During both the 1971 Sylmar and 2014 Napa earthquakes, homes built atop a fault suffered major damage as their foundations were torn apart, while homes just a few hundred feet away suffered far less damage, said Tim Dawson, senior engineering geologist with the California Geological Survey.
Larger buildings straddling a fault could be more vulnerable and completely collapse, Dawson said.
"We don't want to put essential facilities on top of active faults, such as fire stations, hospitals, schools," he said.
The Santa Monica earthquake fault zone is generally 1,000 feet wide, and runs west of Beverly Hills for more than eight miles, with the zone running through the Westfield Century City mall and the Mormon temple.
Earthquake faults can change land in ways that made them appealing generations ago for developers who were unaware of the seismic risk.
It was past earthquakes that made Santa Monica Boulevard a perfect spot for a major roadway and the Red Car trolley line -- a flat area just below a hillside, Dawson said.
Prehistoric earthquakes threw up the north side of Santa Monica Boulevard, Dawson said, forming the dramatic hillside perch that the Mormon Temple now calls home and moving it west toward the ocean. The area to the south of the boulevard is getting relatively lower and has also been shifting to the east.
California law does not require existing buildings in the zones to be altered. But it prohibits new development on top of fault lines.
State scientists have various degrees of certainty about the fault's location. Some drawn paths are considered accurate based on detailed geology investigations on specific plots of land to identify a fault. Other sections of the fault are considered approximately located.
Yet other parts of the fault's path are inferred, in which geologists connect the dots between two points along the fault and "we see a fault at point A, and there's good evidence there's a fault at Point B," Dawson said. "Faults are typically continuous features -- they don't typically stop."