Surely people can get at the electricity powerlines "just by digging" too?
I'm not sure you understood the point. In what way is a 220V electric cable (those are the only ones they bury, but hell: I'll spot you the high tension lines if you like) or a truck (as jobeirne mentioned above) anywhere near as dangerous, as a box of post-fissile nuclear material?
Even discounting the possibility of terrorism, think of the possibility of poisoning the groundwater after an earthquake.
But it's probably a bad idea to have a nuclear reactor in an earthquake zone anyway, regardless of whether it's underground or not.