I think the 1960 event was two different main shocks / earthquakes, not a fore- or after-shock. But I can't find a reference offhand - the first one became at best a footnote after only eight hours when the 9.5 hit.

The one good thing about having two in 1960 - all of their responders had gone to their stations to deploy to the first quake by the time the second occurred - they were already near their equipment. Also, anyone close enough to feel the first was already out of and away from buildings when the 9.5 happened.