This one has a little bit of everything

For quite a while, the Did you feel it? link on the USGS web site has given the general public a chance to report earthquakes. This allows the seismologists to get a quick fix on the location and intensity of a quake before their instruments can produce more precise results -- seismic waves take time to travel through the earth.

This is a nice bit of crowdsourcing, somewhat akin to Galaxy Zoo, but it depends on people getting to the USGS site soon after they feel an earthquake. Some people are happy to do just that, but it's not necesarily everyone's top priority. So now the USGS has started searching through Twitter for keywords like "earthquake" or "shaking", and they're finding enough to be useful. The tweets range from a simple "Earthquake! OMG!" to something more like "The ceiling fan is swaying and my aunt's vase just fell off the top shelf," which gives some idea of magnitude.

As with Twitter in Iran, tweets are a great primary source of information, but you need to sift through them to get useful data. As with Google Flu, mining tweets doesn't require active cooperation from the people supplying the data. Rather, it mines data that people have already chosen to make public. In the case of Google Flu, Google is trying to use its awesome power for good by mining information that people give up in exchange for being able to use Google. (you have read Google's privacy policy, haven't you?) With Twitter, the picture is much simpler: The whole point is that you're broadcasting your message to the world.

It should come as no surprise that tweets about seismic activity are much more useful if you know where they came from (though even the raw timestamp should be of some use). Recently (November 2009), Twitter announced that its geotagging API had gone live. This allows twitterers to choose to supply location information with their tweets. The opt-in approach is definitely called for here, but even so there are serious questions of privacy. Martin Bryant has a good summary, to which I'll add that information about your location over time is a very good indicator of who you are.