When there's a gigantic earthquake, why doesn't the shaking start a cavalcade of earthquakes worldwide?

The answer is pretty simple if you think about it. It's happened before but of course it wasn't world wide. The deeper and more powerful an earthquake is the more likely it is to trigger more earthquakes. One earthquake creating havoc on the world isn't among us. It's in the skies. If for some reason a large astroid struck earth it would most likely do something similar to triggering just about every fault that is ready to be triggered worldwide!