The tripods emerging from the ground cause earthquakes. Earlier in the movie, we're given the exact magnitude, so this will be easy.
Enter 6.5 into the earthquake calculator and have you 3.5481338923358E+14 joules, or 84.802435285272466103 kilotons.War of the Worlds (2005) said:
Final Results
Tripods cause earthquakes = 84.802 kilotons