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SF Bay Area is likely to suffer a "cluster" of large earthquakes instead of just one

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  • SF Bay Area is likely to suffer a "cluster" of large earthquakes instead of just one

    Several Earthquakes are Bigger Threat to San Francisco Bay Area - WSJ.com



    Several Earthquakes Are Bigger Threat to San Francisco Bay Area
    Multiple Major Temblors, Not a Single Huge One, Pose a Bigger Risk, Study Says
    By
    Tamara Audi
    Updated May 19, 2014 7:00 p.m. ET

    For decades, Northern California has been bracing for the "Big One"—an earthquake of magnitude 7.0 or larger expected on one of the many fault zones that snake through the region. But new research suggests the area is more likely to be rattled by a cluster of large, damaging earthquakes than a single major temblor.

    The study, to be released this week in the Bulletin of the Seismological Society of America, sheds new and somewhat disturbing light on the potential hazards faced by the San Francisco Bay Area.

    "Everyone is still thinking about a repeat of the 1906 quake," said lead author David P. Schwartz, referring to the Great San Francisco Earthquake, a magnitude 7.9 quake that sparked a massive fire and killed 3,000. "But what happens if every five years we get a magnitude 6.8 or 7.2? That's not outside the realm of possibility."

    Scientists say the new information should shift the region's focus to preparing for the possibility of a potentially devastating quake on one fault followed by one on another fault within the same decade. A number of major faults in the Bay Area haven't seen large quakes in decades, so they haven't had a chance to catch up to the movements of the Earth's tectonic plates, the scientists behind the study said.

    "So, as the region is recovering from one event, another event happens," said Mr. Schwartz, a geologist with the U.S. Geological Survey. "In a sense, it's a much more difficult hazard to deal with."

    The study reveals a previously unknown tally of very large quakes occurring close together in the Bay Area going back to the year 1600. From around 1690 to 1776, six large quakes ranging from magnitude 6.6 to 7.8 on five different faults rumbled through the region, according to evidence of past fault movement unearthed in excavations. After that period, there was a marked decrease in the number of large earthquakes on major faults in the area. That cluster of quakes released an amount of energy comparable to the 1906 quake, the study says.

    "It is an important study that has important data that needs to be incorporated into our thinking about earthquake forecasting," said Thomas Jordan, director of the Southern California Earthquake Center. The professor of geophysics at the University of Southern California said the data show that "earthquakes are not occurring randomly in time, but in fact are occurring in clustered sequences."

    The research comes as San Francisco undertakes an ambitious plan to withstand the next large quake. Over the past 15 years, the city has spent more than $10 billion to upgrade public buildings and infrastructure to prepare for such a shock, according to city officials. Taxpayers will be asked to approve a $400 million bond measure on June 3 to continue those efforts.

    The scene after the 1989 Loma Prieta quake in San Francisco Agence France-Presse/Getty Images

    Earthquake-safety officials said the new information isn't likely to alter those plans, because they already take into account the large aftershocks that typically follow major quakes, and said they have understood that because of multiple faults, multiple quakes are a possibility. "From our perspective, it doesn't really matter if it's 'The Big One' or 'The Big Ones,' " said Patrick Otellini, director of earthquake safety for San Francisco. "Our approach is mitigating against both scenarios."

    San Francisco sits on the boundary of the North American and Pacific tectonic plates, portions of the Earth's crust that slowly grind against each other, creating faults and building up energy from that movement. That energy is released in the form of an earthquake when too much stress builds up on a fault.

    The U.S. Geological Survey says there is a 63% chance of one or more magnitude 6.7 or larger quakes striking the Bay Area in the next 30 years.

    According to a 2010 hazard report commissioned by San Francisco city officials, a 6.9 quake on the Hayward Fault east of the bay could cause as many as 120 deaths and 2,300 injuries in the region. A 7.2 quake along a northern section of the San Andreas Fault, an 800-mile-long fracture stretching from the Southern California desert through just south of San Francisco, could kill 300, injure 7,000 and cause $30 billion in property damage.

    It was along the San Andreas that the 1906 quake, the most powerful in Northern California's recorded history, broke. That quake had the effect of releasing enough pent-up energy to "relax" other faults around it, according to Mr. Schwartz, the study's author. Since then, the region has been "seismically quiet," meaning that stress has been rebuilding on the fault system over time.

    San Francisco's Loma Prieta quake in 1989 killed 63 people, injured 3,757 and caused $6 billion in property damage. But that magnitude 6.9 event released a "tiny fraction" of energy on the fault system, Mr. Schwartz said.

    Since 1906, the pressure on the fault system has been rebuilding, seismologists said, and must find its release—as a cluster of large quakes, as the new study suggests, or possibly like the massive 1906 rupture.

    Mr. Schwartz believes the break is more likely to be similar to the cluster now known from the 18th century. "All these other faults have to catch up" with the movement of the plates, Mr. Schwartz said. "In a way, 1906 delayed them and they have some catching up to do. How else will they do it?"

    Write to Tamara Audi at [email protected]
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