ANCHORAGE - In a shift that's baffled scientists, the earth beneath part of Southcentral Alaska has been inching southward for the past three years.
Researchers say the discovery raises fundamental questions about the movement of the tectonic plates beneath the area. That's because the region is supposed to be moving northward.
"This event was completely unexpected," said geophysicist Jeff Freymueller of the Geophysical Institute at the University of Alaska Fairbanks. "We had no idea that the earth could do such a thing."
In 1998, scientists taking annual global positioning system measurements discovered that sites spread over about 5,800 square miles of Anchorage and the Matanuska-Susitna area had begun easing southeast. The movement was swiftest that year, gradually slowed and appeared to have stopped by summer 2001.
"We can see the effect of this from somewhere toward the end of Turnagain Arm to as far north as Talkeetna," Freymueller said. "Presumably, it's going to start moving northward again."
Freymueller and five other researchers presented the findings, titled "The Great Alaska Earthquake' of 1998-2001," last week at the annual meeting of the American Geophysical Union in San Francisco.
If this big creep had hit all at once - say, over a minute - it would have shaken Anchorage to its foundations, probably generating a magnitude 7 earthquake similar in strength to the one that struck the San Francisco area in 1989, Freymueller said.
To get some sense of energy involved, the 6.7 quake that struck Northridge outside Los Angeles in 1993 caused an estimated $20 billion in damage to 250,000 buildings and killed 61 people, according to The New York Times.
But no quake of that size has struck this area recently. And that's part of the problem.
"It's really a puzzle," Freymueller said. "The earth seems to have been behaving as though there had been a big change in stress, but if so, where did it come from? It's a big change in stress that you would expect from a significant earthquake, but there was no significant earthquake."
Understanding the mystery on the surface requires a brief look at the action about 20 miles beneath Anchorage. For eons, the North American tectonic plate has been slowly riding over the top of the Pacific plate, squashing it down into the earth under Alaska and the Aleutian Chain.
When the plates lock up and stop moving past each other, stress builds. When locked plates suddenly give way, the jolt shakes the earth. The slip on Good Friday 1964 generated a magnitude 9.2 quake, the second-largest ever measured.
The same plates have been locked up under Prince William Sound since at least 1993 and possibly since 1964, Freymueller said. So as the Pacific plate slowly descends into the earth toward the northwest, it pushes Anchorage and areas north of the city in the same direction. Through 1997, that surface movement was measured to be about two-fifths of an inch a year. Then it reversed.
So why did the land scoot southeast?
A partial explanation may be that a big chunk of Pacific plate - roughly the same portion that ruptured during the 1964 quake - suddenly sped up, causing the surface to creep in the opposite direction.
"For reasons that we don't understand, at least not fully, (the Pacific plate) just started moving much faster than it had before and is now slowing down to its original rate," he said. "It roughly doubled its speed. It went from moving along a couple inches per year to about four inches per year."
But geophysicists can't yet explain why this section of the Pacific Plate suddenly surged forward with no accompanying earthquake or other geological signs on the surface like uplift or an offset.
So what does all this say about Anchorage's earthquake risk?
"It's more puzzling than good or bad," Freymueller said.
"I think the good news is the likelihood of a '64 type earthquake right under Anchorage is rather small. But where you get the bad news is whether this sort of event might trigger another earthquake (farther away). At this point, it seems to be confined to this region."