JAKARTA, Indonesia—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 …