Hot fluids and yes, magma, are ubiquitous beneath the Great Basin and High Lava Plains. Unusually high heat flow across the region (this is the initial point of impact of the Yellowstone Hotspot!) is a major reason that earthquakes here occur in swarms rather than one big "one and done" event. The upper crust in this region is comprised largely of layers of basaltic material, interspersed with granitic bodies of various sizes which may govern the maximum magnitude (i.e. fault length/depth) of events here which generally range around M5.0. Around the larger (and perhaps cooler) granitic bodies, event magnitudes may occasionally reach M7.0+. The persistent "precursory" earthquake activity(beginning this past Spring), followed by the now "intense" seismicity is classic geothermal/magmatic-related earthquake activity. As mentioned by geologists/geophysicists in the press, while not exactly common occurrences, similar swarms have occurred in the region before,...and I must add that while none yet have culminated with surficial volcanic activity, magma/magmatic fluid injection likely accompanies most, if not all earthquake sequences in this region.

This guy 's comment sums it up nicely.