San Francisco's weak foundations leave it vulnerable to earthquakes
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San Francisco lies next to a major fracture in the Earth’s crust. The San Andreas Fault forms the tectonic boundary between the Pacific Plate and the North American Plate. It extends roughly 1,200 kilometres (750 miles) and splits California in two.
San Francisco witnesses minor earthquakes on a regular basis, due to its close proximity to both the San Andreas and Hayward Faults, even though neither physically passes through the city itself.
The San Andreas Fault caused both the 1906 and 1989 earthquakes that devastated California.
With a magnitude of 7.9, the 1906 earthquake killed more than 3,000 people and destroyed more than 80 percent of San Francisco.
The 1989 Loma Prieta quake, with a magnitude of 6.9, saw 63 people lose their lives along California’s Central Coast and a further 3,757 injured.
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A Discovery Channel documentary explained the risk posed to the city in the event of another “big one”.
Jeremy Wade’s ‘Mysteries of the Deep’ series featured a section which looked at the Golden City and its foundations.
The documentary’s narrator explained 1848 sparked the beginning of the California Gold Rush, where thousands of “wannabe prospectors” flocked to the area via San Francisco.
One historian explained the new arrivals, as well as many crews, simply abandoned their ships and headed for the gold fields.
Marine historian Michael Tuttle added: “I think at one point there were over 900 vessels just abandoned in San Francisco harbour.”
Scuttling ships is the deliberate sinking of vessels by allowing water to flow into the hull.
Peter B Campbell, a maritime archaeologist, explained: “Often they’re filled with rocks or they’re filled with sediment. They have big piles driven through them in order to keep them in place.”
The areas in between the sunken ships are filled with refuse, dirt and “all kinds of things” to create land.
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Through a loophole in San Francisco law, those responsible for sinking a ship could claim it as land.
Naturally, people dashed to sink the ships, given that they would then be owners of waterfront property.
Now, 170 years on, these boats are the “skeleton of the city’s past” and the foundations of one of the world’s technology hubs, the narrator explained.
However, this comes with its own risks. The city has expanded far beyond its natural limits.
The Marina, Mission Bay and Hunters Point neighbours, as well as parts of the Embarcadero, are built entirely on landfill.
Marine archaeologist Rob Rondeau said: “When you build a city like San Francisco on fill that’s comprised of old shipwrecks, it’s not the most stable platform and what also adds to the challenge is when you’re living on an active fault line like the San Andreas Fault.”
Saturated and partially saturated soil significantly loses strength and stiffness after applied stress, such as shaking during an earthquake.
This can cause extensive damage to the buildings built upon it, as San Francisco’s Marina district witnessed during the 1989 quake.
Maritime historian Alexander Clarke warned that San Francisco could be especially at risk for this reason, should a significant seismic event happen in future.
A study published in the journal Nature in 2006 found that a sufficient stress level has been reached on the San Andreas fault for an earthquake of a magnitude greater than 7.0.
The risk, the study concluded, could be increased more rapidly than previously believed. However, in the 15 years since its publication, there has not been a substantial quake, although the ability to predict major earthquakes remains elusive.
Reflecting on San Francisco’s position, Mr Clarke said: “You start thinking ‘hang on, is this going to hold in the next seismic event?’”
Mr Tuttle echoed this, and the documentary concluded with him saying: “I wouldn’t want to be there for the next big one”.
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