According to Reuters:

Retrofitting Los Angeles’ water infrastructure to better withstand a major earthquake could cost up to $15 billion over the next 20 years, the city’s water engineers said in a report released on Monday.

The report prepared last fall looked at the cost of retrofitting the city’s entire massive system, with 7,200 miles of pipe, something water officials say they are unlikely to do, but it provides a starting point as the city looks to shore up its aging infrastructure.

The city’s aging water system has been of concern recently in Los Angeles, after water main breaks in several places, including Westwood near UCLA and the Hollywood Hills.

… aging pipes were currently being replaced at a cost of about $100 million per year.

The city is considering increasing that amount to up to $300 million per year, upgrading both for reliability and seismic improvements. At that rate, the pipes will turn over roughly every 150 years, Adams said, an improvement over the current cycle of once every 280 years.