STEVE INSKEEP, host:

This is MORNING EDITION from NPR News. I'm Steve Inskeep.

RENEE MONTAGNE, host:

And I'm Renee Montagne.

Before sunrise in San Francisco, this morning, thousands of people remembered the great earthquake and fires of 1906, which destroyed the city and killed three thousand people.

A wreath was laid in a downtown plaza at 5:12 AM, Pacific time-the minute the quake is believed to have started. There was a moment of silence, then fire stations throughout the city sounded their sirens in tribute.

INSKEEP: That earthquake, 100 years ago today, was blamed on the mighty San Andreas Fault, which slices along 800 miles of coastal California. Its slippage created one of the strongest earthquakes ever recorded in the continental United States: a magnitude 7.8. It was felt as far north as Oregon and as far east as Nevada.

Almost immediately, scientists fanned out across the landscape to record what happened. And they mapped what had been a little-known fault. They produced a report that gave birth to modern earthquake science in this country.

NPR's John McChesney reports their information is still being used by scientists today.

JOHN MCCHESNEY reporting:

A hundred years of erosion and human development have made traces of the 1906 quake hard to find. But 30 miles north of San Francisco you can still see dramatic evidence of how the San Andreas ruptured back then.

It's preserved at the Point Reyes National Seashore on what's called The Earthquake Trail.

Mr. JOHN BOATWRIGHT (Geophysicist, U.S. Geological Survey): Well we're looking at a fence which appears to have been torn in half. This area is one of the places where the slip is the largest.

MCCHESNEY: The fence lies at right angles across the fault, and it is split: one section 18 feet to the north of the other.

In 1906, the two gigantic, tectonic plates that rub against each other at the San Andreas Fault, shifted. Our guide, geophysicist John Boatwright, puts his palms together and slides one forward, the other back.

Mr. BOATWRIGHT: Basically, if the earthquake occurred now, and you were on the other side of the Fault, I would go to the north and you would go to the south.

MCCHESNEY: And Boatwright would be standing on the Pacific plate, and I on the North American plate.

The offset fence here was noted in an important scientific report written a century ago by John Boatwright's predecessors. That's known as the Lawson report, after Andrew Lawson, the geologist who sent 25 scientists out just after the quake.

Boatwright reads from the report about a legendary incident recorded close to this sight.

Mr. BOATWRIGHT: (reading from the Lawson Report) 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. But there was abundant testimony...

MCCHESNEY: Well, it never happened like that. A farmer later admitted that he squeezed a dead cow into that crevasse. But this tall tale is a rarity in the hefty, two-volume tome, the largest set of seismic effects ever compiled in a single report.

Early seismometers were unable to register the strength of such a powerful earthquake, so instead, the scientists interviewed witnesses, examined damages to buildings, landslides. No detail was too small.

Mr. BOATWRIGHT: In many places in Lawson, you have areas where the only effect recorded is the milk that was poured into a shallow pan, you know, every morning before dawn, by your milkman, is sloshed out of the pan.

MCCHESNEY: The report loosely assigned shaking ratings to different areas, and Boatwright refined and expanded on those. Sloshed milk is obviously pretty low. People or horses bowled over, higher. Treetops snapped off by whiplash, even higher.

When his boss at the U.S. Geological Survey asked John Boatwright to find out where the ground shook the most and the least in 1906, the Lawson Report became his guidebook. He visited dozens of sites mentioned in the report, to see what evidence of shaking still remained.

Mr. BOATWRIGHT: Yeah, look...

MCCHESNEY: As a result, Boatwright has become on expert of graveyards. That's right, graveyards, where he finds not only a record of death, but also a dramatic demonstration of the shattering force of 1906.

Mr. BOATWRIGHT: We're a little bit to the west of a town called Graton. This is a very small, rural cemetery that apparently got damaged quite severely in the 1906 earthquake.

MCCHESNEY: On a tiny hill, almost hidden by dark green, live oaks, the graveyard is silent except for a strutting rooster. First thing you notice here, many of the gravestones are lying broken on the ground. Not the work of vandals, says Boatwright.

Mr. BOATWRIGHT: We're looking at one of my favorites, which is Thaddeus Ames (ph), who died in, what is this, 1876. And the headstone's broken in three parts. Everywhere you look, you can see broken headstones and they all predate the 1906 earthquake.

MCCHESNEY: What's this tell you?

Mr. BOATWRIGHT: It tells me that it shook really strongly here.

MCCHESNEY: Nearly half the pre-1906 headstones here are damaged. You'd expect them to shatter when they fall, but surprisingly, some still standing five inch thick slabs of marble were simply snapped in half by the fierce whiplash caused by the 8,000 mile an hour shock wave. This tells Boatwright that some of the strongest shaking happened here. And this is 50 to 60 miles away from the epicenter near the Golden Gate. One result, gravestone makers here changed their materials.

Mr. BOATWRIGHT: It's nearly impossible to find a post-1906 marble headstone. After 1906, they went to lower and more squat granite headstones.

MCCHESNEY: Now, after either confirming or adjusting Lawson with his own research, John Boatwright has accomplished his assignment. He sits in his office in Menlo Park, California and clicks on a new digital shake map of the 1906 earthquake.

Looking at this map there's all these little tiny almost invisible squares superimposed. So each one of those squares has...

MR. BOATWRIGHT: Each one of those squares has an intensity associated with it.

MCCHESNEY: So take us to that little cemetery that we were in Graton.

Mr. BOATWRIGHT: Indeed, this is the intensity estimate which is intensity 8 to 9 for Graton.

MCCHENSNEY: That's on a scale from 1 to ten. Nearby Sebastopol was given the highest intensity rating for this quake, 9.5; Santa Rosa, nearly destroyed in 1906, was almost as high. The city of San Francisco, which was much closer to the epicenter, wasn't shaken as severely. And that's also true of other areas laying 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 using Boatwright's data and a brand new three-dimensional map of the earth's crust in Northern California.

Ms. MARY LOU ZOBACK (Geophysicist): The question is, where is the shaking going to be the strongest? Are there vulnerable infrastructure pipelines, things like that that we should be addressing and fixing now? And we know the answer to that is yes.

MCCHESNEY: Zoback says that 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.

Ms. ZOBACK: In the period leading up to 1906 there were lots of earthquakes in the San Francisco Bay area. So earthquakes were a part of daily life around the turn of the century.

MCCHESNEY: Then came the big one, followed by a long period of relative quake quiescence in Northern California. Zoback says the Lawson report was the first to recognize that over time strain builds up along a fault, followed by a big release. Now, there's some evidence that the fault is again being stretched to the breaking point.

Ms. ZOBACK: And 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 in 1989. So are we now beginning to move into another very active seismic period?

MCCHESNEY: 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 the one in 1989 which killed 62 people. A new report just released says a quake equal to 1906 could kill as many as 3,500 people and cause $150 billion in damages.

John McChesney, NPR News, San Francisco.

INSKEEP: You can see a shake map of the 1906 earthquake and see some of the places that John Boatwright visited in retracing the quake by going to npr.org.

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