If news or an event is earth-shattering, it is so surprising or so important that it feels as if the ground under you has broken. People use it for huge public news — a war, a fall from power, a discovery that changes a field — and also for a private shock that turns a life around. Though actually we stretch it a lot: headlines and adverts call quite ordinary things earth-shattering to make them sound bigger than they are, so listeners learn to take it with a pinch of salt. It is informal and a bit dramatic, more at home in speech and newspapers than in careful school writing. It does not mean the earth is really breaking; that would be an earthquake.