The earthquake and some 100 aftershocks that hit Southern California on Friday caused some structural damage, power outages and water main breaks. Minor injuries were reported.
But things could have been worse -- much worse -- had the earthquake been a 7.5 magnitude. Some experts say it could be even worse than the so-called "Big One" that could occur along the San Andreas fault, the Los Angeles Times reported.
The Puente Hills thrust fault runs from the suburbs of northern Orange County, through the San Gabriel Valley and under downtown Los Angeles before ending in Hollywood.
Lucy Jones, a U.S. Geological Survey geologist, said an earthquake along the fault could be particularly hazardous because it is a horizontal fault, which means more intense shaking could be felt over a larger area than vertical faults like the Newport-Inglewood or Hollywood.
The USGS and Southern California Earthquake Center predict a massive quake on the Puente Hills thrust fault could kill between 3,000 and 18,000 people and cause up to $250 billion in damage.
Shaking from an earthquake in the heart of Los Angeles -- where there are older, more vulnerable buildings -- could be so intense that heavy objects would be lifted into the air.
"That's the type of shaking that will hit all of downtown. And everywhere from La Habra to Hollywood," Jones said.