A 3.O magnitude earthquake is very small.
Numerous small earthquakes can be a precursor to a big one.
The area produced 8.0 magnitude quakes, all occurring within a three-month period between December 1811 and February 1812. So, if it becomes active it might not be very small.
Yep, its barely measurable and certainly cannot be felt by humans unless they are right at the epicenter and it is shallow.
It would be scary though to live near the New Madrid fault, glad I live a long way from it in California.
You are more than correct. The maximum displacement of a 3.0 earthquake is 1 mm. I'm more that a little dubious of how they detected that 4 km down. Maybe they are referring to the surface displacement.