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Report on Krakatau (Indonesia) — July 1995


Bulletin of the Global Volcanism Network, vol. 20, no. 7 (July 1995)
Managing Editor: Richard Wunderman.

Krakatau (Indonesia) Unusually loud sounds shown on seismic records

Please cite this report as:

Global Volcanism Program, 1995. Report on Krakatau (Indonesia) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 20:7. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199507-262000



6.102°S, 105.423°E; summit elev. 155 m

All times are local (unless otherwise noted)

During June there were unofficial reports of unusually loud noises heard on the W coast of Java . . . . On 27 June, GMU scientists visited islands around Anak Krakatau and heard some very loud sounds; only some of which correlated to visual activity at Anak Krakatau. The observers compared their observations to a 1993 visit, when the volcano emitted steam-bearing discharges accompanied by lightning. The eruptions on 27 June 1995 appeared dissimilar because they were ash-rich and without visible steam. In addition, the 27 June eruptions produced string-shaped columns with mushroom-shaped tops; lightning was absent.

The group deployed two seismometers for five hours of observation. A vertical-component long-period seismometer (0.2 Hz cutoff) was put on Panjang Island, 3.6 km W of Anak Krakatau's summit, and a 3-component broadband seismometer was put on Sertung Island, 3.2 km WNW.

Typical seismograms, showing two of the three components recorded on Sertung Island, appear on figure 11. In one case, a low-frequency seismic signal arrived ~8 seconds prior to a sharp onset, reaching amplitudes of 0.6 mm/sec in the vertical component (figure 11). The second seismometer also recorded ~8 seconds of seismic signal before the onset of the air wave. Other events also showed the same 8-second delay between the seismic signal and these air waves. The case shown was correlated with a small eruption that generated a loud sound and ultimately spawned an ash cloud of undisclosed dimension. Assuming a shallow source for the eruption, the travel times for first arrivals of the strong impulsive signals across the 3.2 km source-to-receiver distance were on the order of 10 seconds, roughly the velocity of sound in air (0.33 km/sec). Thus, the strong impulsive signals were probably due to pressure waves transmitted through the air.

Figure (see Caption) Figure 11. Seismic record from Anak Krakatau showing the vertical component for a typical event received on nearby Sertung Island. The data were collected on 27 June 1995 (t = 0.0 sec corresponds to 1049 local time). Approximate arrivals of the seismic and air-wave signals are indicated. Courtesy of Wahyudi and A. Brodscholl.

Geological Summary. The renowned volcano Krakatau (frequently misstated as Krakatoa) lies in the Sunda Strait between Java and Sumatra. Collapse of the ancestral edifice, perhaps in 416 or 535 CE, formed a 7-km-wide caldera. Remnants of that volcano are preserved in Verlaten and Lang Islands; subsequently Rakata, Danan, and Perbuwatan volcanoes were formed, coalescing to create the pre-1883 Krakatau Island. Caldera collapse during the catastrophic 1883 eruption destroyed Danan and Perbuwatan, and left only a remnant of Rakata. This eruption caused more than 36,000 fatalities, most as a result of tsunamis that swept the adjacent coastlines of Sumatra and Java. Pyroclastic surges traveled 40 km across the Sunda Strait and reached the Sumatra coast. After a quiescence of less than a half century, the post-collapse cone of Anak Krakatau (Child of Krakatau) was constructed within the 1883 caldera at a point between the former cones of Danan and Perbuwatan. Anak Krakatau has been the site of frequent eruptions since 1927.

Information Contacts: Wahyudi and A. Brodscholl, GMU.