We analyze 153 bursts of earthquakes in southern California and Japan. The burst patterns are similar in southern California and Japan; they fill a spectrum between “swarm-like” sequences without obvious mainshocks and mainshocks with Omori-law-abiding aftershocks. In agreement with our previous work, the “swarm-like” sequences in Japan have more events, are more voluminous, and tend to expand with time, when compared to “mainshock-aftershock” type sequences. In both regions, we find that the sequences starting with their largest events tend to be much shorter in duration. Bursts within 50 km of volcanoes are similar in character to those elsewhere except they tend to have longer duration. We hypothesize that swarminess is a proxy for fluid pressure redistribution and/or aseismic slip driving the seismicity bursts, and conversely, the mainshock-aftershock-style sequences have end-member behavior that results solely from a cascade of elastic failures. The complexity of the spatial seismicity distribution does not correlate with the style of swarm observed, indicating that fluid conditions and composition are likely more influential than geometry in determining the patterns we observe.