Aftershocks are most common immediately after the mainshock; their average number per day decreases rapidly as time passes. Aftershocks are most likely to be felt in the first few days after the mainshock, but may be felt weeks, months, or even years afterwards. In general, the larger the mainshock, the longer its aftershocks will be felt.
Aftershocks tend to occur near the mainshock, but the exact geographic pattern of the aftershocks varies from earthquake to earthquake and is not predictable. The larger the mainshock, the larger the area of aftershocks. While there is no "hard" cutoff distance beyond which an earthquake is totally incapable of triggering an aftershock, the vast majority of aftershocks are located close to the mainshock. As a rule of thumb, a magnitude 6 mainshock may have aftershocks up to 10 to 20 miles away, while a magnitude 7 mainshock may have aftershocks as far as 30 to 50 miles away.
Intensity map.
According to the United States Geological Survey (USGS) a magnitude 5.6 earthquake, lasting about 30 seconds, struck at 20:04:54 (PDT) on October 30, 2007, with the epicenter being five miles north, northeast of Alum Rock California and nine miles northeast of the center of San Jose.  The actual coordinates given by USGS put it along the border of San Jose and Milpitas in the hills near neighborhoods in both cities. The quake was felt as far away as the California communities of Sacramento, Sonoma, and Los Banos. It was the most powerful quake in the San Francisco Bay Area and Silicon Valley since 1989.

At least ten aftershocks had been reported by 8:35 p.m with magnitudes ranging from 1.3 to 1.8.

Phone service, including cellular phone service, was reported to be down in some areas around the epicenter. Residents and business owners also reported their homes shaking.

According to the USGS, damage could be "moderate to heavy" and Rafael Abreu of the USGS said that the earthquake is considered "moderate," but so far there are "no injuries." The quake was reported at a depth of 9.2 km (5.7 miles).

USGS predicts a 30% chance of strong (magnitude >5) aftershocks in the next seven days, with a 5-10% probability of aftershocks stronger than the main quake. Additionally, USGS predicts approximately 15-40 small (magnitude 3-5) aftershocks.

The USGS reports that the quake was centered on the Calaveras Fault, and was the most powerful earthquake on that fault since the 1984 Morgan Hill earthquake.