The quake on the Hayward Fault was initially reported a higher magnitude but later downgraded.
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"The last big earthquake on the Hayward fault happened about 150 years ago, in 1868. In fact the 150th anniversary is coming up -- we think that earthquake was a magnitude 6.8 or so. Through geologic excavations across the Hayward fault we've learned that there are earthquakes on at least the southern part of the Hayward fault every 140, 150, 160 years," said Keith Knudsen, U.S. Geological Survey.
A USGS official said the possibility of this morning's earthquake being a foreshock to a larger quake is only about 5 percent. There has been at least one aftershock and officials expect there to be more in the upcoming days.
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BART officials say there are no initial reports of damage to the system, but out of an "abundance of caution" the very first trains were run at a very reduced speed for inspections. Normal train service has since resumed.
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