Nearly I am no expert.
Richter magnitude scale - Wikipedia, the free encyclopedia
The table in that is quite good at giving you a feel to how much bang it is in relation to bombs.
You can see a 5 is about the same as a 480 kiloton nuke going off. But 4 is 15ktons
Now as the Nagasaki fat boy nuke had a yield of 21kton. To give you a feel for the numbers.
Logarithmic scales are a bit of a pain to get your head round. Its easier if you can relate to something else which you can get your head round.
And the reason for the negative I think is because of a spurious reading that confused it. Possibly something local to the sensor.
I think they have a quality method which involves cross referencing three sites which if they all agree about the power and where it is, it then gets recorded as happening. Which then removes the likes of that reading