The DMC database contains event information from several catalogs. Magnitudes and locations are often contributed to a catalog by several sources. When you conduct a search of the DMC event database, you will see all of the catalog entries available (for the parameters set) unless the “preferred” catalog option is selected. Setting the preferred option results in the selection of only those entries from the most accurate catalog available for the time period requested. (See an explanation of these catalogs on the event search help page) However, selecting preferred does not always result in just one entry per event being displayed because there may be more than one magnitude reported for the same event. This can be frustrating if all you are interested in is the number of events. To help address this problem, the DMC has upgraded the SeismiQuery Event Search to include a grouping of events by location and time.
How Events are Grouped
Entries can be associated by event location and time parameters. These parameters can be changed by the user but the default parameters work in most instances. If you use the default values, all event entries that fall within 5 seconds of each other and are within 1 degree are considered to be one event. All the entries are listed and grouped by color (see example below). A total number of event entries and groups are listed at the top of the results page. In the example results below, there were a total of 1191 event listings but really only 699 individual events. It is not uncommon to have multiple magnitude or catalog listings for one event occurrence.
When “MakeEvent Map” is selected on the results page, one event per group is selected for the map. The event listing selected is the one with the largest magnitude for that group. If there are two listings with the same “largest” magnitude in a group, the first one listed is selected to be mapped.
by Deborah Barnes (IRIS Data Management Center)