There have been no reports of major damage following a 4.0 earthquake that struck near Alamo in unincorporated Contra Costa County at 9 p.m. Friday.
The quake was centered 2 miles east northeast from Alamo and 4 miles north northwest of Danville, according to the U.S. Geological Survey. It had a recorded depth of about 10 miles, the USGS said.
USGS seismologist David Oppenheimer said the quake was along a fault line that does not have a name because it does not rupture the surface.
Oppenheimer said the last time the fault was active was in April 1990 when a sequence of quakes shook the area for three to four weeks.
Oppenheimer said that sequence included 18 quakes of at least 3.0 and caused some structural damage along the fault line.
But he said it's too early to know if this quake will have a similar result. Oppenheimer said the 10-mile depth of the tremor means there was more rock between the quake and the surface, resulting in shaking that was not felt as strongly.
By 10 p.m., there had already been three aftershocks, none of which registered as more than a 1.9.
The USGS reported that the initial tremor was felt as far away as Modesto, Watsonville and Napa.
The quake caused Bay Area Rapid Transit trains to stop systemwide for nine minutes, said spokesman Linton Johnson, who added that stopping trains after an earthquake for about five minutes is protocol.
After stopping the trains, BART officials let the trains run again but at a slower speed so that any damage could be reported back. Johnson said there had been no reports of structural damage to the BART lines.
By 10 p.m., BART was still experiencing delays up to five to 10 minutes.
Officials with the Oakland International Airport, the closest airport to the quake's epicenter, said they did not find any damage when they checked the runways and there were no delays reported.