When we find a bogus event, usually by studying the seismograms, we delete it and careful observers may notice that an earthquake has disappeared. This often happens after a large earthquake when our systems don't realize that all of the seismograms were created by a single event. In other cases problems in our telemetry systems that bring the data from our seismometers to our computers create glitches that also can create bogus events. For these reasons it is very important to remember that this data is preliminary and when events disappear they weren't real to begin with. Seismologists evaluate the hypocenter location and the focal mechanism of an earthquake to decide if the earthquake occurs on a named fault.
Research shows that many earthquakes occur on small, un-named faults located near well known faults.
Loma Prieta earthquake occurred on small, subsidiary faults within a few hundred meters of the mainshock rupture plane. On other fault segments like the San Andreas fault near Parkfield, most of the earthquakes occur on the San Andreas fault. It is difficult to automate this decision process, and it would be misleading to imply on the basis of only distance that an earthquake occurs on a named fault.