Dr. Lucy Jones of the U.S. Geological Survey said 200 earthquakes of magnitude 5 or larger were recorded within the sequence, with 20 of them being foreshocks that occurred in the two days before the main shock on Friday.
Jones said on average, the largest aftershock in a sequence is 1.2 units of magnitude smaller than the main shock, which will mean a 7.7-magnitude aftershock in Japan's case.
"We've had one 7.1. We've had about 30 magnitude-6s. No one would be surprised if there's a magnitude-7 aftershock within this sequence," Jones said.
Authorities say Japan's strict building codes helped limit the damage and saved more lives from being lost.