Can "Mega Quakes" really happen?
Theoretically yes, but realistically the answer is probably no. The magnitude of an earthquake is related to the length of the fault on which it occurs. That is, the longer the fault, the larger the earthquake. A fault is a break in the rocks that make up the Earth's crust, along which rocks on either side have moved past each other. No fault long enough to generate a magnitude 10 earthquake is known to exist.
Now for the history lesson - the largest earthquake ever recorded was a magnitude 9.6 on May 22, 1960 in Chile on a fault that is almost 1,000 miles long.
Scientists, however, can't rule out a 'Mega Quake' because they've only been measuring earthquakes for 100 years, which is a blink of on eye in geologic time. I also want to point out that the magnitude scale on which earthquakes are measured is open-ended, meaning that science has not put a limit on how strong an earthquake could be.