Sometimes scientists set up experiments, expecting a result — but they don’t get that result. These are called negative results. How an earthquake that didn’t happen is still advancing science?
In the 1970s, experts predicted a major earthquake on part of California’s San Andreas fault — near Parkfield — by 1986.
But 15 years after the predicted earthquake should have come, it still hasn’t come. Ross Stein is a geophysicist at the U.S. Geological Survey in Menlo Park, California.
Ross Stein: … the absence of a Parkfield earthquake has caused not only soul searching, but a new appreciation of the scientific issues that underlied the first forecast. The Parkfield prediction was based on what’s called “time predictable earthquake occurrence” …
The theory involves looking at how quickly the two land plates at the fault slip past each other. Experts have had to modify their theories to include the effect of nearby earthquakes.
Ross Stein: … some of us think that what likely kept that next Parkfield earthquake from occurring was another shock, the Coalinga shock in 1983, that occurred about 25 kilometers east of Parkfield and we think that earthquake had the effect of reducing the stress on the Parkfield segment. …
Now, Stein says, the effect of Coalinga might be over. Scientists still expect an earthquake at Parkfield soon. The site is closely monitored by researchers, so even if it happens 20 years from now it’ll be worth the wait.
Scientists were eager to study this spot for clues that might help them predict earthquakes.
The prediction was based on the fact that large earthquakes seemed to start at this spot about every 30 years. Ross Stein says, “the absence of a Parkfield earthquake has caused not only soul searching, but a new appreciation of the scientific issues that underlied the first forecast. The Parkfield prediction was based on what’s called ‘time predictable earthquake occurrence,’ which is that if you know how much the slip occurred in the last earthquake and you know the long term slip rate of the fault, you can figure out how long you should have to wait until the next one occurs and that has turned out not to be true here.”
In other words — if the two land plates that meet at the fault were pushing against each other at a steady speed — pressure would build up steadily until it reached a certain threshold — and the fault would snap. The pressure would be relieved and the plates would again start pushing against each other, starting the cycle over.
Earthquake researchers are more cautious these days about making predictions about Parkfield. But Stein says, “it still makes sense to park on this spot and wait it out. If we wait 20 years, we’ll get a bigger shock and we’ll learn more from it. If it happens tomorrow, it’ll be, let’s say a six and a half and we still have a fabulous instrumentation package that’s going to record it.”
Ross Stein pointed out that he doesn’t personally have instruments at the Parkfield site. And he won’t have his own instruments in the upcoming deep drilling project. Instead, he analyzes data that others collect.
In a few months, a team of researchers headed by Bill Ellsworth of the US Geological Survey hope to begin drilling a hole more than 4 kilometers deep into the fault north of Parkfield. The so-called SAFOD project is expected to take four or five years. When completed, instruments will be placed at a part of the fault that tends to produce small earthquakes every year or so — but is still not too far from the spot in Parkfield where the big quakes originate every few decades. Scientists will also sample rocks and fluid from the fault zone.