Nicholas,

Sutherland is valuable for several reasons in combination. Low seismicity is valuable not just because of not many quakes, but because even minor tremors can mess up images and readings, much as I could ruin your ability to take a decent picture by jogging your elbow. When one deals with the size and nature of telescope they have there, multiple instruments that combine as one, a tremor that some other large telescopes would not notice, could ruin readings that depend on keeping parts at relative alignments of a fraction of the wavelength of the light collected.

Also there is the lack of light pollution from cities (visit Sutherland some night near new moon to see what a really dark, starry night looks like! Marvellous!)

And there is still air and usually little cloud cover. "Good seeing" astronomers call it. And by and large steady temperatures, dry air and so on.

And, as you say, certainly, low seismicity...