Great earthquake shakes off theories.

On March 25, the world's largest earthquake in 4 years rattled the ocean bottom between Antarctica and Australia, a region so remote that the tremor went unfelt except by nearby penguins and other wildlife. The great quake, which measured magnitude 8.2, has stymied seismologists who are trying to understand a normally stable patch of the seafloor turned so trembly.

"It's really kind of a befuddling earthquake because it seems to violate a lot of the usual rules," says Douglas A. Wiens, a seismologist at Washington University in St. Louis.

Most giant earthquakes occur in distinct seismic zones, where two of Earth's surface plates scrape against each other. The March quake, however, struck within the Antarctic plate nearly 350 kilometers from the nearest border with another plate, says Wiens. Seismologists call these sorts of events intraplate earthquakes. The recent tremor was the largest intraplate quake ever recorded in the oceans, Wiens and colleagues report in the July 28 Eos, a publication of the American Geophysical Union.

The researchers raise a number of possible theories to explain the quake, but "none of the ideas are really that attractive," says Wiens. Some intraplate earthquakes occur because the ocean crust cools as it ages, but this process hadn't produced such a large tremor in the past. Researchers have identified other intraplate quakes as delayed after-effects of the last ice age; when the glaciers melted, the formerly weighed-down crust rebounded upward and put stress on the ocean floor. Wiens, however, rejects this idea because the March quake hit some 400 km north of Antarctica, a long way from the area that was depressed during the last ice age.

Compounding the mystery, seismic records show no evidence of any other earthquakes in the region going back to the 1960s.

Answers could come from more detailed studies of the seismic waves that crisscrossed the globe after the March quake, says Wiens. Researchers would like to send a ship to the region to probe the seafloor and set down temporary seismometers to record aftershocks, but the chances of getting to such a remote location are slim.