The San Andreas Fault: The Last Stretch of Submarine Mountains

How an ‘ancient landslide’ keeps threatening a railroad, homes in San Clemente

In the 1980s, an area of coastal mountains in San Clemente, California, was under the influence of a geological catastrophe that had wiped out many parts of the world’s second-oldest human-made structure — the giant, 300-foot-high San Andreas Fault.

The San Andreas Fault lies on the coast of Southern California, and the only way it ever got there is by sliding along the ocean floor. It is one of the oldest, biggest and most complex faults on the face of the Earth, moving about 200 miles (320 km) over the past 200 million years as part of the Pacific “Ring of Fire.”

To date, about one-third of that fault is still sticking out into the ocean. This ancient fault line is home to the world’s last stretch of submarine mountains. And, because it stretches for miles around, it is one of the most dangerous places on the planet to build a structure.

For years, the land where those mountains lie was a quiet, unassuming community of houses and a small airport. But on the night of August 16, 2011, that quiet, unassuming place exploded with gunfire when a large, ancient earthquake rolled through the town, sending an avalanche into the small town of La Jolla.

The quake rattled the ground and damaged hundreds of homes. The San Andreas Fault shifted slightly and threw chunks of mountainside out to sea. A small pocket of land that was once a town became an island of volcanic rock.

“The quake was so strong that it threw off the tops of some of the houses on [the] peninsula,” said Michael Davis, a professor at San Diego State University who studies the earth’s surface, earthquakes and tsunamis.

“I saw big chunks come up out of the ground. It was about the size of a car. There was at least that much of

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