A small earthquake shook northern California, delivering a "quick, short shake," according to one witness in Oakland.
Reuters said the US Geological Survey registered the quake at magnitude 4.0 on the Richter scale, centred one mile north of Piedmont.
According to the USGS, the quake was felt most strongly on the East Bay, including Oakland, Berkeley and surrounding areas. Lesser shaking was felt in San Francisco.
While there were no immediate reports of damage or injury, one weatherman was certainly put off his stride by the tremor.
But for most people, there was little to report.
— Mashable (@mashable) August 17, 2015
This map shows exactly where the earthquake struck.
California has 23.4 per cent of all earthquakes in the US and has reported more than 6,500 in the past year.
The largest one in the last 12 months was a 6.0 magnitude quake in American Canyon in August when a tsunami bulletin was issued.
Bay Area Rapid Transit said it held trains to check the tracks on the public transit system.
"Although many calls continue, no reports of damage or injury in #Oakland," Oakland police Lt Chris Bolton said on Twitter.
Residents said the quake lasted only a few seconds and sounded like a tree limb cracking, the Los Angeles Times reported.
The quake was centered near the junction of the 24 and 13 freeways. Seven aftershocks followed.
It appears to have occurred along the Hayward fault, which experts have long said could produce a devastating quake. The fault runs below heavily populated areas of the East Bay. The fault has produced some small quakes recently, including a 4.0 in Fremont in July.
Studies have estimated that a massive quake on the fault could cause $32bn in damage and leave 20,000 dead and injured.Reuse content