After the 1971 Sylmar earthquake, California began an ambitious effort to map faults across the state.

Over the next two decades, officials published 534 maps of active earthquake faults. New construction was prohibited on top of these fissures because previous quakes showed that buildings could be torn apart during violent shaking.

But the mapping campaign has slowed to a crawl — with many dangerous faults still undocumented.

Since 1991, only 23 have been drawn. Because of budget cuts, none were completed between 2004 and 2011, according to records reviewed by The Times.

State officials said there are still about 300 maps to draw and even more to revise — including some in heavily populated areas of Southern California. That represents about 2,000 miles of faults statewide.

The slow pace affects public safety. The ban on building atop faults is  enforced only for those formally mapped by the state; the regulations don't cover faults not on California's official map. 

It has become an issue in recent months because of several new developments planned along the Hollywood fault, which runs through the heart of the famed entertainment district. A high-rise development has been approved by the Los Angeles City Council along the fault area, and another large mixed-use project nearby is already under construction.

State law requires new buildings located near faults mapped by the state to perform extensive testing to prove their structures are not on top of the fissures.

But the Hollywood fault has not been officially mapped, even though the state has known of its existence for decades.

The state geologist's office said it hopes to complete mapping the Hollywood fault by 2014. After that, officials want to concentrate on the Santa Monica fault in the Westside. Officials are also concerned about several other zones they believe need further seismic research, including San Diego Harbor, Yorba Linda, the San Gabriel and Pomona valleys, Indio, Napa County and South Lake Tahoe.

The state geologist's budget has dropped from $9.1 million in 2001 to $2.9 million for the current fiscal year. State geologist John Parrish said his office scraped up funding from its existing budget to restart the mapping program last year, though he said progress has been slow.

"We try to perform as best we can do," Parrish said.

Many earthquake faults have already been extensively researched by scientists at places such as USC, Caltech and the University of California. When creating a map, the state reviews all this outside research and draws a roughly quarter-mile zone around the fault. Under the state's Alquist-Priolo Earthquake Fault Zoning act, developers in that zone must prove their structure does not sit on top of the active faults.

The 557 maps the state has completed represent various parts of different fault lines, including such well-known ones as the San Andreas, Hayward and Newport-Inglewood.

Experts say it's crucial to complete the mapping to keep new structures away from dangerous areas straddling faults.

"Then there's no mistake [about] the red line," said retired state geologist Robert Sydnor. "It helps at the political level: the city council and the mayor cannot somehow override" it.

James Dolan, a USC earth sciences professor who has experience mapping faults, recalled driving through the Westside when he was doing research on the Santa Monica fault 20 years ago. "I was stunned to see, either right next to or on top of where I was mapping the fault, a big building in excavation," he said.

He called around and found out the fault had not been officially mapped by the state, so the property was not covered by the law.

Seismic safety experts have long studied the destruction caused above faults when big quakes strike. But the Sylmar quake reinforced the risks to officials.

One side of the San Fernando fault moved as much as 8 feet from the other side. Many homes and commercial buildings were severely damaged by the ground fracturing. About 80% of the buildings along the fault suffered severe to moderate damage.