Starfleet assumed that it was just a seismic anomaly. You;d think that if there was a ship capable of getting off the planet the Enterprise wouldn't have been surprised when they arrived. Seeing as Nero has had his ship rammed once already by a less advanced vessel you'd imagine he's not too keen on letting anyone get into a position to do the same. Easier to imagine that any ship attempting to take off was destroyed. The Romulans managed to take out the rest of the fleet in just a matter of minutes. The Enterprise wasn't that far behind them. Wiping out some civilian ships would hardly even take a fraction of their weapon capability.