Simplify the following text for a Grade 8 native English-speaking person.
Original: Scientists later looked back to the hours before the eruption and noticed that several long-period earthquakes , which start out strong and then slowly die out , had occurred in the final hours before the eruption .
Simplified: Scientists later looked back to the hours before the eruption and noticed that several long period earthquakes had occurred . Long-period earthquakes are those which start out strong and then die down very slowly .