Here is a quote I trust for you. The Geologists Haven't a clue themselves.

The Great Basin holds underground water, which does not drain to the Pacific but seeps underground. It is subject to mountain building, being on the western slope of the Continental Divide. Quake swarms occur most often in regions where there is underground water, which gets trapped and then later released as the rock layers slide during either stretch activity or mountain building activity. The water is essentially sloshing around underground, each time causing a minor quake. At times such activity will cause new lakes to form suddenly on the surface, or drain an existing lake. This is a factor of the N American bow, and the mountain building ongoing in the area, not magma movement.