The rule of thumb for seismologists is that an earthquake's largest aftershock will be one magnitude smaller than the main shock, Caruso said. That means a 7.9-magnitude earthquake could hit the region even a year from now. Yet aftershocks are already hitting northern Japan now — 35 larger than magnitude 5.0 and 14 larger than magnitude 6 — according to the UGSS.
Big aftershocks are not unusual. In February, a 6.6-magnitude aftershock ruptured near Maule, Chile — almost a year after what is now the sixth largest earthquake in recorded history, a magnitude 8.8, hit in the same region.
The Japanese earthquake ruptured near the boundary between the Pacific and North American tectonic plates — huge, moving slabs of the Earth's crust. The quake was a megathrust earthquake, where the Pacific plate dove underneath Japan at the Japan Trench. The seafloor was pushed away from Japan sending waves roaring toward Hawaii and the West Coast of the United States.