The History of San Andreas Fault

California’s greatest cities and millions of her citizens live in constant peril. Since records began, there have been 13 large earthquakes along the San Andreas Fault. And now, America’s geologists, her rock detectives, are warning of a potential disaster. In the fall of 2008, more than 300 scientists calculated what a major earthquake would do to Southern California. We have been conducting a special study of a magnitude 7.8 earthquake on the southern San Andreas Fault, large enough to potentially damage tall buildings. Fire will be very significant.

The definitive scientific report presented to politicians was codenamed ShakeOut. It forecast thousands deaths and billions of dollars of damage in the city of Los Angeles, which makes it crucial to investigate the most important question–when will the next big earthquake hit the San Andreas Fault? The latest preparations for disaster are the climax of an investigation that started more than 100 years ago, in the aftermath of the Great San Francisco Earthquake of 1906. The earthquake struck on a Wednesday, just before dawn.

The ground shook violently for 45 seconds, igniting fires that raged unchecked for the next four days. 28,000 buildings, a tenth of the entire city, were destroyed, and more than 3,000 people, one in every hundred of the population, were killed. With a magnitude of 7.8, it is in the top 20 of North America’s strongest ever earthquakes. The scale of the Great San Francisco Earthquake shocked the nation. But no one understood what had made the city shake. Native American myths explained earthquakes as shocks from a battle between warring spirits. Latter-day explorers could not understand the shocks that destroyed their mission buildings.

One spanish missionary wrote, The Earth shook around me from explosions under the ground.

300 years later, and science has still made little progress. Refugees in the ruins of San Francisco still blamed earthquakes on mysterious underground explosions. So, just three days after the earthquake, the state of California asked one of the world’s most famous geologists, Andrew Lawson from California State University, to investigate what had destroyed the city. He and a team of 25 scientists began collecting damage evidence in the city and surrounding countryside.There were roads that had buckled.

Rail tracks that had twisted the most startling evidence of all? That came near the town of Bolinas in Marin county, to the North of San Francisco. This picket fence had an eight-foot gap in the middle. Before the earthquake, it was a solid boundary fence, dividing two fields. But, when he recreated what had happened, Lawson realized that the land had jolted apart and torn the fence in two.

Plotting the evidence on a map around San Francisco revealed a surprising pattern, because connecting the dots drew a straight line, and, at every point, the Earth moved in the same way. On the coast to the North, inland to the South. This line of weakness was the culprit they were searching for. South of San Francisco, the suspect line ran underneath a lake, the Laguna De San Andreas. So now, the earthquake perpetrator at last had a name.

Professor Lawson, who, a decade earlier, had identified cracks in the Earth here as a harmless rift, now rechristened it the San Andreas Fault. In modern-day San Francisco, the buildings, the roads and the railways have long since been repaired. But, if you know where to look, evidence of the 1906 quake can still be found.

Geologist Charlie Paull follows in the footsteps of Lawson’s team, seeking signs of the havoc from 1906. He finds it on the cliffs at Mussel rock, 12 miles south of San Francisco.

“The cliff is not here by accident, there is a very good reason why this cliff is here. Half a mile or so of the shore face apparently fell off in the 1906 Earthquake, and if you look down below us, there’s a big rotated block, that’s near the present-day shoreline, and it is just inboard of the San Andreas Fault, the San Andreas Fault is, about a quarter of a mile offshore here, and of course, that’s one of the major crustal junctions, um, on this side of North America,” said Paull.

Modern computers can now trace how damage waves spread out across the city. And that pinpoints where the quake originated along the San Andreas. It was offshore, about two miles out to sea from the Golden Gate Bridge. So, to continue tracking the fault, the investigation must head out to sea. Marine Geologists use remote operating vehicles, mini-submarines, to map the sea floor.

“What you would see is subtle variations in the topography, or topography that would not naturally line up. So there might be a line on the ocean floor that is higher or lower on one side, and you can use various techniques to determine that this actually is a fault, instead of some other process,” added Paull.

Running south across the seabed, the San Andreas finally runs out of ocean, and hits the land. This broken line of rocks, stretching in from the sea, marks where the San Andreas hits land, 12 miles south of San Francisco.

“We are here at Mussel Rock and we are essentially standing on the San Andreas Fault right now. And if there was an earthquake, I do not know what would happen right here, but I would not want to be here,” added Paull.

This Pacific coastline, where cliffs crumble slowly into the sea, is the boundary between two of the Earth’s massive continental plates. Separated by the San Andreas Fault, two vast, separate blocks of the Earth’s crust lie directly alongside each other.