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Okmok reported activity

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EVENT SPECIFIC INFORMATION

Event Name : Okmok 2009/3

Start:March 2, 2009 Observed

Seismicity with no confirmed eruption: BibCard
Eruption Type:Not an eruption.
Other""

Description: From McGimsey and others (2014): After the 2008 eruption, "activity at Okmok remained at background levels until the morning of March 2, 2009, when a series of relatively strong, tremor-like events were recorded on the local seismic network. The largest tremor burst occurred at about 21:11-21:18 UTC. These events were recorded on average about once per hour during a 24-hour period, but were not detected on adjacent networks. A clear satellite view on February 28 revealed a 1-pixel thermal anomaly in the caldera near the location of Cone D, on the northeastern floor of the caldera. Clouds obscured satellite view of the area on March 2, and any surface manifestation accompanying the seismic activity went unobserved. In response to this abrupt, anomalous seismicity, AVO upgraded the Aviation Color Code/Volcano Alert Level to YELLOW/ADVISORY on the evening of March 2, 2009. A pilot flying low over the caldera on March 3 made observations and took several photographs. Several of the photographs appear to show an indistinct flowage deposit down the flank of New Cone [Ahmanilix], a 400-m (1,300 ft) high tephra cone (Schaefer and others, 2012) developed inside the caldera at the primary eruption site (see fig. 25 of Neal and others, 2011 for location). A clear satellite view on March 9 revealed an area of broad, but apparently shallow, slope failure on the west-northwest side of New Cone, which partly filled the pair of shallow pits located between the cone and the new lake west of Cone D. Material - likely pumice - also spread out into the lake but had been windblown to the eastern shore.

"Deformation (uplift) of the caldera floor began in August 2008 following cessation of the eruption. The GPS data from September 1, 2008 to March 1, 2009, show a linear progression of 9 cm of uplift in the center of the caldera (J. Freymueller and T. Fournier, UAFGI, written commun., May 3, 2009, AVO internal log entry 29436). Although seemingly rapid, the uplift was not considered unusual compared to the considerably more rapid inflation pulse that occurred in 2002-03 during a non-eruptive episode (J. Freymueller, UAFGI, written commun., May 3, 2009, AVO internal log 29436).

"Seismicity remained at low, to near background levels for the next several weeks, and satellite views showed no activity, thus prompting AVO to lower the Aviation Color Code/Volcano Alert Level to GREEN/NORMAL on March 20, 2009. No significant changes or activity occurred for the next 2 months. Then, on May 24-25, 2009, a series of tremor bursts were recorded, including a 15-minute-long high amplitude signal that registered across the entire Okmok network. The activity ceased almost as quickly as it began, and the network returned to near background seismic levels.

"Activity at Okmok remained at background levels through the remainder of 2009. The only point of interest was that satellite imagery in June compared with earlier imagery revealed extensive erosion of the 2008 eruption features within the caldera (J. Larsen, UAFGI, written commun., 2009, AVO internal log entry). Many of the collapse pits had become filled with erosional debris, the flanks of New Cone became heavily rilled, and the crater floor enlarged as sediment began filling and raising the bottom. The two lakes adjacent to New Cone also increased significantly in size."

From Neal and others (2014): "The rapid inflation [during 2009] slowed somewhat by the middle of 2010. In 2011, in inflation at Okmok continued, increasing over recent (2010) rates (5-6 cm over a 12-month period ending in September 2011), but still at a lower rate than was recorded following the 2008 eruption (M. Kaufman, UAFGI, written commun., September 7, 2011). In summary, the inflation at Okmok continues - albeit in pulses - with rates in the 5-7 cm/yr range (J. Freymueller, UAFGI, written commun., September 28, 2013)."

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