NASA Earth Observatory reports on an ongoing swarm of earthquakes near Hawthorne, Nevada. The quakes are concentrated over an area of about 10 square km (4 square miles) near Buller Mountain. Since April 10, 400 events with magnitude 1.0 and higher have been recorded. The most powerful with a magnitude of 4.6.
This map shows locations of earthquakes from March 5,2011 to April 27.
NASA has this to say about their origin:
Although the earthquake swarm has been adjacent to the Aurora-Bodie Volcanic Field, the earthquakes are tectonic in origin, not volcanic.
…The Basin and Range stretches from the Sierra Nevada in the west to central Utah in the east, and the complex topography is caused by the uplift, stretching, and thinning of the North American crust. The stretching crust results in the frequent earthquakes and occasional volcanism in the area.
The Nevada Seismological Lab maintains an up-to date map and list of the sequence.