The 25-mile Puente Hills thrust fault, which slices below Los Angeles neighborhoods, downtown skyscrapers, bridges and freeways, could cause catastrophic damage.

A week ago today, a magnitude 5.1 earthquake on the fault broke water mains, cracked foundations, and triggered a rock slide. Experts say a 7.5 would be devastating — and if the Big One struck, it could kill as many as 18,000 people and displace 750,000.

That’s because much of Los Angeles’ infrastructure isn’t earthquake-proof. Bridges aren’t built to modern earthquake codes and hundreds of buildings are made of vulnerable concrete.

Retrofitting the city’s structures would be a time-consuming and costly endeavor. It took three years and a quarter billion dollars to retrofit LA’s Hall of Justice — and it would cost $400 million to shore up 11 critical bridges.