Description: From Cameron and others (2020): "AVO seismologists noted seismic tremor at Okmok Caldera (herein called Okmok volcano to include associated volcanic features exterior to the caldera) in September 2016. This activity did not lead to significant unrest, and the Aviation Color Code and Volcano Alert Level remained GREEN and NORMAL, respectively. Tremor episodes began on September 19 and were frequently noted during the following week. Tremor episodes occurred at a rate of 5-10 per day and were detected on seismic stations within the caldera but not on those located outside the caldera. The locatable tremor episodes occurred in the east side of the caldera, and relative lag times suggested a source near Cone D. Although depths were not well constrained, they often seemed to be shallower than 5 km; however, occasional and more broadly occurring events seen across the network often had depths of greater than 25 km in the same region of the caldera.
"Inflation coincident with the tremor was not initially noted with the GPS network, but re-evaluation found a westward motion on GPS sites OKCE and OKNC. This motion is not consistent with the typical inflation signal at Okmok volcano (Jeff Freymueller, University of Alaska Fairbanks Geophysical Institute, written commun., 2017). Seismic and infrasound alarms were crafted for the volcano to assist with monitoring, but no further activity ensued. Pulses of tremor were sporadically noted through October, and by early November the instances of tremor were no longer observed."