Submarine eruption during 28-29 November 2021

Iliwerung is constructed on the southern rim of the Lerek caldera on the south-facing peninsula on Lembata Island. The summit lava dome was formed during an eruption in 1870. Beginning in 1973-1974, submarine eruptions began from the lower ESE flank Hobal vent, located 2 km from the summit crater. Since then, activity originated from this vent during eruptions in 1983, 1993, 1999, and 2013. The eruption in August 2013 that consisted of diffuse fumarolic emissions, incandescence at sea level, discolored water, and shallow volcanic earthquakes (BGVN 40:05). This report updates activity through December 2021 and describes a submarine eruption during 28-29 November 2021. Information primarily comes from Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM, or the Center of Volcanology and Geological Hazard Mitigation).

A submarine eruption was observed during 28-29 November 2021, which prompted PVMBG to raise the Volcano Alert Level (VAL) to 2 (on a scale of 0-4). Residents reported that the ocean surface was bubbling up to heights of less than 1 m at 2152 on 28 November from a spot 30 m from the shore. This activity lasted about an hour and originated about 1 km SW of the Hobal submarine vent. Bubbling up was also observed at 0517 the next day on 29 November. Additional bubbling water and a plume was noted at 0829 that rose 100 m. Intermittent tremors were also detected, beginning on 29 November and continuing through the end of December.

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/).

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

August 1983 (SEAN 08:08) Cite this Report

Submarine explosions

Explosions occurred 17 and 18 August from a submarine vent (Hobal) S of Iliwerung. No casualties were reported.

Its previous eruption began with submarine activity in December 1973; by July 1974, there were two small islands at the eruption site and a third emerged in August. All have subsequently been eroded away. In July 1979, about 50 x 10⁶ m³ of Iliwerung were removed by a landslide. About 1/3 of this material entered the sea, and (probably aided by additional submarine slumping) generated tsunami that killed hundreds of people.

Information Contacts: L. Pardyanto, VSI; AFP.

August 1993 (BGVN 18:08) Cite this Report

Submarine eruption produces 100-m-high water columns

A submarine eruption in the Alor Strait was reported on 16 September by a Japanese cargo ship. Crew members felt three shocks at around 0630, and observed a rising steam cloud or plume soon after. Explosions occurred at 0656 and 0855. Each explosion produced a water column ~100 m high, followed by a cone of upwelling water; no other unusual phenomena were observed after the explosions. From ~6.5 km away, the ship located the explosion site at about 8.538°S, 123.592°E, [2 km ESE of the Iliwerung summit]. The VSI confirmed that an eruption had occurred from the submarine Hobal vent at Iliwerung....

Information Contacts: JMA; M.A. Purvawinata, VSI.

October 1993 (BGVN 18:10) Cite this Report

Follow-up on Hobal vent eruption and 1979 tsunami

In September, Hobal vent on the [ESE] submarine flanks of Iliwerung volcano (figure 1) produced an eruption that broke the ocean surface (18:08).... During the eruption pyroclastic material sank within ~200 m of the point where it penetrated the ocean surface, consequently no samples were obtained. This suggests that the erupted material lacked sufficient closed vesicles to allow it to float.

Figure 1. Map showing Iliwerung volcano, Hobal vent, and the known extent of the 1979 landslide that triggered a tsunami. Note that N is to the upper right. Courtesy of VSI.

Depth to the Hobal vent has never been measured, though in 1979 the VSI estimated it at 50 m. The bathymetry of this region is also significant because of its relevance to tsunamis. A 1979 landslide NW of Iliwerung reached the sea (figure 1), and probably aided by additional submarine slumping, triggered a tsunami that killed hundreds of people. Several areas evacuated prior to the 1979 landslide and tsunami still remain evacuated. Relevant aspects of the tectonic setting, and the effects of a 1992 earthquake-triggered tsunami on the N side of Flores Island are discussed by Yeh and others (1993).

