There were lots of swarms like the Tres Pinos swarm you mention, some were
not listed - like the Mammoth Lakes swarm of 42 events on 29 June at exactly
the same time as the Yellowstone swarm. These were the largest swarms in
both places in more than a year. Also not mentioned was the Spokane swarm
which started on June 25. The Ml > 3 earthquakes are therefore the tip of
an iceberg representing the Tres Pinos, Rohnert Park, Truckee, Big Bear, Gilroy,
Spokane, Mammoth Lakes, Yellowstone and Los Banos swarms among others
which were not mentioned because they did not contain an event of M>3. These swarms of
earthquakes began on June 24, the day after the Mw 8.4 Peru earthquake. Seems
a good place to point a finger, if you are looking for a starting point.