In the past two days, malfunctions in the network of sensors that detect earthquakes in Northern California have issued three false alarms, forcing the U.S. Geological Survey to make a series of embarrassing retractions.
Just after midnight Friday morning, a magnitude 6.7 quake struck off the coast of Alaska. When its waves reached sensors operated by the Northern California Seismic Network, they were mistakenly interpreted as a 5.1 temblor near the Oregon border in Lewiston, officials say.
"There was a ghost in the machine," Don Drysdale, spokesman for the California Geological Survey, said Friday. "There was no earthquake."
The error occurred again Saturday morning after a magnitude 7.8 quake struck off the coast of Japan and the same Northern California sensors misinterpreted its wake as a 4.8 magnitude shaker near San Simeon and a 5.5 magnitude rumble near Brooktrails.
"When the waves from these big quakes hit [the Northern California] network they think it's a local quake," said John Bellini, a USGS geophysicist. "They have some kind of filtering system, but it's not working properly."
USGS officials said they are working to repair the problem and prevent future false alarms.