The 2017-19 activity at Mount Agung in Bali (Indonesia): Intense unrest, monitoring, crisis response, evacuation, and eruption


In the course of the fifty years for the reason that 1963 eruption, virtually no native earthquakes have been recorded on the CVGHM community at Mount Agung, and seismic power was dominated by cultural noise from the south flank of the mountain. As of 2017, the seismic monitoring community consisted of two short-period stations on the south and southwest flanks of Mount Agung ~4 and 5 km from the summit and 4 short-period stations within the Batur Caldera (Fig. 1A). All through the disaster, the first information streams used to watch unrest have been real-time seismic information from the CVGHM community and earthquake hypocenters from the Indonesian Meteorology, Climatology, and Geophysics authority (BMKG).

The CVGHM community was used to make visible observations, conduct each day earthquake counts, and compute RSAM (Actual-time Seismic Amplitude Measurement). Though many hypocenters have been additionally manually computed utilizing the CVGHM community throughout the disaster, these have been used primarily to confirm and complement BMKG options and weren’t constantly catalogued. The outline of exercise under is a quick abstract of the noticed seismicity from all information sources.

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A swarm of earthquakes (M2.3-3.9) was recorded in mid-Could 2017, situated NW of the Batur caldera, with a most reported depth of MMI III. After a number of months of gradual will increase, earthquake charges and seismic power elevated quickly between 16 and 22 September 2017 from tens of earthquakes per day to lots of of earthquakes per day (Fig. 2). Felt reviews and seismic-wave-arrival occasions on native stations recommended that the noticed volcano-tectonic (VT) earthquakes have been situated between Mount Agung and Batur Caldera (i.e., NW of Agung). Nonetheless, regional hypocenter options produced by BMKG initially recommended that the occasions have been nearer to Mount Agung (Fig. 3). Seismicity peaked on 22 September with >800 earthquakes of magnitude >1 recorded by the CVGHM seismic community (Fig. 2B). Earthquake magnitudes additionally elevated, with a M4.2 (BMKG) that occurred on 26 September. These earthquakes have been all high-frequency, VT earthquakes.

VT occasion charges decreased considerably on 20 October (Fig. 2B) and continued to lower via early November. Throughout October and November 2017, we augmented the seismic monitoring community by including six broadband digital stations and one short-period digital station at websites close to the mountain to enhance detections and places throughout the community (Fig. 1A).

In late October, earthquake hypocenters started to unfold to the N and NE of Mount Agung whereas persevering with to happen to the NW. By early November, earthquake charges had dropped to regular ranges of ~300 earthquakes per day with massive M3+ occasions nonetheless widespread. Whereas earthquake charges decreased throughout this time interval, RSAM ratios of the closest two stations confirmed a sign of magma migration towards the summit crater and RSAM values confirmed a refined however persistent long-term development enhance, a development that continued into the preliminary phreatomagmatic eruptions in late November (Fig. 3). On 8 November, 2017 ~22:00 UTC, BMKG recorded a M4.9 and a collection of aftershocks situated ~10 km NE of Mount Agung (Fig. 3). Shortly afterwards, small, low-frequency (LF) and VT earthquakes proximal to the summit have been famous usually. The primary clear indicators of tremor (~40-120 second period; broadband 1-10 Hz) have been recorded by the early hours of 12 November UTC. On reflection, by this time, magma was clearly invading the higher ranges (<5 km) of the Mount Agung edifice. VT and LF earthquakes continued at low charges and RSAM values regularly elevated via the primary phreatomagmatic eruption on 21 November, however the eruption itself was not recorded seismically. Extra tremor was recorded a day after the 21 November phreatomagmatic eruption, and VT and LF occasion charges continued at low ranges. The onset of magmatic eruption was preceded by a swarm of twenty-two bigger LF earthquakes on the morning of November 25 native time, though the onset of lava effusion, which was first detected in satellite tv for pc information the identical day, was not recorded seismically.

