The next time your flight is delayed at San Francisco International Airport, you might have something besides the weather or maintenance problems to blame. It could be what scientists call "subsidence." In simple terms, SFO is sinking.

Built on landfill, the unsettled ground beneath SFO has long created problems for the airport operations staff. But with an increase in runway traffic over the last decade, the surface cracks caused by subsidence are now creating a problem for travelers forced to endure flight delays, according to aviation sources and records reviewed by the NBC Bay Area Investigative Unit.

The next time your flight is delayed at San Francisco International Airport, you might have something besides the weather or maintenance problems to blame. It could be what scientists call "subsidence." In simple terms, SFO is sinking. Senior investigative reporter Stephen Stock reports on a story that first aired Feb. 18, 2019.

SFO UNDERWATER

In 2018, earth research scientist Dr. Manoochehr Shirzaei released a study in conjunction with NASA that analyzed satellite data to measure sinking land throughout the San Francisco Bay Area. Shirzaei found that the land under SFO is sinking more than 5 millimeters per year, a significant drop geologically speaking. At that rate, Shirzaei and his team calculate that more than half of SFO's runways will be completely underwater before the year 2100. Furthermore, the runways are sinking at different rates, causing the runways on top to crack and wear out faster than normal.

"Those differential movements result in horizontal stresses, which opens cracks," Shirzaei told NBC Bay Area. "When there is precipitation, when you have heavy rain, those areas accumulate water."

NBC Bay Area obtained runway repair records dating back to 2016 and found several instances of linear cracking called "alligator cracks." Shirzaei believes these fractures are the result of ground settling.

The records show about two dozen runway repairs in the past three years, with a slight increase in the number of repairs over that time. Several of those repairs were classified in the records as an "emergency," meaning they were not anticipated by regular maintenance and were serious enough to demand immediate attention from repair crews.

SFO Runway Repairs, 2016-2018

Runway repair costs at San Francisco International Airport totaled at least $457,526.82 from 2016-2018, according to SFO work orders obtained by NBC Bay Area. Points are based on approximate locations described in the work orders and do not reflect the exact boundaries of cracks or other imperfections.

BIG CRACKS, BIG DELAYS

Former SFO operations supervisor and NBC Bay Area aviation consultant Mike McCarron told the Investigative Unit that these cracks can cause big problems for passengers when they occur on busy travel days.

"You have to take a runway out of service and repave it," McCarron said. "All airports have to go through and repave runways eventually, [but] SFO because they are on [landfill] it's a lot more problematic."

Between January 2016 and January 2019, maintenance crews shut down a portion of SFO's runways for a total of 90 days to make repairs at a cost of $457,526.82. That works out to one repair closure every four days during that time span.

Departure data from FlightAware shows 1,103 canceled flights and 13,217 delayed flights during those repair days.

Last year in particular had several significant delays caused by cracks.

June 25, 2018, crews were issued a work order for an emergency asphalt repair on runway 28L. On that day, according to FlightAware, there were 31 canceled flights and 410 delays.

Oct. 10-30, 2018, runway 1R showed "several areas of alligator cracking" near taxiway F. FlightAware data shows 85 canceled flights and 1,272 airport delays.

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LONG-TERM SOLUTION

SFO spokesman Doug Yakel told NBC Bay Area the airport recently conducted its own study to measure runway sinking and developed a strategy to address the issue. Those plans include building new seawalls to mitigate rising sea levels and sinking land.

"Our hope is maybe in about five years' time to actually begin physical construction activity such that about 10 years from now that mid-century protection is already in place," Yakel said.

In the meantime, Yakel said the airport plans to continue conducting routine maintenance.

"Our ongoing runway maintenance program addresses those differences and that differential settling that we may see in the area," he said. "The other thing to remember is we periodically repave our runways and every time we do it, we're actually heightening the level of our runways in comparison to the waterline."