The U.S. Geological Survey agency monitors earthquake activity around the world. It reports the geographic location and intensity of an earthquake and anyone can subscribe to this data through Twitter, RSS feeds and KML files for Google Earth.
Then there are desktop software like Quake Alert that poll the USGS website and will pop-up an alert as soon as a new “big” earthquake is reported anywhere in the world.
The Twitter and RSS feeds will report earthquake activity worldwide but if you are only interested in receiving email alerts for your own geographic region, you should use the Earthquake Notification Service.
Here you have a Google Map and all you have to do is draw a custom polygon over regions that you would like to monitor for earthquake activity. You can mark one or more regions on the map, set the notification magnitude threshold (like only send me alerts for earthquakes with magnitude of 5 and above) and save your profile.
The USGS website says that information for U.S. earthquakes is generally available within 2-8 minutes but for earthquakes outside the U.S., it may take up to 20 minutes to report the activity. Twitter may thus be a faster medium to get earthquake news but as USGS says – “it cannot provide quantitative data such as location and magnitude.”