Weak explosions, some with ash, from the crater lake during July and August 2023

The Dempo stratovolcano in SE Sumatra, Indonesia, has had many small explosions reported since 1817. The active crater contains a 400-m-wide lake was the source of phreatic explosions in 2006, 2009, 2017, and most recently in May 2022. This report covers activity from June 2022 to December 2023, based on daily and special reports of the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG), and satellite data. Throughout the reporting period, the Alert Level remained at 2 (on a scale of 1-4), and the public were reminded to stay 1 km away from the crater and as far as 2 km on the N flank.

PVMBG reported that at 2115 on 25 July 2023, an explosion produced a white-and-gray ash plume that rose at least 2 km above the summit and drifted S and SW. Additional ash explosions were recorded at 1547 and 2215 on 26 July, the latter rising 2 km above the summit crater and drifting S and SW, based on ground observations. Another explosion was reported by PVMBG at 2105 on 21 August, but no ash emissions were observed. Webcam images posted in daily MAGMA Indonesia reports showed possible patches of discolored water or material floating on the surface of the crater lake during 27-29 August (figure 8).

Figure 8. Webcam image of Dempo’s crater lake on 29 August 2023, showing material floating on surface. Courtesy of MAGMA Indonesia.

Most Sentinel-2 satellite views of Dempo during the reporting period were obscured by weather clouds. When observations were possible, the crater lake surface typically looked milky gray, indicating high levels of particulate matter. However, during several days in March and April 2023 the lake took on a blue appearance.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); Copernicus Browser (URL: https://dataspace.copernicus.eu/browser).

Information is preliminary and subject to change. All times are local (unless otherwise noted)

August 1999 (BGVN 24:08) Cite this Report

Earthquakes 18-19 September increase alert status

At 1810 on 18 September an earthquake was felt in an area around Dempo. The Volcanological Survey of Indonesia (VSI) reported its Modified Mercalli (MM) Intensity as MM IV. The following day three more earthquakes were felt: the first event took place at 0900 (MM II), the next at 1155 (MM I), and the last at 1800 (MM I). Recorded seismicity during these days indicated 30 events/day with amplitudes of 20-30 mm and arrival time differences (S-P) of 2-3 seconds on average. Volcanologists took this to mean that the earthquakes were local tectonic events. No visible signs of changed activity were apparent at the crater. The VSI took the precaution of increasing the condition of alert to Level II.

Information Contacts: Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

January 2009 (BGVN 34:01) Cite this Report

Sudden phreatic eruption on 1 January 2009

According to a report by Indonesia's Center of Volcanology and Geological Hazard Mitigation (CVGHM), Dempo had a phreatic eruption on 1 January 2009. The event resulted in a strong sulfur odor and an ash (or cinder) rain that was noted as far as ~ 10 km from the summit. During 27 December 2008 to 1 January 2009, fog prevented direct observations of the summit; clouds over the summit area are a common occurrence. The one available photo of the eruption showed a whitish-colored plume rising perhaps a few kilometers over the summit.

On 1 January authorities raised the alert status from 1 (normal) to 2 (alert, "Waspada" in Indonesian), on a scale that ranges from 1-4. Visitors and residents were advised not to go within a 2-km radius of the summit. A 2 January report from CVGHM noted that both local volcanic and regional tectonic earthquakes were recorded during 27 December 2008 through 1 January 2009 (table 1).

Table 1. Highlights of Dempo seismicity recorded during 27 December 2008 to 1 January 2009. Courtesy of CVGHM.

As of early 2009, no thermal anomalies had been measured over Dempo by the MODVOLC infrared satellite system for at least the last 5 years. ASTER images from 2001-2007 show extensive cloud cover.

Information Contacts: Center of Volcanology and Geological Hazard Mitigation (CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://vsi.esdm.go.id/); Hawai'i Institute of Geophysics and Planetology (HIGP) Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/).

March 2009 (BGVN 34:03) Cite this Report

Phreatic eruption in September 2006 sent mud 300 m

Our most recent report on Dempo (BGVN 34:01) discussed a phreatic eruption on 1 January 2009. This eruption prompted Indonesia's Center of Volcanology and Geological Hazard Mitigation (CVGHM) to raise the alert level from 1 (normal) to 2 (alert) on a scale of 1-4. A few months later, on 23 March 2009, the CVGHM lowered Dempo's Alert Level to 1, based on visual observations of the crater lake during 5-6 January and 2-4 March, and decreased seismicity since the 1 January phreatic eruption.

