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Why do some earthquakes disappear from the map/list?

The USGS and networks contributing to the Advance National Seismic System (ANSS) take great effort to provide accurate and timely earthquake information. Occasionally our systems produce erroneous information that is released to the public via our web pages or Earthquake Notification System. These mistakes are generally promptly identified by seismologists, removed from our web pages, and “delete” e-mails are sent through ENS. The event webpage will have a note on it indicating that the event was deleted and in some cases the reason for the erroneous event and subsequent removal.

In the interest of rapidly providing earthquake information to the public, most of the information about earthquakes that occur in the USA is automatically posted to the web and ENS if it meets quality standards. There is a trade off between the speed of our earthquake notifications and number of false alarms in the same way that any kind of "breaking news" story may have substantial changes or corrections as more information is received. The faster we release earthquake locations and magnitudes, the more likely it is that the information may be erroneous. Experience demonstrates that imposing more restrictive quality standards prevents the release of legitimate earthquake information.

There are multiple causes of false earthquake reports. Automatic systems are particularly prone to errors following large earthquake when earthquake location algorithms misidentify reflected and refracted seismic waves created by a single earthquake. In this case, one earthquake can turn into "events" located in areas far from the earthquake. In other cases, noise in legacy analog telephone circuits that bring the data from seismometers to computers can be misidentified as earthquakes. Software optimized to locate local earthquakes by ANSS “regional” seismic networks in the USA occasionally may mislocate a large earthquake occurring on the other side of the Earth (e.g., China) deeply beneath the seismic network. Adding to this complexity, there are multiple seismic monitoring networks that contribute their earthquake locations and magnitudes to the ANSS system. These networks use different data and algorithms to locate the earthquakes, and sometimes the spatial separation of the contributed locations is so large that our systems interpret the independent solutions as distinct earthquakes of similar magnitude and location. In this situation, a delete message will be sent for one of the earthquake solutions but an earthquake did occur.

We are continuously improving our automatic systems and manual procedures to reduce the number of false earthquake reports. However, with the advent of rapid distribution methods like RSS/ATOM feeds and the re-distribution of our alerts through social media sites, our errors are more widely seen and more difficult to retract completely. For these reasons, it is very important to remember that this data is preliminary and users should check our web site for the most recent updates.