John Rundle is a professor of geology and physics.
He told Capital Public Radio's Insight that powerful aftershocks from Friday's quake are moving closer to Japan's capital. (CLICK TO HEAR INTERVIEW)
"The thing that's worrisome about this situation to me is that initially the earthquake of course occurred in the Sendai area, the main shock, and the aftershocks in the first hours were generally confined to that region. But in the past several days those aftershocks have migrated south towards Tokyo."
Rundle says an earthquake of magnitude nine is typically followed by one aftershock of magnitude eight, ten aftershocks of magnitude seven and many smaller aftershocks.
Rundle also says there is historical evidence of major earthquakes off the coast of Japan being followed by another similarly large earthquake nearby within a relative short period of time.