After the onset of effusion, earthquake charges and RSAM values continued at pre-eruptive ranges till a major enhance on 8 December. Fluctuations in seismicity weren’t correlated with adjustments in visible observations of eruptive exercise right now. Though Mount Agung started producing common, discrete explosions early on within the effusive part, not one of the explosions have been recorded seismically on the CVGHM community till 23 December. After this date, virtually all explosions at Agung have been recorded on the CVGHM seismic community. Prior to every explosion, nevertheless, earthquake charge or power will increase have been both absent or, in some instances, too refined to reliably forecast subsequent explosions. Beginning after the primary lava extrusion on or simply earlier than 25 November, tremor episodes lasting 30-90 minutes occurred sporadically, however weren’t usually correlated with eruptive habits. There may be convincing proof (repeated incidence throughout afternoon rains, comparatively excessive frequency content material, and visible observations of rain clouds on the summit) that means these episodes have been associated to rainfall on the summit, plausibly because of interplay of rainfall with scalding rock via rising cracks within the crater lava. Throughout essentially the most intense part of the eruption, transit of lahars was seismically recorded on the N and S flank of the volcano. These lahars have been thought to have originated by rainfall on ash that was deposited on the higher flank of the volcano throughout the preliminary explosive exercise throughout the interval of roughly 21-30 November.

After essentially the most intense part of eruptive exercise in late November, seismicity decreased. Although charge will increase in LF seismicity culminated in Strombolian-type explosions on 19 January 2018, and enormous (M3+) VTs continued in February and March, general earthquake charges decreased to tens of occasions per day or fewer. On 23 June 2018, a small swarm of VT and LF seismicity started and elevated till an explosion on 27 June and extra lava extrusion and ash emissions on 28-29 June, which was accompanied by monochromatic tremor. On 2 July 2018, Strombolian exercise was recorded as a collection of seismic explosion indicators. Seismicity related to intermittent explosive exercise continued via the current (June 2019).

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Deformation of Mount Agung is monitored by a community of 5 steady GNSS stations (Fig. 1A) that was put in in 2012. By 2014, all the websites had ceased transmitting information, however they have been revived in late 2017, and a few information extending again to 2016 have been recovered. Floor displacements previous and accompanying 2017-2018 eruptive exercise occurred in a number of discrete episodes, as exemplified by the point collection from station REND (Figs 2C and 3D situated ~12 km south-southwest of the volcano’s summit. Previous to the onset of the seismic swarm in mid-September, two intervals of obvious inflation have been evident, in February-March 2017 and once more throughout August-September 2017. Throughout each intervals, movement of operational stations was away from Agung (Fig. 3A), with the later inflationary epoch being the bigger of the 2 (for example, southward movement of REND was ~5 mm in February-March and ~20 mm in August-September). The primary episode was not accompanied by seismicity. The second was accompanied by slowly rising seismicity, and no important deformation occurred throughout the intervening months. A easy Mogi model9 of the displacements suggests a strain enhance at 10-20 km depth, though the few information factors don’t allow a extra detailed evaluation. The deformation shouldn’t be obvious in InSAR information spanning the time interval, in all probability as a result of small magnitude of the displacements10.

The speedy enhance in seismicity in September was accompanied by a major change in deformation in any respect websites (Fig. 3B). Station REND, for instance, started shifting north in direction of the volcano’s summit. InSAR outcomes spanning September-October recommend the emplacement of a dike at ~10 km depth between Agung and Batur10 whereas GNSS stations—notably REND—are in line with a mix of dike intrusion to the northwest of Mount Agung and deflation of a deeper supply (the identical supply that inflated in February-March and August-September). A co-eruptive episode of deformation in November 2017 coincided with the onset of lava extrusion and is in line with deflation of a supply beneath Mount Agung, though the info can’t distinguish the depth of this supply. From mid-December 2017 via April 2018, floor deformation was minor. From Could to mid-June 2018 shallow inflation was detected, adopted by extrusion of lava and a rise in explosion frequency in late-June to July 2018.

