Preliminary Seismic, Infrasound and Lightning Observations of the July 2008 Eruptions of Okmok Volcano, Alaska

Okmok volcano began to erupt July 12, 2008, following an 11 year hiatus. The previous eruption in 1997 was from Cone A whereas the new activity occurred on the north flank of Cone D, a structure that had not been active for 800 years. Seismic activity at Okmok is monitored by a network of eight short-period and four broadband seismometers. The eruption was preceded by a swarm of earthquakes lasting just 5 hours, with events large enough to be located only occurring in the last hour. The bulk of these events occurred under Cone D with a few near Cone A. The eruption began with the onset of continuous tremor at 19:43 UT, which increased abruptly at 19:48 UT and lasted about 12 hours, strongest at about 22:00 UT. The tremor was strong enough to appear on stations out to 260 km distance. The ash cloud quickly grew to an elevation of 16 km or more. Infrasonic waves from the eruption were recorded on the I53US infrasound array in Fairbanks as three groups of waves starting at 21:44 UT, 01:14 UT July 13, and 05:41 UT July 13 and lasting 29-95 minutes. The time of flight is estimated to be 94 minutes along a great circle path. The waves were strongest between 0.1 and 0.5 Hz and had amplitudes of 0.1-0.3 Pa at the array. Low-pass filtered broadband seismic data showed extremely long-period waves with a period of 540 sec starting at about 22:45 UT. However, these waves, which were also visible in GOES satellite images and are thought to be gravity waves, have not yet been found in the infrasound data. Vigorous lightning was observed in the eruption column by observers at Fort Glenn, 12 km from the vent, on July 12 and several occasions after that. Unfortunately no instrumental data were obtained for the lightning.