Note: Map shows the area with a shake intensity of 3 or greater, which U.S.G.S. defines as “weak,” though the earthquake may be felt outside the areas shown. The New York Times

A minor, 3.9-magnitude earthquake struck in the San Francisco Bay Area on Monday, according to the United States Geological Survey.

The temblor happened at 7:46 p.m. Pacific time about 3 miles northwest of Dublin, Calif., data from the agency shows.

U.S.G.S. data earlier reported that the magnitude was 4.

As seismologists review available data, they may revise the earthquake’s reported magnitude. Additional information collected about the earthquake may also prompt U.S.G.S. scientists to update the shake-severity map.

An aftershock is usually a smaller earthquake that follows a larger one in the same general area. Aftershocks are typically minor adjustments along the portion of a fault that slipped at the time of the initial earthquake.

Quakes and aftershocks within 100 miles

Aftershocks can occur days, weeks or even years after the first earthquake. These events can be of equal or larger magnitude to the initial earthquake, and they can continue to affect already damaged locations.

When quakes and aftershocks occured

Source: United States Geological Survey | Notes: Shaking categories are based on the Modified Mercalli Intensity scale. When aftershock data is available, the corresponding maps and charts include earthquakes within 100 miles and seven days of the initial quake. All times above are Pacific time. Shake data is as of Monday, March 17 at 11:12 p.m. Eastern. Aftershocks data is as of Tuesday, March 18 at 12:02 a.m. Eastern.