Alaska Volcano Observatory

From Coombs and others (2019): "Without a local seismic network, precursory seismicity was only recognized retrospectively. Wech et al. (2018) show that seismicity first occurred in September in the form of volcano-tectonic earthquakes, mostly on September 28-29 (swarm S1of Tepp et al. 2019). Wech et al. (2018) interpret these events as likely caused by magmatic intrusion into the middle to upper crust. A smaller earthquake swarm occurred in early October, with sporadic earthquakes continuing until the eruption in December (Tepp et al. 2019).
"From December 12 through March 13, explosions occurred at a mean rate of once every 58 h (2.4 days). Many of the explosions during this period were preceded by repeating earthquakes that accelerated into an explosion, characterized by increasing magnitude and becoming closer together in time, described by Wech et al. (2018) as 'slow-clap' seismicity and also described in Tepp et al. (2019) and Tepp and Haney (2019). On the basis of T-phase character, Wech et al. (2018) interpret these as occurring in the shallow crust.
"Seismicity remained at fairly low levels in October and November. In early December, days before the first detected infrasound, earthquakes with weak T-phases again suggested mid- to shallow-crustal magma movement (Wech et al. 2018). On December 12, the first infrasound signal from Bogoslof marked the beginning of the eruption. The signal, later recognized as event 1, was accompanied by a weak seismic signal, but no cloud was observed in satellite images. Infrasound for event 2, later the same day, has a relatively high frequency index (FI; ratio of high-frequency to low-frequency infrasound), suggesting a subaerial vent (Fee et al. 2019). On December 14, AVO received an email report from St. George, 308 km north-northwest of Bogoslof, about intermittent sulfur smell, likely corresponding to activity at Bogoslof. A Sentinel-2 satellite image taken minutes after event 3 on December 14 captured intense steaming from a subaerial vent and new pyroclastic deposits on the island and suspended in the surrounding ocean (Fig. 4a). Events 4 and 5 occurred on December 16 and 19. These first five explosions were not detected in real time, only after retrospective analysis of data streams. Thus, we have no direct observations of the character of these explosions.
"Satellite imagery beginning December 14 shows that uplift of Bogoslof Island occurred and may have been associated with cryptodome emplacement. A Sentinel-2 satellite image from December 21 (Waythomas et al. 2019a) shows an approximately 300-m long oval-shaped raised mass just north of Castle Rock (a remnant lava dome from the 1796-1804 eruptive period; see Fig. 3). This feature was not present in a Sentinel-2 image from December 14 (Waythomas et al. 2019a; their Fig. 5a). Preliminary analysis of DEMs generated by stereo satellite pairs shows that this feature continued to grow throughout the eruption (A. Diefenbach, written comm. 2019).
"No elevated surface temperatures were seen in satellite monitoring checks at Bogoslof until January 19, and neither the uplifted block nor surrounding areas were visibly steaming in December or January in a way that would be consistent with lava effusion (Figs. 3 and 6). Samples of the material exposed along the steep, eastward face of the uplift (see Waythomas et al. 2019b) are trachyandesite lava with 58-60 wt% SiO2 and are a different composition than the dominant juvenile basalt magma erupted in 2016-2017 (Loewen et al. 2019). Given the lack of thermal signature, as well as the composition of this feature, we suggest that this uplifted feature is likely the roof above a shallow cryptodome.
"Several pilots observed a volcanic cloud in the vicinity of Bogoslof on December 21 (Fig. 4b), and were the first alert to AVO that Bogoslof had become active. A pilot later the same day reported seeing 'a new land mass' in the island cluster. This event, event 6, began a period of activity that lasted a little less than a month, with explosive events 6-22 occurring at a rate of about one every 40 h. They produced volcanic clouds of heights up to 11 km, with variable durations (2 to 102 min), and about half produced detected lightning (Table 1). Most (80 %) consisted of single seismic pulses.
"Some of the only direct observations of activity occurred during event 9 on December 23, when observers aboard a Coast Guard vessel in the area reported ash emissions, lightning, and the ejection of incandescent lava and fragmental material that lasted about an hour. The cloud from event 16 on January 5 rose to 11.8 km asl and was observed by numerous pilots and mariners (Fig. 4c).
