September 27th, 2014-
USGS-September 2014 – CALIFORNIA – The Long Valley Caldera is experiencing a large seismic swarm. As magma moves through the earth, it displaces and fractures rock along the way. This movement causes earthquakes that can be recorded with seismometers at the surface of the earth. Seismic monitoring is the most used technique for volcano surveillance. Volcanic earthquakes often provide the initial sign of volcanic unrest. Their signals differ from typical, tectonic, earthquakes because they tend to be found at depths shallower than 10 km, are small in magnitude (< 3), occur in swarms, and are restricted to the area beneath a volcano. Harmonic tremor, or volcanic tremor, is the name for the continuous, rhythmic seismic energy associated with underground magma movement. At Long Valley Caldera, there are currently 61 seismometers that make up the seismic network used to determine earthquake location and energy of movement with time.
The first instrument was installed in 1974 and additional instruments were added throughout the 1980s and 1990s. Between 2000 and 2003, the seismic network was updated to include additional, more modern instruments. More than 200 more earthquakes have erupted in the area in a 24 hour period. Additionally, some earthquakes were now reported at shallower depths. Rodger Wilson, who is following this area for tens of years, hasn’t seen this activity since the 1990′s! We have the impression however that the frequency of the earthquakes has seriously declined the last couple of hours. The seismicity at Mammoth Lakes California has even increased compared to this morning. Below all earthquake epicenters during the last 24 hours. Depth of the hypocenters still at +10 km.
Earthquake swarms are a regular phenomenon at Long Valley but nobody knows where the magma will move next. We will have to wait and see if this latest swarm indicates a massive movement of magma and might be an early-warning sign that Long Valley might be moving towards an eruption. The last eruption at the volcano is said to have occurred 700,000 years ago and is long over-due. –ER, USGS