By Jonathan Amos
Science correspondent, BBC News, San Francisco
The Great California Shakeout was a drill and not a real quake event
Tweets are being used by the US Geological Survey (USGS) to get instant public reaction to earthquakes.
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.
However, the survey stressed that the social networking tool would only ever supplement the existing scientific reporting systems which determine shake effects.
"It is a speed versus accuracy issue," explained Dr Paul Earle.
"Twitter messages start coming out in the seconds after an earthquake whereas, depending of the region, scientifically derived information can take 2-20 minutes," he told BBC News.
Dr Earle has been describing the Twitter Earthquake Detection project here at the American Geophysical Union's (AGU) Fall Meeting, the world's largest annual gathering of Earth scientists.
It seems that sharing their experiences is one of the first things tweeters are most keen to do following a quake.
For example, typical messages after a magnitude 5.1 event offshore of New Zealand earlier this month included:
• Wellingtonians - was that an earthquake or just a very big truck going past our building?
• that was a bigger earthquake the mirror shook in the bathroom and the floor moved ..... Scared :(
• Just had a Earthquake, biggest one ive ever seen, not huge, but enough to really shake our house and everything on my desk, good old NZ
• My monitors were shaking like the water in Jurassic Park, kinda awesome. #earthquake #wellington
"Twitter provides a stream that dumps out the tweets continuously," explained Michelle Guy, a software engineer working on the USGS project.
"We put a filter on that stream, looking for key words like 'earthquake' or 'quake'. We download it 24/7."
The USGS continuously collects, geo-codes (where the information is supplied, perhaps by a GPS enabled device) and stores the tweets.
When the national seismic network detects a quake, the new system then checks back to see if there was a significant increase in messages following the event.
If there was, it can send interested scientists emails that summarise where the tweets were coming from and the text from a sample of them.
The USGS development team concedes there can be a lot "noise" in the data stream.
For instance, the filter has had to be tuned to ignore references to the popular video game "Quake" and an ice cream known as the "Oreo Brownie Earthquake".
"Because there is a lot of noise in this data and we don't believe this system could ever be used to initiate a critical response such as shutting down a nuclear power plant, but what it may do is give us an initial heads-up in a region which doesn't have a dense seismic network that further scientific evaluation is needed," said Dr Earle.
A paper describing the project will be published soon in Seismological Research Letters.
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