Why was Anak Krakatau’s boom loud in West Java but silent in Banten?

Booms and tremors reported across parts of West Java during Anak Krakatau’s eruption were caused by acoustic waves from the volcano, Indonesia’s Geological Agency said.

Geological Agency chief Lana Saria said the timing of the reports matched the volcano’s increased activity on September 4 and 5. Signals recorded at monitoring stations in West Java strengthened the conclusion that the sounds came from the eruption.

Low-frequency acoustic waves can travel long distances under certain atmospheric conditions. They may be heard as booming sounds and felt as vibrations, Lana said. Similar effects have been reported during eruptions at Mount Kelud, whose booming sounds reached Central Java, and Mount Lewotobi Laki-laki, whose eruption was heard as far away as Maumere and Alor Island.

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The timing and the eruption of Krakatau Volcano, together with the signals at monitoring stations in West Java, strongly support that the phenomenon originated from acoustic waves from the Krakatau Volcano eruption.

Anak Krakatau’s activity has increased since July 2026. Authorities raised its status from Level II, Alert, to Level III, Standby, on July 2. At 11 p.m. local time on September 4, the volcano entered a continuing eruption that included lava fountains, rumbling and booms. Its highest activity was recorded on September 5.

The Geological Agency is monitoring the volcano through seismic networks, visual observations, deformation measurements, gas emissions, satellite imagery and field reports. The government urged people in affected areas to rely on official updates and avoid unverified information about Anak Krakatau.

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These low-frequency acoustic waves can travel far under certain atmospheric conditions and be perceived as booms and vibrations.