The Gerlach, Nevada M=4.1 is the second largest quake in the past month, part of a swam of 231 events near the Nevada-Oregon border. The swarm lies at the junction of the Warner Valley fault and the Guano Valley fault zone, neither of which is very active.
Swarms are most common in geothermal areas, such as the Geysers in northern California, the Salton Sea in southern California, and the Coso area in eastern California. The hot water tends to make faults more slippery, permitting them to creep, accompanied by sustained bursts of earthquakes.
The underlying cause of many of these geothermal areas are the stretching and thinning of the Earth’s crust in the Basin and Range province (whose northwest boundary is the brown curve in the Faulds et al., 2011 map), which brings the hotter lower crust closer to the Earth’s surface, where it heats, mobilizes, and mixes deep crustal fluids and shallow groundwater.