Seismic data were acquired by VSI prior to the September Hobal eruption. The data were received at Lamaheku seismic station, at ~75-m elevation on the W flank of Iliwerung, 4.5 km from the Hobal vent. The seismic record is incomplete, but for July to September 1992, the number of volcanic B-type earthquakes averaged more than 70/month, reaching ~96 in September 1992. A gap in the record occurred in early 1993, but later in the year B-type earthquakes progressively increased from ~30 in May to 60 in September. Thus, the B-type events increased prior to the September eruption, but at least one interval with more B-type events passed without reports of concurrent eruptions.

VSI has informed local governments about the hazards of future eruptions, and has established a volcanic danger area that includes at least four communities on Iliwerung. Local VSI staff continue to monitor the region, and as of 1 November, were in the process of installing a new seismic station.

Reference. Yeh, H., Imamura, F., Synolakis, C., Yoshinobu T., Liu, P., and Shi, S., 1993, The Flores Island tsunamis: Eos, Transactions of the American Geophysical Union, v. 74, no. 33.

Information Contacts: W. Tjetjep, VSI.

June 2003 (BGVN 28:06) Cite this Report

Increase in felt earthquakes during October 2001

During 17-26 October 2001 the Volcanological Survey of Indonesia reported an increase in felt earthquakes. A total of 2-11 events occurred per day, with Mercalli magnitudes of I-II. The earthquakes S and P wave arrival times differed by between 1 and 35 seconds. Visual and instrumental monitoring revealed a lack of significant changes. The Alert Level was increased from 1 to 2 (on a scale of 1-4). No further reports were issued through at least May 2003.

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

May 2015 (BGVN 40:05) Cite this Report

Submarine flank eruption during August 2013 accompanied by crater fumarolic emissions

A submarine vent [~2 km ESE] of Iliwerung, named Hobal, erupted in December 1973-August 1974, August 1983 (SEAN 08:08), September 1993 (BGVN 28:06), and May-September 1999. No further activity was reported from this volcano until August 2013 when Hobal had another short eruption on the 20th.

The Center of Volcanology and Geological Hazard Mitigation (CVGHM) reported that observers at the observation post 6 km from Iliwerung saw diffuse fumarolic emissions from the crater during the mornings from 1 to 19 August 2013. The various seismic signals fluctuated during the month, but 81 shallow volcanic earthquakes were detected between 1606 and 1741 on 19 August. The Alert Level was raised to 2 (on a scale of 0-4).

At 0714 on 20 August 2013 an eruption from the submarine Hobal vent ejected dense white plumes to 2 km altitude that drifted S. Incandescence at sea level was observed at 0746, and the water around the eruption site turned yellow. Fishermen and tourists were not permitted within a 2 km radius of the activity.

Although seismicity had decreased on 19 August, the observers reported that the diffuse morning fumarolic crater emissions had continued through 1 September. On 2 September the Alert Level was lowered to 1 (on a scale of 0-4).

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

December 2021 (BGVN 46:12) Cite this Report

Submarine eruption during 28-29 November 2021

Iliwerung is constructed on the southern rim of the Lerek caldera on the south-facing peninsula on Lembata Island. The summit lava dome was formed during an eruption in 1870. Beginning in 1973-1974, submarine eruptions began from the lower ESE flank Hobal vent, located 2 km from the summit crater. Since then, activity originated from this vent during eruptions in 1983, 1993, 1999, and 2013. The eruption in August 2013 that consisted of diffuse fumarolic emissions, incandescence at sea level, discolored water, and shallow volcanic earthquakes (BGVN 40:05). This report updates activity through December 2021 and describes a submarine eruption during 28-29 November 2021. Information primarily comes from Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM, or the Center of Volcanology and Geological Hazard Mitigation).

A submarine eruption was observed during 28-29 November 2021, which prompted PVMBG to raise the Volcano Alert Level (VAL) to 2 (on a scale of 0-4). Residents reported that the ocean surface was bubbling up to heights of less than 1 m at 2152 on 28 November from a spot 30 m from the shore. This activity lasted about an hour and originated about 1 km SW of the Hobal submarine vent. Bubbling up was also observed at 0517 the next day on 29 November. Additional bubbling water and a plume was noted at 0829 that rose 100 m. Intermittent tremors were also detected, beginning on 29 November and continuing through the end of December.

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/).