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New findings shake up quake history

Earth Observations: California, USA
Photo: San Francisco after the Hayward Fault earthquake of 1868.
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Feb 06, 2007
Scientists have long known a big earthquake on the Hayward Fault — which runs through some of the most densely populated parts of the Bay Area, including Fremont and Union City — could kill hundreds, destroy tens of thousands of homes and close more than a thousand roads.
But recent research suggests that may be an underestimate.
After looking at records of a huge quake that occurred on the fault more than 135 years ago, a Bay Area scientist has concluded that it was bigger than the one the region has been preparing for.
While it may not sound like a huge difference — a magnitude 7.0 quake vs. a magnitude 6.7 — it is. With the way quakes are measured, each increase by a full number, 6.0 to 7.0, for instance, the power of the quake is increased by a factor of 10, which translates into much more damage.
Assessing the 1868 quake is important because a repeat of that quake is considered the most likely and devastating scenario for the Big One in the Bay Area. It was known as the "great San Francisco earthquake" until the 1906 temblor came along, and it's the only major quake on the fault in historical times. Scientists estimate there is a 27 percent chance a quake of magnitude 6.7 or greater will occur on the Hayward Fault by 2032.

Graphic : 1868 Hayward earthquake intensity map (PDF)

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