The agency detected activities of Mount Anak Krakatau eruption on Friday (Dec 21), Rahmat Triyono, the head of the earthquake and tsunami center of BMKG, said in a statement here, Monday.
The government had earlier declared the alert level 2 of Mount Krakatau which produced ash column reaching a height of some 400 meters above its crater and 738 meters above sea level.
The eruption column was dark, thick and inclined toward north.
"Earlier, we had issued an early warning of waves reaching height of between 1.5 meters and 2.5 meters from 7 a.m. Western Indonesian Standard Time (WIB) on Dec 22 to 7 a.m. WIB on Dec 25, 2018," he said.
On Saturday (Dec 22) at 8:56 p.m. WIB, Mount Anak Krakatau erupted and triggered a collapse of flank measuring 64 hectares.
The eruption was recorded by seismograph censor (CGJ) of BMKG in Cigelulis, Pandeglang, and several other censors existing in Banten and Lampung areas.
The agency`s automatic earthquake processing system did not process the data further because the recorded tremor was not tectonic earthquake signal.
The early warning system owned by BMKG currently is for tsunami induced by tectonic earthquake, while the Sunday Strait tsunami on Dec 22, 2018, was caused by volcanic eruption of Mount Anak Krakatau, or the Child of legendary Mount Krakatoa.
Besides, BMKG does not monitor activities of Mount Anak Krakatau nor other volcanoes, because they are monitored by the Center of Volcanology and Geology Disaster Mitigation of the Geology Department of the Energy and Mineral Resources Ministry, he said.
On Dec 23, 2018, BMKG confirmed that the Sunda Strait tsunami was triggered by a magnitude 3.4-volcanic tremor whose epicenter was locoated at 115.46 degree eastern latitude and 6.10 degree southern longitude, and at a depth of one kilometer.
Reporting by Desi Purnamawati
Editing by Fardah Assegaf, Andi Abdussalam