 A young geologist named Kerry See started work that would lead to a new approach to earthquake prediction based on the prehistoric record. The theory of plate tectonics told him that the buildup of strain along the fault was steady. Perhaps there was some regularity in the release. Another 10 meter earthquake might take place when another 10 meters of strain had built up. Kerry needed to figure out the rate at which the strain was accumulating. A stream that had been moved off course by a series of earthquakes offered a tantalizing clue. We recognize that this channel at one time flowed across the fault in a straight line, but it has been offset now 130 meters so it now has a dog leg in it along the fault and then continues to flow out into the valley. If Kerry could determine how long ago the creek ran straight across the fault, then by simply dividing the distance it had moved by the number of years it had taken, he would arrive at the rate of strain accumulation. The history of the now dry stream bed could be read in the way that sediments had been deposited over the centuries. A trench 12 feet deep has been cut across the fault. A sedimentary record of the past 3,000 years is exposed. Each layer is identified and marked. Breaks in the layers are the result of earthquakes. Eventually samples will be collected and earthquakes dated. Individual earthquakes can be identified with the help of a diagram, like this one made at a site called Pallet Creek. As the sediments have been accumulating over the past two millennia, they've recorded the story of the San Andreas Fault. The reason they've done that is that every hundred or a couple hundred years the San Andreas Fault breaks and disrupts these sediments as you can see here. Now let's look for example at this particular place. This layer was the ground surface in 1480 AD, about the time that Columbus was discovering America. So you can imagine a little Native American boy standing here about four feet high on this old marshy, boggy ground. If he had been standing right here, what he would have seen looking out this way is the ground suddenly dropped and shifted this way during the great earthquake. What formed was a scarf about a foot high, or about 30 centimeters high here. You can see this layer has been broken off from its continuation over here. In the succeeding 100 or 200 or 300 years, these layers of sand and silt and peat accumulated and buried that fault scarf completely. So here we have a beautiful example of a buried fault scarf produced by a paleo earthquake or an ancient prehistoric earthquake about the time that Columbus was setting foot in North America.