The name Dempo applies to both the larger complex and to a peak that sits adjacent to a neighboring peak called Marapi. The latter volcano name applies to several different volcanoes in Indonesia and is easily confused with the very prominent volcano Merapi (central Java). The Marapi cone in the Dempo complex contains a ~ 400 m diameter crater lake, the source of both the 2006 and 2009 eruptions.

The remainder of this report discusses the phreatic eruption that occurred in September 2006. This eruption had not been previously discussed in the Bulletin, and CVGHM reporting on the subject has recently come to our attention.

In the month before this eruption, teams from the CVGHM had visited several times. On 13 August 2006, a team prepared a map of Dempo and reported that the condition of the water in Marapi's crater lake was normal and clear or slightly blue in color, with no bubbling. On 4 September 2006, a team climbed to the peak of Dempo and reported that activity was normal other than some bubbling at the E edge of the crater lake.

2006 phreatic eruption. On 25 September 2006, a phreatic eruption occurred that expelled water from Marapi's crater lake and propelled mud onto the area around the peak up to a radius of 300 m.

On the next day, the inspector for Dempo, Mr. Mulyadi, accompanied by six friends, inspected the volcano and its lake. According to Mulyadi's team, the lake water was bubbling and had changed to a grayish color. Acrid sulfurous emissions were accompanied by a hissing sound. The NW crater wall was covered by mud from the eruption. Many of the phreatic deposits around the crater lake were only about 0.5 cm thick, although in several other places they were thicker (figures 1-4). A visit on 4 October 2006 found the deposits not yet eroded, owing to a lack of rain since the eruption.

Figure 1. Photo of the plume resulting from Dempo's phreatic eruption on 25 September 2006. This is one of a set of multiple photos taken of a rising plume. Photo by Fredy, a local resident. Courtesy of CVGHM.

Figure 2. (top) Pre-eruption and (bottom) post-eruption scenes from the saddle between the cones of Dempo and Marapi looking upslope towards Marapi (which contains the source vent in a steep-sided crater not visible from this perspective). The shots were taken on 7 and 26 September 2009, respectively. Freshly deposited mud and evidence of ejected crater-lake water in the foreground (bottom) represents distal deposits from the 25 September eruption; note peson at right for scale. Large blocks were not from this eruption. Courtesy of the Dempo inspection team, CVGHM.

Figure 3. Pre- and post-eruption photos looking into Marapi crater at Dempo, taken on 7 (top) and 26 (bottom) September 2006. The lake is on the order of 400 m across. The bottom photo portrays the crater's mud-covered walls and sediment-covered lake. Comparison of both photos indicates that after the eruption the lake surface had dropped, consistent with discharge of water and mud. Camera look-direction unstated. Courtesy of the Dempo inspection team, CVGHM.

Figure 4. Post-eruption conditions on Dempo's Marapi cone seen from a point a few meters back from the crater rim. Widespread gray-to-brown mud covered the rim and upper crater, creating a desolate scene. The large angular blocks on the rim were placed there in previous events, not the 25 September 2009 eruption. Unstated direction; photographers shadow and gear for scale. Courtesy of the Dempo inspection team, CVGHM.

Information Contacts: Center of Volcanology and Geological Hazard Mitigation (CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://vsi.esdm.go.id/).

December 2017 (BGVN 42:12) Cite this Report

Phreatic explosion from the crater lake generates a dense ash plume in November 2017

Activity at Dempo on Sumatra in recent years has consisted of brief phreatic eruptions, most recently single-day events on 25 September 2006 (BGVN 34:03) and 1 January 2009 (BGVN 34:01). There were no additional reports from the Center of Volcanology and Geological Hazard Mitigation (CVGHM), also known as Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG), until a brief episode of unrest in late April 2015, Another typically short phreatic explosion took place on 9 November 2017.

Activity during 2015. On 29 April the Alert Level was raised to 2 (on a scale of 1-4) by PVMBG following observations of diffuse white-gray plumes on 27 April rising to 50 m above the crater. Seismicity had increased during April compared to the previous month (figure 5). A Detik news report on 30 April quoted the PVMBG Head of the Western Volcano Field of Observation and Investigation, Hendra Gunawan, as saying that there had been tremor recorded over the previous four days. No ashfall was reported by PVMBG, and a phreatic eruption was only mentioned in the 29 April notice as a potential danger.

