Earthquake severity during supereruption at Yellowstone caldera?
Volcanic earthquakes aren't actually all that strong. That's quite easily understandable by just looking at length scales: In the largest earthquakes that happen on earth, tens of kilometers of subduction zones rupture at once, releasing the stress that has built up from two plates moving past each other (but being locked) for decades. 

Even large calderas cannot produce faults of this length that are sufficiently long and linear and can rupture in the same quake. Consequently, earthquakes in volcanic zones are usually much smaller than the largest ones we experience on earth.

The global effects of supereruptions are not due to earthquakes or tsunamis, but due to the gases, ash and sulphur that ends up in the atmosphere and that leads to global cooling for several years.