Ross Stein, Temblor
The two swarms are 20 km (12 mi) apart, and both lie on the west side of the Hilton Creek Fault, in what is known as the "Sierran Block." While the 13 August swarm was 3-5 km off the end of the Hilton Creek Fault and with a focal mechanism somewhat different than expected for the Fault, today's quake is directly on one of the fault strands, and has a focal mechanism consistent with tension perpendicular to the Fault. Today's quake was felt in the skiing, fishing, and cycling resort community of Mammoth Lakes, California.
On the basis of its length, the Hilton Creek Fault is easily capable of a M~6.5 earthquake. So, while the significance of these swarms is difficult to assess, this unfolding sequence suggests that the Hilton Creek Fault, or perhaps a section of the 'Sierran Block' that it bounds, is under stress, and so well worth watching.
Data from: Northern California Seismic System: U.C. Berkeley, USGS Menlo Park, and the Nevada Seismological Laboratory
Hill, David P., and Emily Montgomery-Brown (2015), Long Valley Caldera and the UCERF Depiction of Sierra Nevada Range-Front Faults, Bull. Seismol. Soc. Amer., doi:10.1785/0120150149http://temblor.net/earthquake-insights/second-earthquake-swarm-near-mammoth-lake-1174/