Figure 5. Seismicity recorded at Dempo from 1 January to 29 April 2015. The types of earthquakes reported are HBS (Hembusan, puff or emission events), Trm (tremor), VB (shallow volcanic type B), VA (volcanic type A), TL (local tectonic), and TJ (distant tectonic). Courtesy of PVMBG.

Observers reported that during 1 June-9 September 2015 no plumes were seen and seismicity was low. On 10 September PVMBG lowered the Alert Level to 1.

Activity during 2017. Staff at the PVMBG Dempo observation post reported that no plumes rose from the crater during January and February 2017, but some diffuse white plumes during 1 March-4 April rose no higher than 50 m. Seismicity increased significantly above background levels from 21 March to 4 April (figure 5). On 5 April PVMBG raised the Alert Level to 2 based on visual and seismic data, but did not report any phreatic eruptions.

Figure 6. Seismicity recorded at Dempo from 31 December 2016 to 6 April 2017. The types of earthquakes reported are HBS (Hembusan, puff or emission events), TRE (tremor), VB (shallow volcanic type B), VA (volcanic type A), TL (local tectonic), and TJ (distant tectonic). Courtesy of PVMBG.

According to PVMBG a three-minute-long phreatic eruption began at 1651 on 9 November 2017 and generated a dense ash plume that rose to 4.2 km altitude, about 1 km above the crater rim, and drifted S. Ashfall and sulfur gases were reported in villages on the S flanks, but there was no damage to property or injuries. The Alert Level remained at 2, with a 3-km-diameter exclusion zone; the Aviation Color Code was at Yellow.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Detiknews (URL: https://news.detik.com/).

June 2022 (BGVN 47:06) Cite this Report

Phreatic explosion on 31 May 2022

The Dempo stratovolcano in SE Sumatra, Indonesia, has had many small explosive eruptions reported since 1817. The active Marapi crater (750 x 1,100 m) in the summit area contains a 400-m-wide crater lake (figure 7). Recent activity has consisted of single-day phreatic explosions in September 2006, January 2009, and November 2017. Indonesian’s Center of Volcanology and Geological Hazard Mitigation (CVGHM), also known as Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG), is the monitoring agency.

Figure 7. Satellite image of Dempo on 30 May 2022, showing the 400-m-wide crater lake within the Marapi summit crater. Sentinel-2 image with natural color rendering (bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

PVMBG reported phreatic explosion similar to those of recent years at 0154 on 31 May 2022. This event lasted 239 seconds and produced ashfall in areas within 5 km, including Pagar Alam Utara district (E) and North Dempo District; ash deposits were as thick as 1 mm. The Alert Level remained at 2 (on a scale of 1-4), and the public were reminded to stay 1 km away from the crater and as far as 2 km on the N flank.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

September 2023 (BGVN 48:09) Cite this Report

Weak explosions, some with ash, from the crater lake during July and August 2023

The Dempo stratovolcano in SE Sumatra, Indonesia, has had many small explosions reported since 1817. The active crater contains a 400-m-wide lake was the source of phreatic explosions in 2006, 2009, 2017, and most recently in May 2022. This report covers activity from June 2022 to December 2023, based on daily and special reports of the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG), and satellite data. Throughout the reporting period, the Alert Level remained at 2 (on a scale of 1-4), and the public were reminded to stay 1 km away from the crater and as far as 2 km on the N flank.

PVMBG reported that at 2115 on 25 July 2023, an explosion produced a white-and-gray ash plume that rose at least 2 km above the summit and drifted S and SW. Additional ash explosions were recorded at 1547 and 2215 on 26 July, the latter rising 2 km above the summit crater and drifting S and SW, based on ground observations. Another explosion was reported by PVMBG at 2105 on 21 August, but no ash emissions were observed. Webcam images posted in daily MAGMA Indonesia reports showed possible patches of discolored water or material floating on the surface of the crater lake during 27-29 August (figure 8).

Figure 8. Webcam image of Dempo’s crater lake on 29 August 2023, showing material floating on surface. Courtesy of MAGMA Indonesia.

Most Sentinel-2 satellite views of Dempo during the reporting period were obscured by weather clouds. When observations were possible, the crater lake surface typically looked milky gray, indicating high levels of particulate matter. However, during several days in March and April 2023 the lake took on a blue appearance.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); Copernicus Browser (URL: https://dataspace.copernicus.eu/browser).