From the Gutenberg-Richter law we know that the frequency of an earthquake is a power law, so virtually any magnitude is possible on earthquake event. But the earth has a finite size so there must be an upper theoretical limit on the earthquake's magnitude. What is this upper limit?
Because the largest earthquakes are caused by the motion of faults in the earth's lithosphere, the upper limit to earthquake magnitude is going to be related to the contact area between tectonic plates, not the size of the entire earth. Some geophysicists believe the magnitude 9.5 earthquake in Chile was likely close to the maximum size possible.