Two morning earthquakes with magnitudes of at least 3.5 struck near San Francisco on Friday and Sunday, according to the United States Geological Survey.

A 3.5-magnitude earthquake near Concord in the East Bay shook the area on Sunday, two days after a quake of similar magnitude struck in the Pacific Ocean off the coast of San Francisco on Friday, data from the agency shows. Residents across the region reported mild shaking but no immediate damage from either earthquake.

U.S.G.S. data for both earthquakes earlier reported that each one was as strong as a magnitude 3.7 quake. As seismologists review available data, they often revise the earthquake's reported magnitude and update shake-severity maps.

The location of Friday’s earthquake recalled one of the biggest earthquakes of all time, a 7.9-magnitude quake that nearly destroyed San Francisco in 1906.

Friday’s quake, and its much more powerful relative, likely both occurred along the San Andreas fault, said Robert Skoumal, a research geophysicist at the United States Geological Survey. Preliminary location data from the U.S.G.S. suggested that the two may have shared more than a fault line — both occurred at very nearly the same location, about two miles west of Golden Gate Park.

Mr. Skoumal said it was not possible to determine with certainty whether the two quakes shared precisely the same epicenter because of changes in measuring techniques over the last century. “Back in 1906, we didn't have the best seismic instruments out there,” he said.

Earthquakes of similar size are also fairly common in the area west of San Francisco, he said. “We've had dozens of magnitude threes just over the past several decades along this one little patch,” he said.

And while the risk of earthquakes in the San Francisco area is serious, Friday’s small earthquake did not signal the imminent arrival of a more destructive quake, he said. “I wouldn't say this particular earthquake should make anyone worried,” Mr. Skoumal said.

Aftershocks in the region
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. 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.