Distant sensing and ash samples

Satellite tv for pc information offered frequent views of Mount Agung’s summit crater and edifice. Steaming within the crater was first reported in September 2017. Excessive decision satellite tv for pc information confirmed that steaming had been intermittently seen since a minimum of September 2016. Satellite tv for pc information doc will increase within the quantity and space of steaming and episodic ponding of water that emanated from a talus pile close to the bottom of the NE crater wall starting as early as 14 September 2017. After the primary explosive exercise on 21 November, satellite tv for pc information detected a brand new 100-m-diameter crater centered within the bigger summit crater that served because the conduit for subsequent eruptions. Ash samples from the 21 November occasion embody minor juvenile elements, however are dominated by remobilized edifice lithic materials (Fig. 4D,E). Collected bulk ash samples have been analyzed for his or her major-element chemistry and had bulk chemistry of andesite. Sequential sampling revealed an obvious enhance from 55 to 59 wt.% SiO2 in bulk composition of the erupted ash from 22 November 2018 to 29 November 2018. Semi-quantitative analyses of juvenile glass confirmed an andesitic composition. A small lava circulation was first noticed inside this crater on 25 November and by 27 November had lined the crater ground (Fig. 5). When lava effusion slowed considerably, lower than every week later, the lava circulation had lined the ground of the crater and reached a most thickness of about 121 m and a quantity of about 24 million m3. At this level, the lava had reached about one-third of the peak of the low level within the crater wall, situated alongside the south rim. By 5 December 2017, following a one-week pause in exercise, new fractures had begun to kind over the central a part of the lava circulation. Because the fractures grew wider, imagery recommended that molten lava had flowed in from under to seal the fractures. Over the following a number of months, explosions continued to switch the lava floor, creating new explosion pits and depositing coarse eruption particles on the lava floor. Localized inflation of the central vent space floor was noticed shortly earlier than one of many explosions. Satellite tv for pc imagery revealed {that a} new interval of lava extrusion, which started on 28 June 2018, produced new materials that lined practically the complete November crater lava circulation and added a further ~10 m to its thickness.

Gasoline composition and emission charge

As a result of earlier lack of long-lived fumaroles at Agung since its 1963 eruption, no geochemical monitoring program or devices have been in place previous to the 2017 unrest. Circumstances close to the summit have been thought-about too hazardous for proximal sampling, so common makes an attempt to measure sulfur dioxide (SO2) utilizing ground-based distant sensing strategies started in October 2017 after steam emissions had visibly elevated. Regardless of the presence of a small, persistent plume and reviews of sulfurous-smelling gases from unauthorized hikers, 12 cellular DOAS (Differential Optical Absorption Spectrometry)11 campaigns made between 1 October and 14 November 2017 all did not detect SO2.

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In mid-November, we pioneered the usage of a fixed-wing drone (AeroTerraScan mannequin Ai450) instrumented with a miniaturized multi-GAS12,13 (A number of Gasoline Analyzer System) to acquire airborne in situ measurements of plume H2O vapor, CO2, SO2, and H2S. The drone was launched from 530 m elevation at a location 11 km south of the summit and climbed to ~3,300 m for sampling (Fig. 6). The primary profitable measurements have been obtained at 00:21(UTC, 08:21 native time) on 21 November and revealed a big plume-related CO2 anomaly (ΔCO2 = 36 ppmv; “∆” signifies that the measurements are reported with ambient background subtracted); SO2 was under the sensor detection restrict (~0.05 ppmv; Fig. 2E). Whereas no prior baseline gasoline measurements have been obtainable for comparability, airborne measurement of in-plume CO2 anomalies of this magnitude are uncommon12,14,15,16,17 and these information have been seen as a major indication of unrest. Roughly 9 hours later, the primary phreatomagmatic explosion occurred. Floor-based DOAS measurements the next day (22 November) yielded an SO2 emission charge of 660 t/d (Fig. 2D). Three totally different drone flights on 23 and 24 November discovered massive CO2 anomalies (ΔCO2 = 49-98 ppmv), very low SO2 mixing ratios (SO2, MAX = 0.55 ppmv on 23 November; 0.05 ppmv on 24 November), and hint H2S (<0.17 ppmv on 24 November. These information confirmed that gasoline emissions have been very CO2-rich and S-poor, and common molar CO2/SO2 ratios elevated dramatically from 77 to 824 on 23-24 November previous to the beginning of the primary magmatic explosive part at 9:20 UTC on 25 November (Fig. 2F).

