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One user called Noldorin on the first page of weekly reputation points has a score of 74, even though he hasn't asked/answered a question since July. Maybe people automatically get reputation points in proportion to how much they already have?
The questions and answers a person posts can still get upvoted (thus earning them reputation) even if the person isn't active on the site.
There is no "automatic" reputation, but these sites are meant to provide a repository of good questions and answers which people can search for help; and when people find them often vote for them.