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Why quake forecast maps often fail

U. MISSOURI  / NORTHWESTERN (US) — Three of the largest and deadliest earthquakes in recent history occurred where earthquake hazard maps didn’t predict massive quakes, scientists say.

Researchers studied why the maps failed to forecast the quakes and how to improve them. Developing better hazard maps and alerting people to their limitations could potentially save lives and money, they say.

“Forecasting earthquakes involves many uncertainties, so we should inform the public of these uncertainties,” says Mian Liu, of the department of geological sciences at the University of Missouri.

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“The public is accustomed to the uncertainties of weather forecasting, but foreseeing where and when earthquakes may strike is far more difficult. Too much reliance on earthquake hazard maps can have serious consequences.

“Two suggestions may improve this situation. First, we recommend a better communication of the uncertainties, which would allow citizens to make more informed decisions about how to best use their resources. Second, seismic hazard maps must be empirically tested to find out how reliable they are and thus improve them.”

Maps should be tested against what is called a null hypothesis, the possibility that the likelihood of an earthquake in a given area—like Japan. Testing would also show which mapping approaches were better at forecasting earthquakes and subsequently improve the maps.

In a study published in the journal Tectonophysics, Liu and colleagues at Northwestern University and the University of Tokyo detail how hazard maps have failed in three major quakes that struck within a decade of each other.

  • Wenchuan, China: In 2008, a quake struck China’s Sichuan Province and killed more than 69,000 people. Locals blamed the government and contractors for not making buildings in the area earthquake-proof. Hazard maps bear some of the blame as well since the maps, based on bad assumptions, had designated the zone as an area of relatively low earthquake hazard.
  • Léogâne, Haiti: The 2010 earthquake that devastated Port-au-Prince and killed an estimated 316,000 people occurred along a fault that had not caused a major quake in hundreds of years. Using only the short history of earthquakes since seismometers were invented approximately one hundred years ago yielded hazard maps that didn’t indicate the danger there.
  • Tōhoku, Japan: Scientists previously thought the faults off the northeast coast of Japan weren’t capable of causing massive quakes and thus giant tsunamis like the one that destroyed the Fukushima nuclear reactor. This bad understanding of particular faults’ capabilities led to a lack of adequate preparation. The area had been prepared for smaller quakes and the resulting tsunamis, but the Tōhoku quake overwhelmed the defenses.

“If we limit our attention to the earthquake records in the past, we will be unprepared for the future,” Liu says. “Hazard maps tend to underestimate the likelihood of quakes in areas where they haven’t occurred previously.

“In most places, including the central and eastern US, seismologists don’t have a long enough record of earthquake history to make predictions based on historical patterns. Although bad luck can mean that quakes occur in places with a genuinely low probability, what we see are too many ‘black swans,’ or too many exceptions to the presumed patterns.”

“We’re playing a complicated game against nature,” says the study’s first author, Seth Stein of Northwestern University. “It’s a very high stakes game. We don’t really understand all the rules very well.

“As a result, our ability to assess earthquake hazards often isn’t very good, and the policies that we make to mitigate earthquake hazards sometimes aren’t well thought out. For example, the billions of dollars the Japanese spent on tsunami defenses were largely wasted.

“We need to very carefully try to formulate the best strategies we can, given the limits of our knowledge,” Stein says. “Understanding the uncertainties in earthquake hazard maps, testing them, and improving them is important if we want to do better than we’ve done so far.”

Source: University of Missouri

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