The earthquake swarm under Mammoth Mountain which started on February 3, 2014 has declined over the past few days. The daily numbers of earthquakes, however, are not yet at background levels. Most earthquakes were (are) M1.5 and less.
Earthquake swarm under Mammoth Mountain - Update
Many are less than 1.0. The largest event detected was a M3.1, which occurred during peak activity on February 5 (event was upgraded from reported M3.0 to M3.1 after further inspection of the data). At 5 AM PST on February 8, a second period of heightened intensity similar to that observed on February 5 started, which persisted for about 1 hour. During the hour of heightened activity on the 8th, four M2.0+ earthquakes occurred, with the largest a M2.4. A few folks on the mountain felt the M2.4. CalVO deformation monitoring sensors on Mammoth Mountain show no significant change. The swarm is likely caused by the migration of deep-seated hydrothermal fluids. The migrating fluid pressurizes and weakens pre-existing faults causing rock to crack, producing earthquakes. Preliminary measurements made last week suggest carbon dioxide emissions at Mammoth Mountain fumarole have not changed notably; additional magmatic gas data await laboratory analysis of samples to be collected by the USGS this week.