

    http://www.livescience.com/62746-east-antarctica-many-earthquakes.html

  2. craigm350 says:

    Reblogged this on WeatherAction News and commented:
    One theory of prediction falters but there are others;
    http://quakewatch.net/statistics/

  3. B says:

    Studying for years and not being able to determine a single algorithm to predict quakes is hard. There are those folks who approach the problem differently, whether by planetary mechanics, by solar wind, or by the tilt of the Earth and try to define all parts of the process. A few folks are doing a good job determining the bigger quakes.

    I continue in defining the process of triggering and realize of course without pressure of some kind there will not be an earthquake. Now, with what is happening in Hawaii and now Guatemala and the earthquake swarms that do not want to cease there is much to be learned. Since the Hawaii eruption started the number of earthquakes around the world has dropped considerably. Those quakes whose triggers appear to be related to the solar wind or changes in the magnetosphere have dropped to almost zero. Answering questions related to “how can one volcano relieve crustal pressure almost around the world” is just the beginning of work that needs to be discussed!

    Hopefully, folks who can study geology full time can focus on new developments and find ways to relate the data to their past work.

  4. oldbrew says:

    Something new here? Possibly.
    – – –
    Scientists find pre-earthquake activity in central Alaska
    June 5, 2018, University of Alaska Fairbanks

    At Minto Flats, a magnitude 3.7 quake occurred at a depth of about 10.5 miles, not an unusual event in itself. However, the event was preceded by a 12-hour accelerating sequence of earthquakes and 22 seconds of distinct high- and low-frequency waves in a concentrated area.

    Tape said that this kind of slow event transitioning into a rupture had previously only been seen in laboratory experiments.

    “The rupture process started, then it found a patch of the fault that was ready to go, and that’s what people have not seen. It’s really exciting,” Tape said.

    “The leap we make, and maybe the more controversial thing, is that this emergent long-period signal only seen on top of the fault is a low-frequency signal that can sometimes turn into an earthquake and sometimes not,” Tape said.

    Read more at: http://phys.org/news/2018-06-scientists-pre-earthquake-central-alaska.html

  5. oldbrew says:

    B says “how can one volcano relieve crustal pressure almost around the world”
    – – –
    Maybe a clue is that earthquakes can be detected by sensors (seismometers) on the other side of the world.

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