Report on Egon (Indonesia) — October 2009
Bulletin of the Global Volcanism Network, vol. 34, no. 10 (October 2009)
Managing Editor: Richard Wunderman.
Egon (Indonesia) Update on the 15 April 2008 eruption and April-May 2008 seismicity
Please cite this report as:
Global Volcanism Program, 2009. Report on Egon (Indonesia) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 34:10. Smithsonian Institution.
8.676°S, 122.455°E; summit elev. 1661 m
All times are local (unless otherwise noted)
Since the eruption of 28 January 2004, Egon has frequently undergone phreatic eruptions without any significant increase in volcanic tremor or earthquakes. Our last report on Egon (BGVN 33:08) summarized the gradual decline of activity during April-May 2008. This report overlaps with the earlier one but benefits from better report translation. The closest large city to Egon is Maumere (Flores), ~ 25 km WNW.
The Center of Volcanology and Geological Hazard Mitigation (CVGHM) provided additional information on the April-May 2008 disturbances (BGVN 33:08). A spike in the volcano's activity took place on 6-7 April 2008 (table 3). Reports noted a subsequent decrease in earth movements. On 4-15 April 2008 thin white smoke was seen rising ~ 25-50 m above the crater. This emission was considered a daily activity; however seismicity became evident.
|Date||Deep Volcanic Earthquakes (VA)||Shallow Volcanic Earthquakes (VB)||Tremor Duration (seconds)||Observations|
|05 Apr 2008||3||0||--||Usual daily occurrence of hot air blasts and whitish smoke.|
|06 Apr 2008||38||93||--||Hot air blasts and "whitish smoke," rising 25-50 m.|
|07 Apr 2008||15||2||--||Hot air blasts and "whitish smoke," rising ~35 m.|
|08-14 Apr 2008||6||3||--||Hot air blasts and "whitish smoke," rising ~25 m above the crater and a significant decrease in volcanic quakes.|
|15 Apr 2008||--||--||1290 s||Ash plume to 4,000 m height.|
|20 Apr 2008||--||--||1073.5 s||Ash plume to 2,000 m height.|
|24 Apr 2008||--||--||91 s||Ash plume to 850 m height.|
|28 Apr 2008||--||--||60.5 s||Ash plume to 75 m height.|
On 15 April 2008 a phreatic eruption occurred and CVGHM raised the Alert Level to 3 ("Saga" - on a scale of 1-4). Visual observations indicated that the ash column rose ~ 4,000 m above the crater; however the ash was not identifiable from satellite survey due to cloud cover. The eruption was accompanied by a "grumbling" sound. An ash/cinder cloud reached the city of Maumere (Flores), ~ 20 km WNW. Because of the height of eruptive plume, authorities at Waioti Airport serving Maumere were alerted. The emergency response team, together with the district government of Sikka (Flores) onsite at the villages closest to the eruption, reported that ~ 600 persons from local villages evacuated; they reported no fatalities.
The Darwin Volcanic Ash Advisory Center (VAAC) issued two alerts of the volcanic activity at Egon, on 15 and 16 April 2008. Between 15 April and 11 May 2008, four explosive tremor events were recorded (table 3). Land deformation in the vicinity of the volcano stabilized after 27 April 2008.
During 15 April-10 May 2008, 1-2 deep volcanic earthquakes occurred daily. Between 25 April to 10 May, shallow volcanic earthquakes decreased from 6-20 daily to 1-10 daily. During that time, tremors caused by hot air blasts continued to be recorded, reaching a rather high total range of around 6-47 events per day. The higher values are comparatively large; a normal stasis condition is considered to be a ~ 1-9 hot air blast signals per day. On 12 May 2008, hot air tremors had amplitudes of 2 mm and durations of 5-11 seconds. Whitish smoke could frequently be seen reaching a height of only 10 m above the peak. On 13 May, CVGHM downgraded the hazard status to Alert Level 2 (Waspada).
For the rest of May 2008 and for more than a year, Egon's was relatively quiet. From 4 March to 12 July 2009, type-A earthquakes were recorded at a rate of 1-2 events per day; type-B earthquakes, 1-3 events per day; (except on 6 May when six were recorded). During that interval there were 1-9 hot air blast earthquakes per day and the hot air blasts of smoke were generally whitish in color and were rose to ~ 10 m over the peak. Eruptive earthquakes were absent. Although tremor was still recorded (with an amplitude of 0.5-4 mm), since 4 March 2009, earth movements have decreased. On 17 July 2009, the CGVHM. downgraded the hazard status to Alert Level 1 (Normal).
MODVOLC review of activity shows no thermal indicators of volcanic activity.
Geologic Background. Gunung Egon, also known as Namang, sits astride the narrow waist of eastern Flores Island. The barren, sparsely vegetated summit region has a 350-m-wide, 200-m-deep crater that sometimes contains a lake. Other small crater lakes occur on the flanks. A lava dome forms the southern summit. Solfataric activity occurs on the crater wall and rim and on the upper S flank. Reports of historical eruptive activity prior to explosive eruptions beginning in 2004 were inconclusive. A column of "smoke" was often observed above the summit during 1888-1891 and in 1892. Strong "smoke" emission in 1907 reported by Sapper (1917) was considered by the Catalog of Active Volcanoes of the World (Neumann van Padang, 1951) to be an historical eruption, but Kemmerling (1929) noted that this was likely confused with an eruption on the same date and time from Lewotobi Lakilaki.
Information Contacts: Center of Volcanology and Geological Hazard Mitigation, Saut Simatupang, 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), University of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/).