Magnitude Science

Fremont, Calif. — Authorities say a magnitude 4.0 earthquake rattled the San Francisco Bay Area in California, but no injuries or property damage was immediately reported.

The U.S. Geological Survey says the quake hit shortly after 2:30 a.m. Tuesday. The earthquake, with a depth of about 7 miles, was centered about a mile north of Fremont, which is located about 35 miles southeast of San Francisco.

Two smaller quakes were reported following the initial temblor, according to the USGS. Those quakes had initial magnitudes of 2.6 and 2.7 and were also centered near Fremont.

The Hayward Fault runs from San Pablo Bay in the north to Fremont in the south, passing through the cities of Berkeley, Oakland, Hayward, and Fremont. South of Fremont the fault branches into a complex set of surface faults that connect the Hayward Fault to the central part of the Calaveras Fault. The Hayward and Calaveras Faults may have a simpler connection at depths more than 5 km (3 miles), joining in the subsurface just south of the Calaveras Reservoir (site of the October 30, 2007 M5.4 Alum Rock earthquake). The Hayward Fault may be segmented into a northern and southern segment in the vicinity of Berkeley or Oakland.

The 2003 Working Group for California Earthquake Probability assigned a 27% probability that the Hayward-Rodgers Creek Fault system would produce a magnitude 6.7 or larger earthquake in the next 30 years.

The Fremont Police Department told The Associated Press that there have been no reports of damage or injuries.

The quake comes about 11 months after a magnitude 6.0 earthquake struck the region in late August 2014. It was the largest quake to hit Northern California in a quarter-century. That quake killed one woman and injured hundreds of other people while causing about $400 million in property damage in Napa Valley.