The best SO2 emission charge was measured on 26 November (5,500 t/d) however shortly fell to 180 t/d by 1 December. Gasoline emissions throughout lava effusion in December have been extremely variable (SO2 = 140-1500 t/d; month-to-month median = 390 t/d, n = 14) and magmatic in character; a drone flight on 19 December intercepted a dense plume with clear H2O, CO2, and SO2 peaks (H2O/CO2 = 21, CO2/SO2 = 3.2; SO2, MAX = 26.1 ppmv). The DOAS measurements picked up very low ranges of BrO within the massive 26 November plume (BrO/SO2 = 3E-5). Subsequent information confirmed an rising development as much as BrO/SO2 = 1.8 and 1.9E-4 detected on 17 and 18 December, respectively (Fig. 2G). The rising BrO/SO2 ratios are in line with elevated degassing of shallow magma from the rising lava circulation within the crater releasing HBr, adopted by reactions within the ambiance partially changing HBr to BrO18. Additional DOAS measurements in January and February confirmed that SO2 emissions have been lowering (median January SO2 = 230 t/d, n = 12; February = 220 t/d, n = 4). SO2 emissions briefly jumped to greater than 1000 t/d within the week after the 28 June 2018 extrusion occasion, however then shortly returned to low baseline values (<200 t/d) by the start of August 2018.

Abstract of Primary Timeline

Under, we offer a timeline of occasions as they occurred, although in some instances, as with deformation, they weren’t detected on the time. We body the timeline when it comes to alert-level adjustments in order that the reader can respect the occasions and reasoning that led to these adjustments. The date of alert-level change is denoted within the header for every entry, although key occasions and observations start previous to and after that date.

14 September 2017- Improve to Stage 2: The primary swarm of earthquakes was recorded by the native Agung and Batur seismic community in mid-Could 2017. Determine 2 presents a timeline of observational and geophysical measurements from July 2017 to August 2018. By mid-July 2017—across the time {that a} small thermal anomaly was detected (Fig. 2H, Supplemental Figures Fig. S1)—RSAM values at Agung had deviated from baseline ranges (Fig. 2B), and by mid-August, VT earthquakes have been occurring each day, rising considerably in September. On reflection, we now know {that a} second episode of inflation was detected by GNSS from August-September, in addition to by InSAR10. Uncommon fumarolic exercise within the northeast a part of the summit crater, together with rising seismicity, prompted an alert stage change to Waspada (Stage 2) on 14 September (Desk 1, Figs. 1B and 2A).

18 September 2017- Improve to Stage 3: Water ponding (presumably expelled from the edifice or alternatively condensed from fumaroles) was famous within the crater on 14 September and shaped small deltas within the neighborhood of the fumaroles. Rising fumarolic exercise, a rising thermal anomaly within the crater, and felt earthquakes (M3+) elevated the extent of concern of native populations. Quickly rising seismicity prompted an alert stage change to Siaga (Stage 3) on 18 September.

22 September 2017- Improve to Stage 4: Seismicity continued to speed up quickly and RSAM values peaked on 22 September (Figs 2B and three), prompting one other alert stage change. On reflection, we all know there was additionally a change within the relative movement of GNSS-stations (Figs 2C and three). GNSS stations south of the volcano registered motion towards the volcano, whereas a station to the northwest (CEGI) registered motion away from the volcano. The change to Stage 4 (Awas) triggered evacuations. RSAM values then declined, however elevated seismic occasion charges, together with massive magnitude earthquakes (as much as M4.2), endured. On 7 October, a notable white-colored gasoline plume rose from the northeast crater ground ~1500 m above the summit crater, lasted for about an hour, and was detected seismically (Fig. 3F). This was the tallest plume noticed earlier than the eruption. Unauthorized climbers reported sulfur smells, rumbling noises, and fumarolic exercise from the northeast crater ground. Nonetheless, SO2 emissions have been under the detection restrict as measured by cellular DOAS at 12 km distance (Fig. 2E).

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29 October 2017- Downgrade to Stage 3: Seismic occasion charges declined sharply on 20 October, although VTs began to maneuver nearer to the summit (proximal occasions). With the lower in seismic occasion charges and the lengthy (one-month) period of evacuations, the alert stage was lowered to Siaga (Stage 3) on 29 October. In early November, RSAM values started to extend slowly (Figs 2B and three). On 8 November, an M4.9 earthquake was recorded and was felt by folks (Modified Mercalli Depth, MMI II-V) so far as ~60 km from the volcano. This was the most important recorded VT occasion throughout the disaster interval (Figs 2B and three).

