According to Indonesia's geological agency, the volcanic activities started increasing from Saturday and it could trigger an avalanche of lava and searing gas. About 8 million cubic meters of sand from the volcano's crater has already clogged the Besuk Kobokan river.
Indonesian authorities raised the alert level for the highest volcano on Java island, saying Mount Semeru could blow up again after a sudden eruption earlier this month left 48 people dead and 36 missing in villages that were buried in layers of mud.
Indonesia's geological agency said Saturday it picked up increasing activity that could trigger an avalanche of lava and searing gas, similar to the Dec. 4 eruption, which was preceded by heavy monsoon rains that partially collapsed a lava dome on the 3,676-meter (12,060-foot) mountain.
About 8 million cubic meters (282 million cubic feet) of sand from the volcano's crater clogged the Besuk Kobokan River, which is in the path of the lava flow, Energy and Mineral Resources Minister Arifin Tasrif said.
“As a result, if there is another eruption, it would block the flow path and create new lava flows spreading to the surrounding area,” Tasrif said, adding that the government had set up a new danger map and urged people to obey it. It raised the alert level to the second-highest.
The head of Indonesia's Volcanology and Geological Hazard Mitigation Center, Andiani, said villagers living on Semeru's fertile slopes are advised to stay 13 kilometers (8 miles) from the crater's mouth. She also stopped tourism and mining activities along the Besuk Kobokan watershed.
The search and rescue operations ended on Friday with 36 people still unaccounted for. More than 100 people were injured, 22 of them with serious burns. More than 5,200 houses and buildings were damaged, said National Disaster Mitigation Agency spokesperson Abdul Muhari.
After visiting the area last week, President Joko Widodo pledged to rebuild infrastructure, including the main bridge connecting the worst-hit town of Lumajang to other cities, and move about 2,970 houses out of the danger zone.
Semeru, also known as Mahameru, has erupted many times in the last 200 years. Still, as on many of the 129 volcanoes monitored in Indonesia, tens of thousands of people live on its fertile slopes. It last erupted in January, with no casualties.
Indonesia, an archipelago of more than 270 million people, is prone to earthquakes and volcanic activity because it sits along the Pacific “Ring of Fire,” a horseshoe-shaped series of fault lines.