According the Bestiary, tremorsense senses everything touching the ground within range. No perception check needed, an earth elemental just goes "there is a dude behind this wall 20 ft away in that square".
However, in the Core Rulebook, it states that creatures with tremorsense get a +8 perception check bonus to pinpoint anything on the ground and automatically make (not succeed) such checks.
Which one is correct? I personally feel like tremorsense should not be blindsense with x-ray vision, but a lesser form of blindsense. I'm inclined to go with the Core Rulebook on this, feeling as though it should supersede all other rules.
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You don't normally need perceptions checks unless the other person is using stealth. I would say the same applies to Tremorsense. Unless the guy behind the wall is making an active effort to avoid being seen, Tremorsense automatically perceives him. If he is using stealth and not flying, the person with tremorsense has a +8 to his perception roll.