The San Andreas fault is highlighted in red. It strikes through the heart of Southern California, including the Salton Sea.
Geologists say Southern California is long overdue for a huge earthquake that could unleash widespread damage.
It all comes down to the Salton Sea, which lies to the east of San Diego. The Salton Sea lies directly on the San Andreas Fault and covers more than 350 square miles.
A big earthquake has hit the lake bed about every 180 years. But when officials started damming the Colorado River to reduce floods downstream (including in the Salton Sea), the moderate earthquakes stopped for the Salton.
Sounds like a good thing, right? Not necessarily. Seismologists think the damming stopped moderate stress-relieving earthquakes on the Salton. Now, they fear the pressure is building and the area could be as many as 100 years overdue for a mega-quake quake, measuring 7.5 or larger.