Brittle-failure earthquakes in the lower crust, where high pressures and temperatures would typically promote ductile deformation, are relatively rare but occasionally observed beneath active volcanic centers. Where they occur, these earthquakes provide a rare opportunity to observe volcanic processes in the lower crust, such as fluid injection and migration, which may induce brittle faulting under these conditions. Here, we examine recent short-duration earthquake swarms deep beneath the southwestern margin of Long Valley Caldera, near Mammoth Mountain. We focus in particular on a swarm that occurred September 29–30, 2009. To maximally illuminate the spatial-temporal progression, we supplement catalog events by detecting additional small events with similar waveforms in the continuous data, achieving up to a 10-fold increase in the number of locatable events. We then relocate all events, using cross-correlation and a double-difference algorithm. We find that the 2009 swarm exhibits systematically decelerating upward migration, with hypocenters shallowing from 21 to 19 km depth over approximately 12 hours. This relatively high migration rate, combined with a modest maximum magnitude of 1.4 in this swarm, suggests the trigger might be ascending CO2 released from underlying magma.