On April 18, 1906, the mighty San Andreas Fault -- which slices along 800 miles of coastal California -- slipped, creating a magnitude 7.8 earthquake, one of the strongest ever recorded in the continental United States. Its effects, which were most pronounced in San Francisco, were felt as far north as Oregon and as far east as Nevada.
Little was known about the San Andreas Fault before 1906, but that changed when a team of scientists immediately fanned out across the California landscape to record what happened. They mapped the fault and produced a report that gave birth to modern earthquake science in this country.
Technically, it's called the Report of the State Earthquake Investigation Commission. But it's commonly known as the Lawson Report, after Andrew Lawson, the geologist who organized the scientists just after the quake. They covered hundreds of miles of landscape on foot and horseback, often through hills covered in poison oak. One thing the scientists measured was the offset of roads, rows of trees and fences.
Thirty miles north of San Francisco, there is dramatic evidence of one such offset. At the Point Reyes National Seashore, the Earthquake Trail takes you along a path where blue posts in the hillside mark precisely the location of the fault. At the end of the posts is a fence, which appears to be torn in half.
"This is one of the places where the slip is the largest in the 1906 earthquake," says John Boatwright with the U.S. Geological Survey.
The fence lies at right angles across the fault, and is split, one end 18 feet to the north of the other end. In 1906, the two gigantic tectonic plates that rub against each other at the San Andreas Fault shifted. Boatwright puts his palms together and slides one forward, the other back.
"If the earthquake occurred now and you were on the other side of the fault, I would go north and you would go to the south," he says.
The Lawson scientists didn't know anything about plate tectonics at the time, but their detailed information proved invaluable for later seismologists. Their findings make up a two-volume tome, the largest set of seismic effects ever compiled in a single report. Since early seismometers were unable to register the strength of such a powerful earthquake, the scientists instead interviewed witnesses, examined damages to buildings, landslides, trees that snapped, and evidence of ground cracking.
No detail was too small.
"In many places in (the Lawson Report), you have areas where the only effect that is recorded is the milk that was poured into a shallow pan every morning before dawn by your milkman is sloshed out of the pan," Boatwright says.
The scientists loosely assigned shaking ratings to different areas. Furniture moving was lower than damaged chimneys, for instance.
At least one scientist, though, fell victim to a tall tale. Here's one entry from the Lawson Report about a legendary incident that happened very close to Point Reyes National Seashore: "During the earthquake a cow fell into the fault crack and the earth closed in on her so that only the tail remained visible. At the time of my visit, the tail had disappeared, being eaten by dogs."
The farmer who showed people his cow later recanted, saying he merely pushed the cow (which died of natural causes) into the crevasse. Still, Boatwright is in tremendous awe of the Lawson Report. When his boss at the USGS assigned him to find out where the ground shook the most and least in 1906, he used the report as his guidebook. Boatwright visited dozens of sites mentioned in the Lawson Report to see what evidence of shaking still remained.
As a result, Boatwright has become an expert on graveyards, where he finds not only a record of death but also a dramatic demonstration of the shattering force of 1906. Near the small town of Graton, north of San Francisco in Sonoma County, is a cemetery that was severely damaged in the earthquake.
Many of the gravestones are lying broken on the ground. Boatwright says it's not the work of vandals. One stone bears the name Thaddeus Ames, who died in 1876. It's broken in three parts.
"Everywhere you look, you can sort of see broken headstones and they all predate 1906. It tells me that it shook really strongly here," Boatwright says.
Through Boatwright's own observations and descriptions in the Lawson Report, he's determined that some of the strongest shaking during the 1906 earthquake happened in Graton, 50 to 60 miles away from the epicenter near the Golden Gate. In fact, there was so much damage to headstones in this area, that gravestone makers nearly stopped making marble headstones. Granite became the preferred material.
After three years of research, confirming and adjusting Lawson, Boatwright has accomplished his assignment. He's created a digital shake map of the 1906 earthquake. On a map of Northern California, he has placed tiny squares. Each square has an intensity rating of one to 10. By clicking on one particular square, you can see, for instance, that Graton (near that cemetery) has an intensity rating of 8 to 9. Nearby Sebastopol was given the highest shaking rating for this quake, 9 1/2.
Santa Rosa, nearly destroyed in 1906, was almost as high. The city of San Francisco, which was much closer to the epicenter, was not shaken as severely. And that's also true of other areas lying right along the fault. So what's going on here?
Mary Lou Zoback, Boatwright's supervisor, says that's the kind of question they can begin to ask by using Boatwright's data in a brand new, three-dimensional map of the earth's crust in Northern California.
"The question is, where's the shaking gonna be the strongest?" says Zoback. "Are there vulnerable infrastructures, pipelines, things like that, we should be addressing and fixing now? And we know the answer to that is yes."
Zoback says they hope they'll be able to slide a virtual epicenter along a map of the fault and predict the intensity of shaking depending on where the quake starts. She says there's an urgency to this endeavor. Time is running out.
"In the period leading up to 1906, there were lots of earthquakes in the San Francisco Bay area," says Zoback. "Earthquakes were a part of daily life at the turn of the century."
Then came the Big One, followed by a long period of relative quiet in Northern California. Zoback says the Lawson Report was the first to recognize that, over time, strain builds up on a fault and is followed by a big release. Now there's some evidence that the fault is again being stretched to the breaking point.
"Beginning in the late '60s we started having some small to moderate earthquakes in the Bay Area and then, of course, the magnitude 6.9 (Loma Prieta) in 1989," says Zoback. "Are we now beginning to move into another very active seismic period?"
If we are, we could be looking at big trouble. In 1906, almost 300 miles of the San Andreas Fault broke loose, creating a quake 30 times more powerful than Loma Prieta in 1989, which killed 62 people. A new report has just been released for the 100th Anniversary Earthquake Conference. The report is by a team of experts led by structural engineer Charles Kircher. It predicts that if a quake equal to 1906 occurred, it could kill as many as 3,400 people and cause $150 billion in damages.
This story was produced by NPR's Cindy Carpien.