San Francisco is overdue for a big earthquake - here are the most vulnerable parts of the city

Most of San Francisco rests on trash.

It was added during the latter half of the 19th century, when zealous settlers loaded the bay with landfill as a way to widen the peninsula. When shaken by an earthquake, this terrain essentially turns into liquid - leaving the structures above in peril.

Many of those structures are top-heavy homes and apartments built above cavernous garages that buckle when a tremor strikes. During the 1989 Loma Prieta quake, the bottom floors of hundreds of these apartments collapsed. Many of them are slated to begin retrofits, but hundreds have yet to start construction.

Meanwhile, another big quake is coming. There's a 76% chance that the Bay Area will experience a magnitude 7.2 earthquake in the next three decades.

"It could happen tomorrow or in 30 years," Mary Comerio, an architect who specializes in earthquake engineering research and a professor of disaster recovery and reconstruction at the University of California, Berkeley, told Business Insider.

Here are the neighborhoods most at risk.