This report from the BBC says that
The agency is trawling the messages to find out what people felt during a tremor - whether there was a lot of shaking in their area or not.
There are big spikes in Twitter traffic immediately following a quake and the USGS believes emergency responders might find the information useful.
It could help them assess very quickly the severity of a particular event.
the survey stressed that the social networking tool would only ever supplement the existing scientific reporting systems which determine shake effects.