The sidewalk is still sinking in SF's Mission Bay. It could liquefy during an earthquake.

The sidewalk continues to sink deeper in the Mission Bay neighborhood of San Francisco. The sidewalk continues to sink deeper in the Mission Bay neighborhood of San Francisco. Photo: John King / San Francisco Chronicle Photo: John King / San Francisco Chronicle Image 1 of / 8 Caption Close The sidewalk is still sinking in SF's Mission Bay. It could liquefy during an earthquake. 1 / 8 Back to Gallery

It's a known problem that the sidewalk around a particular part of Mission Bay is sinking. But the gaps are getting worse.

Dramatic new photos of the sidewalk on 4th street, south of Mission Creek, show a problem that is continuing to deepen around the area, sparking concerns around earthquake safety and daily danger for pedestrians.

One of the reasons for the widening gap is that Mission Bay wasn't built on solid bedrock — it was constructed on artificial landfill. The new development was built atop land once known as Mission Swamp. In an earthquake, this soil can turn to liquid. It's also the exact kind of soil that intensifies ground shaking the most, according to the U.S. Geological Survey.

NBC reported in 2016 that the sidewalk around the 1200 block of Fourth Street had sunk as much as six inches. That same year, Jerry Sanguinetti, the chief of street use for the city's Department of Public Works, told NBC the sidewalk was not up to standard, because it was separating from the building itself.

KTVU reported that permits were pulled in June 2017 to temporarily fix the sidewalk because of the sinking. Ramps were also put in to satisfy ADA compliant. They reported that in December, more permits were pulled for "permanent work to begin."

But recent photos show that the problem has not yet been fixed.

Gray Brechin, a historical geographer and visiting scholar at UC Berkeley's Department Geography explained in the June edition of the Potrero View that scientists have been aware of the sinking land in Mission Bay for well over a century.

The soft, sandy soils were the reason why the area's early settlers added artificial landfill to build on top of, the Potrero View reported.

According to the Potrero View, Mission Bay isn't the only neighborhood in the Bay Area that's sinking. Treasure Island, SFO Airport and Foster City are sinking "as fast as 10 millimeters per year."

San Francisco Chronicle Urban Design critic, John King, contributed to this report.

Read Annie Vainshtein's latest stories here. Email her at avainshtein@sfchronicle.com. Twitter:@annievain