Seismologists say an 8.7 magnitude earthquake that struck off the coast of Sumatra, Indonesia, late Monday was the largest of thousands of aftershocks to the devastating quake that struck in late December.

It was smaller than the 9.0 magnitude quake that was followed by a destructive tsunami three months ago. But seismologist Jill McCarthy of the U.S. Geological Survey says the quake that struck late Monday can also be classified as a "great earthquake."

"It occurred near the coast of northern Sumatra, about 205 kilometers west-northwest of a town called Sibolga," she said.

She says most aftershocks are much smaller than the initial temblor, but an aftershock of this size is not uncommon after a larger quake. Her agency monitors earthquakes, but she says it is not equipped to monitor ocean levels.

"However, an earthquake of this size and in this location could possibly generate a tsunami," she said.

The seismologist says aftershocks tend to diminish in number after a big earthquake, but the size of the aftershocks and their effect on the local oceans is unpredictable.