26 November 2017- Improve to Stage 4: In mid-November, LF occasions and tremor appeared, and seismic occasion places moved nearer to the volcano. Drone flights fitted with a multi-GAS above the volcano’s crater detected a CO2-rich plume early on 21 November (Fig. 2E,F). The 2017 Agung eruption started with a small phreatomagmatic explosion on 21 November 9:05 UTC, with ash emissions to 700 m above the summit (Figs. 2I and 4). A reasonable quantity of SO2 (660 t/d) was detected the next day by cellular DOAS, in line with magma degassing (Fig. 2D). Multi-GAS drone flights detected elevated ranges of CO2 on 23-24 November (Fig. 2F). Bigger, steady explosions started on 25 November at 9:20 UTC and satellite tv for pc observations detected the presence of a lava circulation throughout the crater. The ash column reached ~6 km above the summit (~9 km asl) by 26 November (Fig. 2I) and traveled ESE leading to closure of the Praya airport in Lombok (~95 km SE of Agung crater) on 26-27, 30 November and 1 December. On 26 November 23:00 UTC, the alert stage was raised to Awas (Stage 4). The tropical cyclone Cempaka modified the wind instructions, and pulled the ash cloud south and west, forcing closure of the Denpasar’s Ngurah Rai airport (~60 km SW of Agung crater) throughout 26-29 November. Excessive SO2 emissions have been detected by cellular DOAS and the OMI (Ozone Monitoring Instrument) satellite tv for pc. Lightning, loud rumblings, and lahars have been produced (Fig. 4F) on account of rainfall mobilizing ash deposits from late November. Two plumes have been emitted on 26-27 November (Fig. 4B), with a darkish, ash-rich half emanating from the primary crater, and an ample white vapor plume coming from the previous fumarole subject. By 27 November, lava lined the crater ground (Fig. 5C,D) and started to quickly fill the summit crater, till slowing on 29 November; plume heights then declined as effectively. Some ash was deposited across the volcano: it was thicker and prolonged additional within the WSW path in step with the prevailing wind path throughout the largest ash emissions interval. Rainfall-induced lahars have been generated inside 16 drainages on the NNW, N, ENE, SE, S and SW components of the volcano in late November, with essentially the most important circulation the Tukad Yeh Unda river on the SW flank right down to the Badung strait (~30 km from the Agung summit). The continual explosive interval was adopted by a semi-continuous, low-level plume till 4 December, when a interval of frequent (each 30-60 min), aseismic, ash ‘puffs’ (vapor plumes) started (Fig. 2).

10 February 2018- Downgrade to Stage 3: Round 23 December 2017, the common puffing ceased and each day to weekly, discrete, seismically-detected explosions started (pink vertical traces in Fig. 2), producing plumes usually as much as 2.5 km above the summit (~5.5 km asl) and leaving explosion pits within the cooling lava circulation. As explosion frequency ceased, the exclusion zone was lowered to a radius of 6 km on 4 January. Minor Strombolian explosive exercise was noticed on 19 January 2018, after which the frequency of explosions declined considerably. The alert stage was lowered to Siaga (Stage 3) on 10 February. Between February and late June, there have been intermittent discrete explosions, and usually low (however above background) SO2 emissions and seismicity charges (Fig. 2). A swarm of VT occasions on 23 June 2018 preceded a small explosion on 27 June 2018 and was adopted by lava extrusion and steady ash emission on 28-29 June 2018. The continual ash emission to the WSW affected flight operations on the Denpasar, Bali and Jember, East Java airports on 28-29 June 2018 (UTC). At 13:04 (UTC) on 2 July, a Strombolian eruption threw incandescent materials so far as 2-3 km from the summit crater. Although the exclusion zone had been set to a radius of 4 km, hundreds of individuals exterior this zone self-evacuated as a result of worry that incandescent materials would journey farther, and as a result of loud thundering sounds produced by the volcano. Ash emissions from these explosions moved west, inflicting airport closures in East Java (Banyuwangi and Jember) on 3 July. Afterwards, there was a interval of quite a few small explosions, regularly declining in frequency via July 2018 (Fig. 2). Minor seismicity continued. On 29 July and 5 August, two massive earthquakes of M6.4 and M6.8 hit N of Lombok island (<120 km E of Mount Agung). Steady degassing of a skinny white plume was noticed following these earthquakes; nevertheless, no different adjustments in eruptive exercise have been noticed straight following these earthquakes; as a substitute, related low-level explosions proceed via the time of this writing.

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