Simplify the following text for a Grade 5 native English-speaking person.
Original: Most of these earthquakes were magnitude 1.0 or less ; some as strong as magnitude 3.2 were centered 25 kilometers below the surface .
Simplified: Most of these earthquakes were magnitude 1.0 or less ; some as strong as M 3.1 or 3.2 were centered 25 kilometres below the surface .