It generally refers to the strength of an Earthquake. So, 7.8 Earthquake is quite a poweful one. It releases about 7.56 Megatons of Enenrgy, or the equivalent of 7,560,000 tons of TNT. This is about 580 times more poweful that the atomic bomb dropped on Hiroshima by the US in the second WW2.
What is 7.8 magnitude
