According to IGN, there is no mission in “Grand Theft Auto: San Andreas” called GTA San Andreas Fault, but there is a secret area in the game that is often referred to as the San Andreas Fault line. This is an area, not a mission, that is located on the ocean floor. It is an in-game reference to the real life San Andreas fault line.
The developer for the Grand Theft Auto games, Rockstar, is known for putting little Easter eggs in its games, according to IGN. An Easter egg refers to an in-game secret that is hidden. Easter eggs typically do not affect the gameplay in any way, but they do encourage player exploration. In addition to the fault line in “Grand Theft Auto: San Andreas,” Rockstar also put areas into the game that look like they have been damaged by earthquakes. One of these areas is located on the ruined freeway next to the San Fierro freeway. The other area is located in the ruined buildings behind CJ's garage in San Fierro.
IGN states that “Grand Theft Auto: San Andreas” is packed full of Easter eggs. There is even one known as the “No Easter Egg Sign.” This sign is located on the top of the Gant Bridge in San Fierro. It is hard for the player to get up there, but when they do, they are greeted with a sign that reads: “There aren't any Easter Eggs up here. Go Away!” Ironically, the fact that this sign is at the top of the bridge is an Easter egg in itself.