San Francisco's South of Market is slowly sinking

A walking tour being led by geologist David Schwartz (left) on the 600 block of Natoma St. in the South of Market in San Francisco, Calif., showing subsidence areas on Thursday, October 27, 2011. SPUR member Catherine Bauman (middle) with Numpha Ragudo joining the tour. less A walking tour being led by geologist David Schwartz (left) on the 600 block of Natoma St. in the South of Market in San Francisco, Calif., showing subsidence areas on Thursday, October 27, 2011. SPUR member ... more Photo: Liz Hafalia, The Chronicle Photo: Liz Hafalia, The Chronicle Image 1 of / 5 Caption Close San Francisco's South of Market is slowly sinking 1 / 5 Back to Gallery

Everybody knows that San Francisco's South of Market district is on the way up - even in the little back alleys. But it is also no secret that South of Market is on the way down. The streets and some of the older houses, built atop soil and sand fill dumped on old marshland more than a century ago, is slowly sinking.

"It is another version of San Francisco on the move," said David Schwartz, a scientist with the U.S. Geological Survey. "In this case, it's moving downward."

Schwartz, who has been a seismologist for close to 40 years, led a walking tour Thursday through the quieter side of SoMa, the area between Howard and Harrison streets, Fifth to about Eighth Street. It's a neighborhood of wide streets and narrow alleys, away from the glistening towers of the new South of Market district.

Just over a dozen people strolled along with Schwartz, some of them residents of the area, and all of them members of the San Francisco Planning and Urban Research Association. They were city people, interested in the backstories of the city.

They saw San Francisco's version of the Leaning Tower of Pisa - 100-year-old flats and small buildings tilted out of plumb, some leaning toward the building next door, as if they were holding each other up. They saw streets that had subsided several feet; garages sunken below street level. The tour was "amazing," said Catherine Bauman, who lives in SoMa at the Millenium Tower at Fremont and Mission streets. She had never been up the side streets before.

South of Market is a storied part of the city and comes with a history, some of it tragic.

The area was originally bay marsh and sand dunes and was filled in during the 1860s. It was built up rapidly and became a working-class district, mostly Irish.

Schwartz stopped at Sixth and Howard streets. "By 1906, this area was home to boarding houses and cheap hotels, some of them three or four stories high, many poorly built wood-frame structures that collapsed in the 1906 earthquake," he said.

The poor soil - bay marsh and old fill - is 300 feet deep, Schwartz said. In 1906 and in the much smaller Loma Prieta quake in 1989, there was a lot of damage due to liquefaction, when the soil turns to mush.

"Three hundred people died in this area in 1906," he said. Hard to believe during a stroll on a warm October afternoon.

Schwartz led the group up Sixth and then down into the narrow alleys: Shipley, Clara, Langdon, Tehama, Natoma. The whole area was destroyed in the firestorm that followed the '06 quake, but then built up again atop the old soil. Sometimes it was a residential area, sometimes industrial. Today, it's both. And it's all settling, slowly sinking.

At Fifth and Shipley, there is a sunken vacant lot. Up side streets, older buildings lean this way and that. On Howard, between Seventh and Eighth, are two adjoining industrial buildings, one leaning east, the other leaning west. Schwartz pointed out how Howard has settled so much there is a dip in the street, as if it were crossing a sunken stream.

The city has repaved most of the streets several times, raising the street level. But short of major rebuilding, there is not much a homeowner can do.

Still, the area is rising. Many of the old, sinking houses are flanked by new townhouses, condominiums, lofts, pied-a-terres. A new boutique top-floor loft at 250 Clara St. is being offered at $569,000. It's a low price for San Francisco, but a fortune in parts of the country where houses are under water.

A Langton Street loft - "in the heart of SoMa ... a fabulous urban retreat," the ad said - was being offered at $550,000. These buildings, of course, have deep foundations, and could ride out an earthquake. They probably wouldn't sink, but the surroundings might go down.

There are lots of new buildings mixed with the old, and a whole new feel South of Market, but there is an underlying problem. And in the back streets you can see it for yourself.