To the amazement of seismologists, they missed a magnitude-8.0 earthquake last year.

“It is sort of shocking,” said Thorne Lay, a professor of earth and planetary sciences at the University of California, Santa Cruz, who led a team that uncovered the seismic fingerprints months later. “If we hadn’t done the careful analysis, we would have missed it.”

Magnitude-8 earthquakes are known as great earthquakes for their ferocious destructive power, and networks of seismometers can pinpoint much smaller earthquakes.

In this case, however, the great earthquake was hidden by a greater quake.

The greater earthquake, of magnitude 8.1, struck Sept. 29, 2009, off the Samoa Islands in the Pacific. That in itself was a surprise to seismologists, because there have not been any known magnitude-8 earthquakes around the Tonga trench, where the Pacific plate slips beneath the Australian plate, leading seismologists to believe that the fault there is well lubricated and does not build up enough stress to generate great earthquakes.

Still, the two tectonic plates are speeding toward each other there faster than anywhere else in the world, some 10 inches a year.