If an earthquake's strength was 9.9 what would happen? 

The largest earthquake was a 9.5 event along the coast of Chile in 1960. The second largest was a 9.2 along the southern coast of Alaska in 1964. A 9.9 earthquake would be 4/10 of a unit larger than that of Chile, so it would release 10 raised to the power (1.5*4/10) = 4 times as much energy. The fault length would also be about 4 times longer, or 4, 000 km. Although we don't expect an earthquake of this size, one of the places it might conceivably occur would be along the Aleutian/Alaska subduction zone. This would require the entire zone to rupture in one earthquake, which we don't think has ever happened. Such an earthquake would both cause strong shaking for 5 minutes or more as well as generate a large ocean wave (tsunami) that would travel at 800 km/hour across the entire Pacific Ocean. This wave could would have low amplitude over the open ocean but would grow in height as it reached shorelines and could cause a lot of damage to near shore facilities and homes.