Alaska Volcano Observatory

From McGimsey and others (2007): "After almost 4 quiet months, on January 4, 2005, AVO received a pilot report of small bursts of ash from the active cone rising a few hundred meters and drifting east, producing a narrow spoke-like deposit on snow within the caldera (see figs. 31 and 32 in original text). This activity seemingly correlated with a period of continuous tremor recorded on the local seismic network that day, and a weak thermal anomaly was detected in an AVHRR satellite image. AVO upgraded the Level of Concern Color Code for Veniaminof from Green to Yellow. AVO seismologists noticed that weak seismic tremor had begun on January 1 and increased over the subsequent week to levels last observed in May-June 2004. Steam and ash emissions continued from the next several days and residents of Perryville, located 35 km (22 mi) south of Veniaminof, reported incandescence; the caretaker a ta local hunting lodge located west-southwest of the volcano reported seeing intermittent bursts of steam and ash. Beginning on January 8, a persistent thermal anomaly began appearing in satellite images. Then, on January 10, following nearly 48 hours of minor but nearly continuous ash emissions - some bursts reaching to 13,000 ft (3,692 m) above sea level - AVO raised the Level of Concern from Yellow to Orange. The maximum amplitude of the seismicity had by then slightly exceeded that observed during the previous phase of unrest, which ended in September 2004. AVO launched an overflight on January 11. The crew observed nearly continuous low-level ash and steam emission from the central cone and much of the caldera was thinly covered in ash (see fig. 33 in original text).
"Seismic data, web camera views, and satellite images indicated that low-level ash emissions continued for the next 5 weeks. The seismicity was characterized by low-amplitude tremor with occasional larger bursts. Weather permitting, satellite views showed anomalous heat at the summit cone consistent with hot blocks and ash ejection from the vent. The web camera showed intermittent ash clouds with the highest reaching almost to 13,000 ft (4,000 m) ASL. Strombolian eruptive activity was visible to the residents of Perryville during the night of February 3. Then during the week of February 25, seismicity decreased substantially and only minor emissions of steam were observed. AVO reduced the Level of Concern from Orange to Yellow. By the end of the following week, volcanic tremor had subsided and seismic activity was deemed to be at background levels and the Level of Concern was reduced from Yellow to Green (see table 6 in original text).
Excerpts from AVO's information releases during this Veniaminof eruption are available online at: http://www.avo.alaska.edu/archives/veniaminof2005/
The Smithsonian Institution's Global Volcanism Bulletin (v. 30, n. 2) summarizes the event as follows: "AVO raised the Concern Color Code at Veniaminof from Green to Yellow on 4 January because around that time several small ash emissions from the volcano's intracaldera cone were observed on the web camera in Perryville. Ash emissions were visible starting around 0938, but may have been obscured by meteorological clouds in previous images. The discrete ash emissions were small, rose hundreds of meters above the cone, and dissipated as they drifted E. Minor ash fall was probably confined to the summit caldera. Very weak seismic tremor was recorded beginning on 1 January, and increased slightly over the next 2 days. These seismic signals were similar to those recorded during steam-and-ash emissions in April to October 2004. However, there were no indications from seismic data that events significantly larger than those observed around 4 January were imminent.
"AVO raised the Concern Color Code at Veniaminof from Yellow to Orange on 10 January as ash emissions from the volcano's intracaldera cone reached heights of nearly 4 km during 8-10 January [see figure 11 in original online source]. Seismicity remained at elevated levels and satellite images showed a persistent thermal anomaly at the intracaldera cone. On 11 January, the Anchorage VAAC again reported emission of a thin ash cloud to ~ 3 km altitude visible on the Perryville web camera. On 12 January the Anchorage VAAC reported emission of a thin ash cloud, visible on the Perryville web camera, that rose to 3-4 km altitude, extended ENE, and dissipated within ~ 55 km of the volcano. On 14 January, a satellite image showed a thermal anomaly in the vicinity of the Veniaminof summit. Although the anomaly appeared less intense than when first detected on 8 January and volcanism seemed to have declined significantly since 12 January, activity still remained significantly higher than normal with occasional bursts of volcanic tremor.
"During the rest of the month of January, seismic data, web camera views, and satellite images indicated that low-level ash emissions continued at Veniaminof. Seismicity was similar to levels observed during the previous week, consisting of low-amplitude volcanic tremor with occasional larger bursts. During clear weather, satellite imagery showed anomalous heat at the summit cone, consistent with hot blocks and ash being ejected from the active vent. In addition, the web camera showed intermittent ash plumes reaching as high as 3 km altitude. Occasional stronger bursts of seismic tremor during 20-21 January and around 28 January may have indicated plumes to higher levels, but not above 4 km altitude. Veniaminof remained at Concern Color Code Orange.
"Activity during February 2005. On the evening of 3 February, Strombolian activity at Veniaminof was visible by residents of Perryville ~ 30 km from the volcano. Activity was also observed on web camera views and seen by satellite as an increase in radiated surface heat. An increase in seismicity suggested that Strombolian activity may have continued through 4 February while the volcano was obscured by clouds.
"During 28 January to 4 February, seismicity at Veniaminof was similar to levels for the previous week, with low-amplitude tremor and occasional larger bursts. During clear weather, satellite imagery showed anomalous heat at the summit cone, consistent with hot blocks and ash being ejected from the active vent. The web camera showed intermittent ash plumes reaching as high as 3 km altitude. Veniaminof remained at Concern Color Code Orange.
"Low-level Strombolian eruptive activity continued at Veniaminof during 4-11 February. On 9 February, an ash burst rose hundreds of meters above the intracaldera cone. Satellite images continued to show a thermal anomaly in the vicinity of the intracaldera cone, consistent with the presence of hot material at the vent. Seismicity remained above background levels at the volcano. On the morning of 10 February there was a distinct increase in the amplitude and frequency of earthquakes. The increase continued through 11 February. This activity was consistent with more energetic explosions from the active cone, but there were no indications that the bursts rose higher than 4 km altitude. Veniaminof remained at Concern Color Code Orange.
"During 11-18 February, it was likely that low-level Strombolian eruptive activity continued at Veniaminof based on seismic data and satellite imagery. Cloudy conditions obscured web camera views of the volcano, and no ash emissions were observed above the cloud cover. Seismicity remained above background levels at Veniaminof. The character of the seismicity changed slightly during the report period, with frequent periods of continuous banded volcanic tremor occurring, but the amplitudes of earthquakes did not increase. This activity was consistent with explosions from the active cone; however, there was no indication that these bursts rose more than 4 km altitude. Veniaminof remained at Concern Color Code Orange.
"Seismicity decreased substantially at Veniaminof during 18-25 February in comparison to previous weeks, leading AVO to decrease the Concern Color Code from Orange to Yellow. Periods of volcanic tremor diminished, and no discrete events associated with ash bursts had occurred for several days. Only minor steam emissions were seen. AVO received no reports of ash emissions from pilots or ground observers. AVO concluded that given the decline in seismicity, it appeared that the most recent episode of Strombolian eruptive activity at Veniaminof had ended.
"Activity during March 2005. A further reduction in activity at Veniaminof during 25 February to 4 March led AVO to reduce the Concern Color Code from Yellow to Green, the lowest level. For more than a week seismic activity was at background levels, periods of volcanic tremor had ceased, and there were no discrete events associated with ash bursts. Only minor emissions of steam were observed on the web camera and satellite imagery. AVO received no reports of ash emissions from pilots or observers on the ground. They concluded that given the decline in seismicity it appeared that the most recent episode of eruptive activity had ended at Veniaminof."