Your probability is just what percentage of global earthquakes occur in vs out of these bands. What we want is the probability of a M5 within these bands in any 5 day period.
so:
6746 quakes in 10 years
=6746 quakes in 3652 days.

almost 2/day, so almost 10 in 5 days. That seems like a lot-did I do that right? I've seen a lot of posts claiming triggering for M5 quakes (for example) on this page, but it is interesting that we expect that many by chance.
Chris