"A Worldview-3 image from December 25 (Fig. 6b) shows a bilobate submarine vent area and new pyroclastic deposits enlarging the island and beginning to enclose a vent lagoon. On December 29, data derived from US National Imagery Systems indicated a nearly complete ring of pyroclastic deposits around a more circular, submerged, ~ 450-m-across vent area. On January 10, passengers on a helicopter that flew from Dutch Harbor to Bogoslof took photographs and video of the island that showed discolored and roiling water in the crescent-shaped vent area, and the area of uplift from December (Fig. 3b). A Worldview-3 image from January 11 (Fig. 6c) shows continued growth of the island, and abundant meter-scale ballistics on the north and south ends of the island (Waythomas et al. 2019a, their Fig. 7).
"Satellite images, pilot reports, and ground-based photos show that event 23 on January 18 was the first demonstrably ash-rich cloud of the eruption sequence. Previous explosions produced clouds that were white in color with almost no ash signal in satellite data (Schneider et al. 2019).
"Prior to event 23, as with many other explosions on the first half of the eruption, seismic stations on neighboring islands picked up precursory seismicity in the form of repeating earthquakes that became more closely spaced in time during the runup to the explosion (Tepp et al. 2019). These lasted for about 11 h. Infrasound data show that the explosion itself lasted about 80 min and was pulsatory (Lyons et al. 2019b), with seven discrete bursts of strong seismicity (Searcy and Power 2019). Despite the event’s duration, only modest lightning was produced (14 strokes; Van Eaton et al. 2019).
"Pilots reports, visual satellite images, and thewest-facing FAA web camera in Dutch Harbor (Fig. 4d) indicated that the explosion produced a dark-gray ash cloud. MODIS satellite images show the cloud rose as high as 8.5 kmasl before drifting northeast over the Alaska Peninsula. An ash signal in the brightness-temperature-difference (BTD) satellite retrieval was seen along the leading cloud edge, suggesting that the cloud interior may have been opaque (Schneider et al. 2019). This event produced 1.9 kt of SO2 and was the first since event 13 on December 31 to produce more than 0.2 kt SO2 (Lopez et al. 2019).
"A mid-infrared MODIS satellite image collected minutes after the explosion showed a possible "recovery pixel". These occur when the sensor encounters a very hot object and saturates, suggesting lava or hot tephra must have been present above the water surface (D. Schneider, written comm. 2017). These were the first elevated surface temperatures detected by satellite imagery of the eruption. Clouds moved in to obscure the volcano soon after, with no additional views prior to January 20. Because event 23 was not detected on the Okmok infrasound array, we do not have infrasound FI analysis for this event, which for other events provides information on subaerial versus submarine venting (Fee et al. 2019).
"In the week following event 23, five explosions (events 24-28) occurred with an average repose time of 42 h between them (Table 1, Fig. 8). These events had total infrasound durations of 3-15 min, produced modest lightning, observable volcanic clouds, and each up to 0.6 kt of SO2 (Table 1). None showed a clear ash signature in satellite data.
"A Worldview-2 image acquired about 10 h after event 26 shows the island with a lagoon, open to the east, with two circular craters. The one to the northwest had upwelling within it, suggesting it was above the vent for the event 26 explosion (Waythomas et al. 2019a, their Fig. 8).
"About 84 h after event 28, the longest sustained explosion of the eruptive sequence produced significant ash and resulted dramatic changes to Bogoslof Island. Event 29 comprised more than 10 short-duration explosions that were detected in seismic, infrasound, and lightning data, took place over several hours on January 31, and produced several discrete volcanic clouds.
"The event started with no detected precursors, and activity escalated from 08:40 to 09:30, as indicated by increased seismic tremor and high amplitude infrasound signals. At 09:00, a continuous volcanic plume extended for a distance of more than 200 km towards the east-southeast over Unalaska Island at an altitude of 5.9 km asl. Event 29 produced 190 lightning strokes (Table 1; Van Eaton et al. 2019) and the most significant SO2 emission since December 22, 2016 (3.6 kt; Lopez et al. 2019).
