Today's quake occurred at 11:48 a.m., according to the USGS, and had a depth of five miles. After initially estimating a preliminary magnitude of 3.8, the agency quickly downgraded it to 3.7, said USGS seismologist David Oppenheimer.
The quake was centered in the same area as the previous day's quake, which had a preliminary magnitude of 4.1, said Oppenheimer. The pair of temblors occurred on two small, separate fault lines, both located about six miles east-northeast of Milpitas and eight miles north-northeast of Alum Rock.
Initially, seismologists thought the Thursday quake occurred on the Calaveras Fault, which runs through the region. However later in the day they determined it was actually centered on a small, nameless fault that lies perpendicular to the Calaveras, Oppenheimer said.
"Every earthquake has its own aftershock sequence," Oppenheimer said. He recalled a 4.4 magnitude earthquake in the Alamo area in 1990 that recorded 350 aftershocks over 42 days.
"If it continues on like this and has a robust aftershock sequence, it could evolve into a swarm," he said. This phenomenon consists of "lots of earthquakes of similar magnitude; it can go on for weeks."
Conversely, today's could be the last of it, he said.
Within seconds of the earthquake, social media outlets like Twitter were buzzing about the follow-up quake, and many people expressed fear that they might signal an even larger earthquake in the near future.
The probability of that is very small, Oppenheimer said. "About 95 percent of all earthquakes don't have foreshocks."