A record number of powerful aftershocks have continued to jolt the already battered prefectures of Miyagi, Fukushima and Ibaraki after the 9.0-magnitude March 11 earthquake.

The aftershocks have halted reconstruction efforts in towns wiped out by the quake-triggered tsunami, as well as efforts to stabilize the crippled Fukushima No. 1 nuclear plant.

Why do we have so many aftershocks? Why are they so large?