When they examined the cores collected in Alaska they found that microorganisms, and the fauna, changed dramatically less than 15 years before that quake struck due to a relatively quick change in the water and mud along the high tide line. Thecamoebians, which live only in fresh water, disappeared, apparently because of the infusion of salt water as the coast subsided. Other changes in fauna and microorganism populations told the same story.
Looking farther, they found the same pattern preceding the other quakes. First the subsidence, and then the quake just a few years later.
But why subsidence before a quake?
"As the oceanic plate dives under the continental plate it pulls the continental plate down slightly and eventually it pulls it down so much it snaps," Scott says. "But what this precursor seems to be is sort of the bending before the snap. It bends a little, maybe for five or 10 years, and then it snaps."
The researchers think their findings could lead to an early warning system for subduction zone earthquakes.
"It's not a prediction," Lipps notes, because even if subsidence does occur, there's no way to know precisely when the quake is going to hit. But this research suggests it won't be long, probably less than 15 years, and possibly within just a couple of years.
Instruments, like tilt meters that reveal when the surface has tilted slightly, could provide a warning in time to do something about it. City planners, for example, are more likely to require reinforcement of public buildings if they have reason to believe a major quake will hit within a few years. They are less likely to take expensive precautions if the best scientists can do is warn them that quakes have happened in the past, and should be expected within the next few hundred years.
If it's coming soon, "you better get your stuff together," Lipps says.
Of course, all of this will require independent verification, and even if it turns out to be on the money it won't help some areas. The notorious San Andreas Fault in California moves laterally, for example, so there's no reason to believe subsidence occurs there before a big quake.
But some experts believe the Great Quake that is waiting in the wings won't be in California. It will most likely be along the Cascadia Subduction Zone, which stretches from Northern California to British Columbia. That one could kill thousands.
A few years warning won't stop it, of course. But at least it would provide the incentive to do all the things residents of that area should be doing anyway. Batten down the hatches.
Lee Dye's column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.