The Volcanology and Geological Hazard Mitigation Center (PVMBG) says a loud rumble heard by residents of Greater Jakarta did not come from the Anak Krakatau volcanic eruption on Friday evening, though it does not know what it was.

Some internet users in Greater Jakarta said they had heard a loud rumble on Friday midnight till early Saturday morning and assumed that they were hearing the sound of Anak Krakatau after being informed about its eruptions that occurred at 9:58 p.m. and 10:35 p.m.

“[The sound] did not come [from Anak Krakatau], but from some other source, but we still cannot determine the source,” PVMBG western volcano mitigation head Nia Khaerani told The Jakarta Post on Saturday.

Officials at the Anak Krakatau observation post, located 42 kilometers from the volcano, had not heard the rumbling, she said, adding that the sound of the eruption could not possibly reach Greater Jakarta, which was 125 km away, as the eruptions were relatively small.

She said officials at the observation post of Mount Gede in Bogor, West Java, and Mount Salak in Sukabumi, West Java, had heard the rumbling. She surmised it may have been the sound of thunderstorms occurring after the Anak Krakatau eruptions.

“Not only [officials] at Mount Gede and Mount Salak heard the thunder, there were also thunderstorms around Mount Anak Krakatau at the same time as the eruption because of the current rainy season,” she said.

She said the infrasound device installed to record acoustic signals of the mountain had recorded increased activity around 10 to 11 p.m., but the frequency of the sound could not be heard by the human ear.

The PVMBG also said that residents of the islands of Java and Sumatra would not be affected by the Anak Krakatau eruption, as the spewing of rocks and lava only occurred around the mountain, adding that the agency had issued a code orange volcano observatory notice for aviation (VONA).

Volcanology expert and former Energy and Mineral Resources Ministry geological agency head Surono, however, suggested the sound caused by the eruption could have echoed throughout Greater Jakarta, as there was less activity and noise in the area as a result of large-scale social restrictions over COVID-19.

Coincidentally, Friday marked the first day of a partial lockdown in the capital.