Hmm, good question. It has to be far enough that the difference in travel time for the waves is noticeable. Loma Prieta was, what, about 70 miles from Emeryville? I doubt you could generally feel the difference at distances of less than 30 miles or so. But this quote from a Chron article pretty well describes what I'm talking about:
"First there was some up-and-down shaking, and then it went side-to-side, but it was nothing like '89,"
and he was in SJ, about 25 miles from the epicenter.