Very interesting methodology. When we did geophysics in Lake Ontario, we saw massively disturbed sediments. The earthquake has to be about M7 and above to do this, simply because you need a high PGV.
An M7 under Hamilton may have a return period of 1000 or 10000 years. Uncertainties here are a factor of 10, just like climate modelling. :) I tend to think in terms of 1000 years for an M7, and maybe 10000 years for M7.5. An M6 is almost 'every day', and they have popped off quite frequently in the neighbourhood. They don't do much, except to tall towers on swamp, a la Washington.
And yet, we are all happy to do nothing. La La I can't hear you! I don't want to be around to say 'I told you so' in the middle of an earthquake, just like I don't want to say that about 'climate science' when I am freezing to death. :)