In real life, the San Andreas Fault is a large continental fault from San Francisco to Bombay Beach, California, and it marks the boundary between two tectonic plates.
The San Andreas Fault Line can be seen in GTA San Andreas as an underwater ridge, located in the San Fierro Bay. The crack starts just north of Jizzy's Pleasure Domes in San Fierro, and it runs along the seabed and comes to an end at a wall of sandstone that goes back up towards the land. The crack is based on the real-life San Andreas Fault in California (where the game takes place) which is also presumably what State of San Andreas is named after. This is most likely the reason why the Fault Line is in the game. It most likely contributed to the San Andreas Earthquake.
- There is a 2015 movie called San Andreas that features a disaster scenario about this fault line.