|Start:||March 7, 2013 ||Observed|
|Seismicity with no confirmed eruption: ||
|Eruption Type:||Not an eruption.|
From Dixon and others (2015): "No eruptive activity was reported at Okmok Volcano, but seismic and geodetic observations of note were made in 2013. Sporadic tremor episodes and three swarms of earthquakes caught the attention of duty personnel in 2013. Geodetic instruments within the Okmok caldera detected a mid-year pulse of inflation. The Aviation Color Code and Volcano Alert Level remained at GREEN/NORMAL throughout the year.
"On March 7, a 36-hour long swarm of over 1,000 low-frequency earthquakes was recorded on seismograph station OKTU, a station on Mount Tulik just outside the caldera (fig. 37 in original text). These earthquakes were too small to be recorded on adjacent stations and could not be located. The earthquakes formed two groups of earthquakes with similar waveforms (or earthquake families) that began at the same time, with the first earthquake family continuing for the duration of the swarm and the second family lasting for about the first 6 hours (fig. 38 in original text). Family 2 (short-duration family) contained larger events than family 1. After the family 2 (short-duration family) events ceased, the events in family 1 became larger and more infrequent than events earlier in the same earthquake family. Towards the end of the swarm, the event interval became more erratic, and the swarm ended abruptly.
"Beginning in May 2013, the geodetic network at Okmok detected a pulse of rapid inflation, one of the steepest rises at Okmok since the 2008 eruption (fig. 39 in original text). The spatial pattern of the deformation is similar to past inflation events at Okmok, and points to an inflation source beneath the center of the caldera. In a study of ambient noise correlations between the Okmok stations OKNC and OKCE, evidence was found of 0.2-percent decrease in seismic velocity during late August and September within the caldera, indicating a change in composition of the crust sampled by the ray paths (Matt Haney, USGS/AVO, written commun., 2013). The geodetic and seismic evidence suggests an infusion of fluid or gas and, although it is certain that this was a change, it is not clear whether this change was magmatic or hydrologic.
"A swarm of earthquakes began at 01:55 UTC on September 28 (17:55 AKDT on September 27) southwest of Okmok and northeast of Mount Recheshnoi in an active geothermal area. A second swarm occurred at Geyser Bight on October 9, forming a continuous zone of seismicity that extends from Recheshnoi towards Okmok (fig. 40 in original text). Neither swarm has led to eruptive activity and has continued to occur into 2014."