Description: From Cameron and others, 2017: "Low-frequency events at Semisopochnoi Island were first noted on June 1, 2014. Semisopochnoi's seismic network had been returned to service on May 22, 2014, only 2 weeks prior to these events. The swarm began on June 9 at about 18:00 UTC (10:00 a.m. AKDT), and the AVO network recorded about 40 volcanic-tectonic earthquakes over the next 2 days. These earthquakes were small (less than M2) and clustered around Mount Cerberus in the center of the caldera. Depths ranged from 2 to 10 km (1.2 to 6.2 mi).
"Tremor was first noted on June 12, coincident with an increase in the rate of earthquakes recorded. Over the next day, the number of earthquakes doubled over the total located in the previous 3 days. The number of located earthquakes, all volcano-tectonic, doubled again on June 14, marking the peak daily earthquake count of the sequence (223 earthquakes). Throughout the increase in activity, the depth range of the events and epicentral area did not change. On June 13, AVO issued a VAN upgrading the Aviation Color Code and Volcano Alert Level to YELLOW/ADVISORY based on the ongoing swarm . Following the peak in earthquake activity on June 14, the number of located events decreased to 20-30earthquakes located in the last week of June, and further decreases in seismicity continued into July and August.
"On June 23, a M7.9 earthquake occurred in the region, 73 km (46 mi) west of Semisopochnoi Island at a depth of 118 km (73 mi) in the subducting slab. It was accompanied by more than 2,500 aftershocks, 60 with magnitudes of 4.0 or greater. There was no direct causality between the M7.9 earthquake and the Semisopochnoi volcanic-tectonic earthquake (VT) swarm; AVO did not note any change in the rate of VT seismicity at Semisopochnoi following the M7.9 earthquake.
"Zhong Lu of Southern Methodist University evaluated the deformation related to the volcanic unrest. More than 10 cm (4 in.) of inflation occurred between 2003 and June 26, 2014, about one-half of which occurred between June 15 and June 26, 2014. Maximum uplift was approximately in the center of the caldera. Modeled source depth ranges from 5 to 10 km (3 to 6 mi). (Zhong Lu, written commun., 2015).
"The consensus interpretation of the 2014 swarm is that it represents an intrusion of magma. This would be consistent with the style and character of the seismic swarm as well as the deformation signal. Although the seismograph network was not operational before May 22, the absence of significant activity between May 22 and June 9 suggests that the intrusion occurred in mid-June. An earlier pulse of magma possibly was emplaced before May 22, but without an operational seismograph network, this cannot be verified.
"In response to the swarm, AVO instituted heightened seismic watch schedules, requested that NOAA initiate HYSPLIT model ash cloud trajectory runs, and solicited pilots and the U.S. Coast Guard to report any unusual volcanic activity (such as steaming or ground cracks). Daily satellite analysis by AVO staff included careful examination of the volcano using all available imagery. Throughout the sequence, no surface activity was noted in the few clear-weather views of the island. The volcano is remote, and aerial reconnaissance by AVO staff was determined to be too costly to obtain.
"The 2014 unrest was the first significant departure from background at Semisopochnoi since installation of the seismic network in 2005. Background seismicity at Semisopochnoi had been relatively low (fewer than 50 earthquakes located per year until 2014), with short periods of weak seismic tremor. The last confirmed eruption at Semisopochnoi was from a satellitic vent in 1987 (Miller and others, 1998). The volcano, however, was deforming. Lu and Dzurisin (2014) discovered an area about 2 km (1.2 mi) in diameter that subsided during 2004-2010 at a rate of about 10 mm/yr (0.4in/yr). They suggested compaction of young volcanic deposits in the caldera as a mechanism to explain this subsidence; alternative hypotheses include localized subsidence due to depressurization of a shallow hydrothermal system or localized changes in the groundwater table."