4.2-magnitude earthquake rattles Northern California

A 4.2 magnitude earthquake was felt across California's Sacramento County and parts of the San Francisco Bay Area on Wednesday morning, according to the U.S. Geological Survey.

The quake hit southern Sacramento at 9:29 a.m., exposing the towns of Rio Vista and Isleton to "moderate" shaking. It was initially estimated to be magnitude 5.7, but it was downgraded.

There was light shaking across five nearby towns, including Oakley and Discovery Bay, and weak shaking in Sacramento, San Francisco and San Jose, the USGS said.

There were no immediate reports of damage or injury.

An emergency alert instructing people to “Drop, Cover, Hold On” was sent to mobile phones across the area, NBC affiliate KCRA of Sacramento reported.

The San Francisco Bay Area train service, BART, delayed trains for five to eight minutes to inspect tracks.

The earthquake struck at a shallow depth of 6.7 miles from the surface, according to the USGS. The intensity of shaking during an earthquake diminishes as the depth increases. That's why the 4.8-magnitude earthquake that hit California at a depth of 19.2 miles Monday had a lower impact.

The earthquake struck just a day before the statewide Great California ShakeOut earthquake drill, in which 10.1 million people across the state will practice how to be safer during large earthquakes.

According to the California Earthquake Authority, most Californians live within 30 miles of one of the more than 500 active fault lines running across the state.

USGS estimates say there is 75% chance that at least one 7.0-magnitude or greater earthquake will strike Southern California in the next 30 years.