How strong are the strongest Ioquakes?
I did not find any Richter-scale numbers for Io, probably since there are no direct measurements yet, or how atmosphericprisonescape puts it:

Nobody put a seismometer on Io yet, so we don't know. The only other bodies except for Earth, where seismometers have been placed so far are the Moon and Mars.

But I found some interesting comparison at on NASA's SpacePlace: High Tideo on Io!

On planet Jupiter's moon Io (&quot;EYE-oh&quot;), the ground itself moves up and down like an elevator taking people to the top and bottom of a 30-story building!

This would mean amplitudes for the tidal waves (and therefore also for regular Io-quakes) of ${\rm 60 \dots 90 m}$! I am not sure though where that number comes from, but it compares with ${\rm 0.2m}$ tidal waves of the Earth's solid crust which is not really measurable on Earth, except with interferometers like e.g. GEO600.
References

Wikipedia on Tital Heating of Io
Are planet-destroying quakes possible or observable?