"Tephra accumulation at the vent produced a demonstrably dry volcanic edifice for the first time during event 29. Data derived from US National Imagery Systems shortly after the event showed light steaming from an apparently dry eruption crater about 400 m in diameter and as much as 100 m deep below the west crater rim. Whereas most previous explosive events in the sequence, with the possible exception of event 23 on January 18, issued from a vent in shallow seawater, freshly erupted tephra formed an almost 200-m-wide barrier separating the vent from the sea. A Worldview-3 image from about 15 h later the same day (Fig. 6d) shows that the crater had already begun to fill with seawater. As with several of the events, large ballistic blocks were visible along the island’s north southwest shoreline (Waythomas et al. 2019a, their Fig. 9).
"Event 29 resulted in ashfall on Unalaska Island including trace (< 0.8 mm) amounts in the community of Dutch Harbor/ Unalaska. A sample of ash collected in Dutch Harbor comprises free crystals of plagioclase, clinopyroxene, amphibole, and rare biotite, as well as particles that display a range of groundmass textures from microlitic to glassy, and that vary from dense to vesicular (Loewen et al. 2019). The material is consistent with being a mixture of juvenile basalt scoria and non-juvenile lithics.
"Following event 29, a series of smaller explosive events occurred from February 3 to 20 (events 30-36; Table 1). During this period, the inter-event times became more variable, with some pauses of up to 9 days between events. Explosions during this time lasted minutes to a few tens of minutes, produced clouds that rose from 4.6 to 8.6 km asl (Schneider et al. 2019), 0-92 strokes of lightning (Van Eaton et al. 2019), and modest SO2 (0-1.4 kt; Lopez et al. 2019).
"A series of satellite images from January 31 through February 12 shows little change in the island’s morphology after event 29 (Fig. 6d-e; Waythomas et al. 2019a). Elevated surface temperatures detected in two MODIS images from February 6 likely reflected hot new deposits from event 31 on February 4. A high-resolution Worldview-2 satellite image from February 23 also shows little change except for the presence of ballistics particles ejected during events 32-36 (February 13-20) and distributed across the island (Waythomas et al. 2019a). A clear view from March 3 similarly shows only minor changes (Fig. 6f).
"The final event in this time interval, event 36 on February 20, was an excellent example of a Bogoslof explosion with a precursory seismic sequence (see Fig. 4 of Coombs et al. 2018). A classic sequence of coalescing earthquakes served as a prelude to the series of energetic eruptive signals that made up the event. Earthquakes were first detected at 20:42 on February 19. The sequence then maintained a relatively low rate until about 00:55 (February 20) when the rate suddenly increased to about 30 earthquakes per hour. The rate then progressively increased over the next hour almost merging to tremor by 2:00. Earthquakes ceased at 2:07 and after a1-min break transitioned to tremor. The eruptive signals consisted of about 9 blasts that were captured on multiple infrasound arrays resulting in a 40-min long explosion. The resulting plume reached 6.1 km asl and was elongated, stretching to the east-southeast over Unalaska Island. Pilots and observers on Unalaska Island at the time clearly observed the white, ice-rich cloud (Fig. 4e).
"After a 16-day pause, event 37 on March 8 lasted 200 min. It had the largest infrasound energy (Lyons et al. 2019a), seismic tremor magnitude (Tepp et al. 2019), most lightning (1437 strokes; Van Eaton et al. 2019), largest SO2 emission (21.5 kt; Lopez et al. 2019), and most significant ash cloud (Schneider et al. 2019) of any event in the eruptive sequence (though not the highest reduced displacement; Haney et al. 2019a). VIIRS satellite images showed the resulting cloud reached between 10.6 and 13.4 km asl and drifted east over Unalaska Island. Minor ashfall of a few millimeters was reported by a mariner near Cape Kovrizkha (northwest Unalaska Island; Fig. 1) who collected ash from his vessel. Like the ash sample from January 31, this ash contains particles of juvenile basaltic scoria and free crystals with minor amounts of what appear to be volcanic lithics (Loewen et al. 2019). A barely perceptible ashfall deposit was reported at Unalaska/Dutch Harbor.
