That 7.8 quake is the subject of a scientific scenario called the "Great Southern California Shakeout." If such a quake occurred, experts estimate there would be about 50,000 people hurt and almost 2,000 people dead.
"This earthquake is probably going to happen in the lifetimes of at least some of our viewers," said Dr. Lucy Jones, U.S. Geological Survey.
Scientists say a weak spot exists in the Cajon Pass. It's a place where vital state infrastructure would be destroyed.
"The problem is that everything that crosses through the fault here is going to be carried apart just like that and broken," said Dr. Jones.
If a major earthquake was to hit the Cajon Pass area, railroad tracks would sever, trains would derail and major electric lines would go down, leaving the Southland without power.
"We're looking at possibly over 72 hours. That's what we would ask our customers to prepare for. But the likelihood could be longer," said Nancy Jackson, Southern California Edison.
The 200,000 people who use I-15 every day to go to work would be stranded. The ground would also rupture in a way Southern Californians have never seen.
On November 13 there will be a statewide earthquake drill to run this scenario to see if we're ready.
"One of the questions we asked within this scenario: 'Is this going to be a disaster that we can recover from in a few years? Or will it be a catastrophe that's going to take decades to recover from?'" said Dr. Jones.
Scientists say we are overdue for the big one, and hope we will be ready.