Report on Merapi (Indonesia) — October 1996
Bulletin of the Global Volcanism Network, vol. 21, no. 10 (October 1996)
Managing Editor: Richard Wunderman.
Merapi (Indonesia) Peak in volcanism on 31 October results in over 15 pyroclastic flows
Please cite this report as:
Global Volcanism Program, 1996. Report on Merapi (Indonesia) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 21:10. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199610-263250.
7.54°S, 110.446°E; summit elev. 2910 m
All times are local (unless otherwise noted)
Sometime between 1417 and 1930 on 9 August, a pyroclastic flow traveled 3.5 km from the summit down the SSW flank, reaching the upper parts of the Krasak and Boyong rivers. No casualties were reported, and neither A- nor B-type volcanic earthquakes were recorded.
During September white plumes rose above the summit solfarata field. From the second week of September, multiphase volcanic earthquakes, which are dominant at Merapi, rose sharply to 1,398 events/day. This seismic increase accompanied growth of the lava dome. After the 9 August pyroclastic flow, the volume of the remaining lava dome was ~100,000 m3. The dome volume was estimated at 4.5 x 105 and 7.5 x 105 m3 on 24 September and 11 October, respectively, based on Celestron photographs. The growth rate of the lava dome was estimated at 17,050 m3/day on 11 October, much higher than usual (3,000-5,000 m3/day).
Multiphase earthquakes again increased with 3,804 events registered on 24 October. These earthquakes showed harmonic patterns, which had never been recorded before at Merapi. At 1533 on 24 October, four pyroclastic flows descended toward the upper reaches of the Krasak and Boyong rivers, and traveled a maximum of 2.5 km from the summit. No casualties were reported.
Between 0000 and 1800 on 27 October, multiphase earthquakes suddenly decreased with only 32 events recorded. However, multiphase earthquakes increased again on 30 October with 693 events during a period of six hours. From Celestron photographs on 29 October, it was estimated that the lava dome was 11 x 105 m3 in volume with a daily growth of 17,500 m3. The height of the dome also increased from 42 to 48 m during 11-29 October. At 1813 on 28 October, a pyroclastic flow descended into the upper part of the Krasak River, and the next day (0133 and 1633 on 29 October) two more pyroclastic flows followed. At 0714 on 30 October one pyroclastic flow was observed to move down toward the upper part of the Sat River from the 1992 lava dome. The tiltmeter at Station 2 indicated increasing deformation (>310 µrad) from 9 August to late October, consistent with increasing seismic and eruptive activity.
Volcanic activity reached a peak on 31 October. At 1544 a series of pyroclastic flows began to be observed. They traveled from the summit to the upper reaches of the Bebeng, Krasak, Boyong, and Kuning rivers. From 1544 to 1650, seventeen pyroclastic flows were recorded. The biggest ones occurred at 1637 and 1640, respectively, with a maximum runout of 3 km. Between 1718 and 2117, dense clouds hampered direct observations, but seismic information indicated nine small pyroclastic flows. During 2125-2155 thin ashfall (0.5 mm in thickness) was reported at Babadan Observatory (4 km W of Merapi). From 2130 to 2352 seven pyroclastic flows descended into the Krasak, Bebeng, and Boyong rivers, with a maximum runout of 3 km.
Pyroclastic flows decreased the next day (1 November) and ceased on 2 November. Five pyroclastic flows were observed to travel 1 km from the summit between 0000 and 0514 on 1 November. From 2 November, the lava dome showed no signs of significant growth, and the number and released energy of multiphase earthquakes also declined. However, rock-fall avalanches slowly increased.
Geologic Background. Merapi, one of Indonesia's most active volcanoes, lies in one of the world's most densely populated areas and dominates the landscape immediately north of the major city of Yogyakarta. It is the youngest and southernmost of a volcanic chain extending NNW to Ungaran volcano. Growth of Old Merapi during the Pleistocene ended with major edifice collapse perhaps about 2000 years ago, leaving a large arcuate scarp cutting the eroded older Batulawang volcano. Subsequently growth of the steep-sided Young Merapi edifice, its upper part unvegetated due to frequent eruptive activity, began SW of the earlier collapse scarp. Pyroclastic flows and lahars accompanying growth and collapse of the steep-sided active summit lava dome have devastated cultivated lands on the western-to-southern flanks and caused many fatalities during historical time.
Information Contacts: Mas Atje Purbawinata, Director, Merapi Volcano Observatory, Jalan Cendana 15, Yogyakarta 55166, Indonesia; Wimpy S. Tjetjep, Director, Volcanological Survey of Indonesia, Jalan Diponegoro 57, Bandung 40122, Indonesia.