A circle or oval (depending of the incline of the fault) of small earthquakes about the same magnitude with a gap in the center. Named after a reasearcher named (not surprisingly) Mogi.

There is a lot of debate about whether or not such patterns are true indicators of impending quakes of larger magnitude in the center of the donut.

There is also some debate about how tight a circle it has to be to qualify as a Mogi.

Loose definition, but I hope it explains a little.

Cathryn