The Northridge earthquake occurred at 4:30 a.m. local time on January 17, 1994. Northridge is located about 30 km northwest of Los Angeles. This earthquake had a 6.9 moment magnitude. The hypocentral depth was 19 km. The duration was about 10 seconds to 20 seconds. The earthquake occurred along a "blind" thrust fault, close to the San Andreas fault. Note that a blind fault is a fault which does not extend to the surface. In other words, it is buried.
Thousands of aftershocks occurred after the main earthquake. For example, a magnitude 5.9 aftershock occurred about 1 minute after the mainshock. A magnitude 5.6 earthquake occurred 11 hours later. Aftershocks are a concern because they can trigger the collapse of structures weakened by the mainshock.
