Seismologists were surprised in 2004 when a magnitude-9.3 quake and tsunami devastated Sumatra and caused more than 200,000 deaths around the Pacific Rim. They were surprised again in 2011 by Japan's 9.0 quake and tsunami, which killed more 15,000 people and touched off a nuclear catastrophe that continues to this day.
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In each case, experts didn't think the area where one geological plate is diving beneath another — known as a subduction zone — was capable of generating a quake that strong.
Talking about probabilities
A research team led by Yufang Rong, a seismologist at FM Global's Center for Property Risk Solutions, addressed the bigger picture: They ran computerized Monte Carlo simulations using historical seismic data from subduction zones around the Pacific to estimate the maximum earthquake magnitude over a variety of time spans.
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In addition to coming up with the 10,000-year figure for the probability of 9.0 quakes, they estimated that quakes of at least magnitude 8.5 should be expected at least every 250 years, and 8.8 quakes should be expected every 500 years.
In a news release, Rong cautioned that those figures were merely estimates based on statistical distribution. "Just because a subduction zone hasn't produced a magnitude 8.8 in 499 years, that doesn't mean one will happen next year," she said. "We are talking about probabilities."
The estimates don't apply to land-based seismic faults such as the ones that gave rise to last month's 6.0 quake in Napa, California. Nor do they apply to California's more famous San Andreas Fault. But they do apply to the Cascadia subduction zone that lies off the coast of Washington state. That area is known to have given rise to a magnitude-9ish quake back in the year 1700 — a temblor that set off a tsunami so large it raised sea levels on Japan's coast.