In the late afternoon of 17 June 2013 a flurry of earthquakes started in the Long Valley Caldera east of the town of Mammoth Lakes, California (in an area known to geologists as the south moat).
Small Swarm in Long Valley 17 June 2013
The swarm produced about 100 earthquakes over 5 hours before petering out; most were too small to be felt by humans (a magnitude 3.0 was the largest). The earthquakes originated at ~ 8-7 km depths where a small volume of partially molten rock is likely to reside.
Earthquake swarms are common in this part of the caldera, especially so in the 1980s and 1990s. A particularly intense swarm in the latter half of 1997 produced 12,000 events over 7 months, including eight earthquakes in the magnitude 4.0 range. Significant ground uplift accompanied the 1997-1998 swarm resulting in ~ 10 cm of caldera inflation. Yesterday's "mini swarm" pales in comparison, and does not indicate any immediate volcanic hazard within Long Valley Volcanic Center. The plots to the right (click to enlarge) show cumulative earthquake counts in the south moat for the past year as well as the number of earthquakes that occurred during the 17 June swarm. Modest, relatively steady inflation of the resurgent dome, located in the center of Long Valley Caldera, is a feature of the last couple of years, but CalVO deformation monitoring sensors show no changes related to the recent swarm.