"Event 37 also changed the shape of the island and temporarily dried out the vent area, as seen in a Landsat-8 image from March 8 (not shown). In infrasound data, event 37 shows a mix of low and high FI, consistent with an eruption from both submarine and subaerial vent(s) during this event (Fee et al. 2019).
"A March 11 WorldView-3 image (Fig. 6g) shows the west coast of the island grew significantly since March 3 (Fig 6f), with about 250m of new land west of the 1926-1927 dome. A new ~ 150-m wide vent was also observed on the north coast of the island, and ballistics ejecta clustered on the eastern side of the island (Waythomas et al. 2019a, their Fig. 11).
"On March 10 and 11, two multi-hour seismic swarms each produced hundreds of earthquakes as detected on station MAPS (Tepp et al. 2019), but neither led directly to an explosion. A few hours later, a precursory swarm (Tepp et al. 2019) began on March 11 and culminated on March 13 in event 38. This event produced a small cloud that reached as high as 4.1 km asl and drifted south-southwest.
"Following event 38 on March 13, there was a 9-week pause in explosive activity at Bogoslof. The only detected unrest observed during the hiatus was a swarm of volcano-tectonic earthquakes on April 15. The swarm lasted for several hours, comprised 118 detected earthquakes (catalog of Wech et al. 2018) with magnitudes between ~ 0.8 and 2.2, and is interpreted to reflect magmatic intrusion in the mid to upper crust because of the earthquakes’ weak T-phases (Wech et al. 2018).
"Satellite images from this period show the rapid surface reworking and erosion of new volcanic deposits on Bogoslof Island and coastline erosion by wave action (Waythomas et al. 2019a). Photos (Fig. 5a) and a Worldview-3 image from May 11 (Fig. 6h) show that the vent lagoon remained hot throughout the hiatus, evidenced by steam rising from the crater lagoon.
"From May 16 through August 30, AVO detected 32 explosive events at the volcano. In contrast to the events of December 2016-March 2017, few of the explosions in the later phase were preceded by detectable seismic precursors, inhibiting AVO’s ability to forecast eruptive activity (Coombs et al. 2018), though retrospective analysis of hydrophone data showed that weaker precursors were still often present (Tepp et al. 2019). Fewer events produced detectable lightning after event 40 on May 28 (Van Eaton et al. 2019). This second phase also included effusion of two short-lived subaerial lava domes.
"After a nine-week hiatus, Bogoslof erupted without detectable geophysical precursors on May 17 (event 39). This explosion lasted 200 min and produced an ash cloud that reached as high as 10 km asl and drifted south along the edge of a mass ofweather clouds, as seen in GOES satellite imagery (Schneider et al. 2019) and reported by pilots. Trace ashfall (< 0.8mm) was reported in Nikolski, Alaska, 123 km southwest of Bogoslof
(Fig. 1). As with ash samples from the previous two events, this one contains about 40 % free crystals, though the remainder of this sample is richer in juvenile scoria (as opposed to lithic fragments) than previous ones (Loewen et al. 2019).
"Following event 39, an oblique photo showed that the crater lake was breached with a 550-m wide gap along the north shore and that the northeast shore was extended by another 300 m from new tephra deposits (Fig. 5b). Eleven days after event 39, explosive event 40 occurred on May 28, also with no detected precursors. This eruption produced an ash cloud that rose to 10.1 km asl as shown in MODIS satellite images
(Schneider et al. 2019). The cloud drifted to the northeast and was reported by numerous pilots, including a report of 'sulfur' smell in cockpit from a plane about 800 km from Bogoslof. A Worldview-3 satellite image collected about 18 min after the start of the event shows the initial development of the eruption cloud (Fig. 4f; Waythomas et al. 2019a).
"These two explosive events, which occurred just after the hiatus, are among the most energetic of the eruptive sequence. They both produced appreciable SO2 clouds as detected in satellite data (9.4 and 7.7 kt; Lopez et al. 2019) and generated among the highest number of lightning strokes (647 and 719; Van Eaton et al. 2019; Fig. 8). The remnant SO2 cloud from event 40 on May 28 was still detectable over Hudson Bay, over 4000 km east of Bogoslof, on June 2. Of the 25 events analyzed by Haney et al. (2019a), the co-eruptive tremor of these two events had the highest reduced displacement of any in the sequence-both yield values of about 50 cm2, which is comparable to values calculated for eruption tremor from the largest eruptions in Alaska over the past 20 years (e.g., Redoubt in 2009; McNutt et al. 2013).
