The focus of the earthquake which was widely felt in the Bay Area shortly after 9 pm PDT on Friday, September 5, 2008, was located about 10 miles below Alamo (large purple circle). Among seismologists this upscale East Bay community is known as a frequent source of temblors. Located along the Calaveras Fault, which roughly parallels Interstate 680, Alamo hosts a type of earthquake which is rather unique for the Bay Area. Over the decades several intense earthquake swarms occured in this area, the last one in 1990 (green circles). One of the characteristics of swarm earthquakes is that they are not dominated by one main shock and dozens of aftershocks. Instead an earthquake swarm consists of hunderds of similar temblors, which can continue to shake an area over weeks or even months. The September 5 quake with its magnitude of 4.0 appears not to be part of a swarm. Only a few small aftershocks have occurred since then. This quake was the strongest temblor in the Bay Area since the earthquake sequence at Alum Rock near San Jose in the Fall of 2007. Current information about California quakes. (hra001)