Vigorous swarms have occurred in the Pacific Northwest in 2001 near Spokane WA, in 2008 near Maupin OR and off the coast of Oregon, and in 2009 on the Hanford Nuclear reservation.
Swarms typically occur in the crust but do not follow the general pattern of an earthquake burst that begins with a main shock and is followed by numerous smaller aftershocks.
Earthquake swarms are responses to factors we can't see and don't yet have a direct way to measure.
In volcanic and geothermal zones, swarms might be driven by hot water or magma pushing fault seams apart or acting to reduce friction and enhancing the seismic activity in those areas.
Away from volcanic and thermal regions, it is unclear what triggers swarms that don't include main shocks and aftershocks.
It is possible that swarms are driven by tectonic movements so gradual that they take many minutes to weeks to unfold but still are much more rapid than normal plate tectonic motions.