<i>If they’re getting reports of an earthquake in a populated area but no Tweets from there, that’s a good indicator to them that it’s a false alarm.</i><p>The other alternative is that the earthquake was so strong it destroyed all the infrastructure in the area, leaving people unable to even tweet. You have to consider all the failure modes carefully; an analogy that comes to mind is cargo smoke detectors in aircraft - if one shows a fire but the indication disappears shortly afterwards, there&#x27;s a chance the fire destroyed the cabling too.