No active fault under proposed site of Hollywood towers, says city of Los Angeles

The Los Angeles Department of Building and Safety says there’s no active fault underneath the lot where developers plan to build a pair of skyscrapers in Hollywood. That decision was based on geological studies commissioned by the developers, Millennium Partners.

The studies did find evidence of a fault on site, but concluded it was dormant. Still, experts say even a fault believed to be inactive can, in rare cases, reawaken and cause a major quake.

A fault in California is considered active if it moved sometime in the last 11,000 years. That cutoff is arbitrary, and based on the start of the holocene era, which is the most recent geological period, according to geologist Tim McCrink with the California Geological Survey.

"The thinking there is that if a fault has moved in that time period, it is more likely to move again in our lifetime," McCrink noted.

If a fault hasn’t moved in in the last 11,000 years, it's very likely it won't go off again in the foreseeable future, he added. That's not always the case, though.

Researchers are starting to believe that last year’s Napa earthquake was caused was by a fault that was previously deemed inactive.

"I think that West Napa fault maybe turns out to be something moved in the last 20- or 30,000 years before it moved this time," he explained.

He says there are also other cases like this on record. This may explain why Chinese officials define an active fault as one that moved anytime in the last 120,000 years. That's significantly longer than the time period used by California.

The fault under the proposed Hollywood towers last moved 150,000 years ago, according to Millennium’s geologists. However, the California Geologic Survey released a map last year claiming the fault had moved in the last 11,000 years, making it active.

Under state law, developers cannot build on top of an active fault that could rupture to the surface and tear a building in two. City officials have the final say in whether or not a fault is capable of such destruction.

CGS's McCrink said, all this research aside, no fault is ever 100 percent safe, no matter how long since its last big shake. Still, California builders need some reasonable guidelines to work by, hence the state's 11,000-year cutoff for inactive.

McCrink added that the state has no current plans to update the definition for an active fault.