"Cosmo SkyMed radar imagery from May 31 shows a large portion of the north side of the island was removed during May 28 explosive activity, leaving a crescent-shaped bay, open to the north. This configuration of the island remained essentially intact through June 12 (Fig. 6i).
"As seen previously from a March 8 Landsat image, sometime following May 28, intense steaming recommenced from an area just southwest of the vent lagoon. This region, about 300 m in diameter, remained hot and emitting steam throughout the eruption and afterwards (Fig. 6i-l), and may have been the site of shallow magma intrusion or a filled-in vent area.
"Early June brought a series of small explosions and growth of a lava dome that breached sea level on June 5, and was then destroyed on June 10.
"Several hours after a swarm of very small earthquakes on May 31, event 41 was a 5-min long explosion that produced a small, water-rich cloud that reached as high as 7.3 km asl. Following this, cloudy weather prevented clear views of the volcano through June 4. A Sentinel 1-B SAR image from June 4 shows no dome in the crater lagoon (Fig. 7a). Midday on June 5, data derived from US National Imagery Systems indicate that a small protrusion of lava had breached water level immediately between the 1926-1927 and 1992 lava domes in northern portion of vent lagoon. By June 6, low-resolution satellite images show distinctly elevated surface temperatures at Bogoslof, suggesting that hot lava was at the surface (Fig. 8). Sentinel-1 SAR images show the growth of the dome from June 7 through June 9 (Fig. 7b,c). On June 7, data derived from US National Imagery Systems showed that the new dome was about 110 m in diameter. The dome was also seen in a COSMO SkyMed radar image from June 8 (Waythomas et al. 2019a; their Figure 15).
"During the interval of lava effusion, several small explosive events (42-47) occurred that did not disrupt the growing dome as shown by Sentinel-1 SAR data from June 9, which confirmed that dome was still there after event 47 (Fig. 7c). Several of the events (44, 45, and 47) have infrasound frequencies consistent with a subaerial vent (Fee et al. 2019).
"The June 5 lava dome was short-lived, as it was completely destroyed during a long, pulsatory explosive event on June 10 (event 48). This event started with discrete explosions detected on the Okmok infrasound array as early as 8:27 but intensifying from 11:18 to 11:38. Starting at about 12:16, activity transitioned into nearly continuous seismic and infrasound tremor signals for about 40 min. Shorter bursts of tremor continued
until 14:51, for an envelope of activity that lasted several hours. VIIRS satellite images of the resulting cloud showed it reached as high as 9.5 km asl and drifted to the northwest. Satellite data also indicated that at least part of the volcanic cloud was more ash-rich than many of those seen
previously in the Bogoslof eruptive sequence to date, suggesting that the eruption may have fragmented and incorporated the lava dome that was emplaced earlier that week (Schneider et al. 2019). This event generated 31 detected lightning strokes (Van Eaton et al. 2019).
"A Worldview-3 image from June 10, acquired after event 48, shows that the June 5 dome was no longer present (Waythomas et al. 2019a). Another, clearer Worldview-3 image from June 12 (Fig. 6i) showed ballistic blocks distributed uniformly around the island with the highest concentrations in the southeast and southwest sectors-likely remnants of the June 5 dome (Waythomas et al. 2019a). The FI of infrasound
from this event gradually decreases in the last hour of the event, suggesting a change from subaerial to submarine venting after the destruction of the lava dome (Fee et al. 2019; their Fig. 8).
"On June 13, event 49 comprised a series of four explosions that started at 01:44 and ended at about 04:34. Each pulse lasted between 10 and 30 min and generated volcanic clouds that rose to a maximum height of 3.8 km asl and dissipated within about 30 min. Residents of Unalaska/Dutch Harbor reported smelling sulfur, and winds were consistent with a source at Bogoslof. An additional 2-min long explosion was detected in seismic and infrasound data later on 13 June (event 50), with no detected ash cloud.
