17.5 SWARM, finding earthquake swarms

The program is used to identify seismic swarms in a catalog. Input to the program is a CAT file with many events and some manually entered parameters. Output is identified swarms. The output file swarm.out contains all swarms organized as 'events'. In the header line is given the center for area identified and the 'magnitude' is the number of events in the area divided by 10. The rest of the line is information from first event in swarm.

Principle of selection:

The area is divided into a lat-lon grid. Around each grid point, there is a cell with radius small_r. The program first checks the number of events in each cell for the whole catalog. It then checks each cell to find which has more than the minimum number of events to constitute a swarm under the condition that enough events are within the required time window. For each time window, with enough events, a swarm is declared so a swarm lasting e.g. twice the time window will be declared as two swarms. An additional condition is that the number of events is larger than the normalized background activity. The normalized activity is calculated as the activity in the large cell normalized for area to the small cell, and normalized in time to the window for the swarm.