At 4:31 A.M. local time, Monday, January 17, 1994 the Northridge earthquake struck the San Fernando Valley region of Southern California with a moment magnitude measured at 6.7 and focal depth of 19 km. The earthquake was centered 32 km west-northwest of Los Angeles along a south-dipping, blind thrust fault. Little if any surface faulting was produced. The earthquake resulted in 57 deaths, more than 5,000 injuries, and structural damage including instances of partial or complete structural collapse. Estimates of more than $20 billion in property damage make this earthquake the costliest seismic disaster in U.S. history. The earthquake touched off considerable scientific and engineering investigation.