There are three types of earthquakes- ones where the plates pull apart, ones where they push against each other, and ones called strike=slip earthquakes where two plates rub together horizontally. Those are the type that are the most shallow, but also cause the most damage because they happen closer to the surface of the ground and sometimes even tear the ground apart. I don't know for sure but it sounds like the New Madrid fault is having shallow quakes for that reason. A low magnitude e.q. can do a lot more damage than a high magnitude one if it is shallow. It sounds scary that the New Madrid quake was shallow because if there was a high magnitude e.q. there again like there was in the early 1800s it could cause extreme damage if it was shallow.