"There was an 11-day pause in detected explosive activity from June 13-24. During an overflight on June 22, sediment-laden water was visible in the open vent lagoon area, and the area of persistent steaming was visible just east of the December uplift area (Fig. 5c).
"Twelve explosive events occurred from June 24 to July 11 (events 51-62). These were generally short-duration, detected in seismic and infrasound data, and produced little or no lightning (Table 1). Several of these events were closely spaced groups of smaller events. A photo taken on July 3 shows the area of persistent steaming visible from behind the December uplifted block, but no activity at other areas of the island (Fig. 5d).
"Following an almost month-long pause, explosive activity resumed on August 7, with a 2-h long sequence (event 63; Table 1). Detected in seismic, infrasound, satellite, and lightning data, event 63 was longer lived than many of the events in the eruptive sequence and satellite images showed that ash from the eruption formed a continuous cloud that was carried by strong winds south over Umnak Island and then out over
the Pacific Ocean reaching an altitude of 10-12 km asl (Schneider et al. 2019). Event 63 produced one of the largest SO2 masses of the eruption, 5.8 kt, as determined by IASI satellite (Lopez et al. 2019). It also yielded the highest number of lightning strokes during the second half of the eruption (117; Van Eaton et al. 2019).
"As shown in aWorldview-2 image from August 8 (Fig. 6j), event 63 produced significant proximal tephra that expanded Bogoslof Island’s northern coastline and closed off the northfacing lagoon to create a crater lake in the vent region, perhaps even leading to a subaerial vent for some portion of this explosive event. This image also shows a large number of new ballistic blocks, primarily in the east-southeast sector of the island (Waythomas et al. 2019a, their Fig. 18). In infrasound data, event 63 shows a progression from low to high FI, consistent with a shift from submarine to subaerial venting (Fee et al. 2019).
"The final 2 weeks of the eruption were marked by mostly short-duration explosions and concurrent growth of a lava dome. Events 64 through 70 were mostly short-lived (6 min or less, except event 70 which lasted 59 min; Table 1), produced little or no lightning (Van Eaton et al. 2019), and modest SO2 (up to1.2 kt; Lopez et al. 2019). Volcanic clouds from the explosions rose to high altitudes (up to 8.7 km asl; Schneider et al. 2019) despite their short durations.
"A high-resolution Worldview-3 image on August 13 shows a vent region filled with seawater and no lava dome was apparent (Fig. 6k). On August 15, repeating low-frequency seismic events from Bogoslof were detected on Okmok and Makushin networks for about 8 h (Tepp et al. 2019). A photo from an overflight of the volcano on August 15 shows the area of persistent steaming visible since late May, but nothing at the site of the dome that would appear days later in the vent lagoon (Fig. 5e). If the August 15 seismicity was related to magma ascent, it had not yet risen shallowly enough to impact the vent lagoon area.
"A new lava dome was observed in data derived from US National Imagery Systems in the enclosed, water-filled crater on August 18 and grew to about 160 m in diameter and 20 m tall by August 22. A Sentinel SAR view shows the dome on August 20 (Fig. 7d). An oblique aerial photo taken on August 26 shows a vigorous steam plume that likely was generated as hot dome rock interacted with seawater in the vent lagoon area (Fig. 5f).
"SAR images from Sentinel-1 (Fig. 7e) and Cosmos SkyMed (Fig. 7f) on August 27, after event 66, suggest that most of this dome had been removed, with only some northern dome edge remnants remaining. The low-frequency infrasound associated with events 66-69 suggest that the vent was below water (Fee et al. 2019).
"Following the last explosive activity on August 30, there were a few earthquakes detected in seismic and hydrophone data (G. Tepp, written comm. 2019), but seismic activity quieted soon after. In August 2018, AVO added a telemetered seismometer on Bogoslof Island, which has recorded little activity of note.
"Weakly elevated surface temperatures were consistently observed at Bogoslof in low-resolution satellite images through November 2017. High-resolution satellite images from the fall of 2017 show steaming and discoloration on the island (e.g., Fig. 6l). As of 2019, continued erosion has changed the shape of the island (Waythomas et al. 2019b), similar to what occurred following previous eruptions."