The Government of the Day says there is no earthquake risk. AECL states that an earthquake would only cause a small poof of radioactivity that gives you a free Cat Scan. Who can argue with these grand statements?
AECL doesn't publish it's assumptions for the accident scenario, so we can't tell. I suspect it's rather benign, just as assuming the coolant boils away slowly, thus cooling the rods. GoD simply doesn't believe in earthquakes, period!