The devices automatically shut off the natural gas that flows into a home or building during an earthquake. They typically cost between $200 and $500 each.

Hal Bernson, the city Councilman who represents the San Fernando Valley neighborhoods that suffered most in the 1994 Northridge earthquake, shepherded the bill through the council. He noted that leaking gas caused several fires and other problems in the aftermath of the '94 quake, and that requiring buyers or sellers to install the valves before a transaction can close escrow will ensure that most buildings will eventually be retrofitted.

Some real estate organizations, including the Southland Regional Association of Realtors, opposed the measure. They say that if the valves are truly needed to protect public safety, then all buildings—not just those that are being sold—should be retrofitted immediately.