BNPB reported that residents continued to be impacted by the collapses and large pyroclastic flows on Semeru’s SE flank on 4 December. As of 6 December, there were 781 people spread across 21 evacuation shelters and heavy ashfall prevented aid from easily reaching Pronojiwo Village, according to BNPB. Kajar Kuning was the worst affected village.

PVMBG reported that lava continued to erupt from the summit vent during 4-9 December, though activity generally declined. Ongoing thermal anomalies were identified in satellite images, though the intensity had decreased. The rate of deformation had declined based on tiltmeter data. Four pyroclastic flows moved as far as 6 km down the SE flanks, and avalanches of material traveled 300-500 m SE. At 0521 on 8 December a gray-to-white ash plume rose around 300 m above the summit and drifted N. At 0536 a gray ash plume rose 400 m and drifted N, as reported by a ground-based observer. A dense gray ash plume rose 500 m and drifted N at 0521 on 9 December. Later that day, at 1200, the Alert Level was lowered to 3 (the second highest level on a scale of 1-4) and the public was warned to stay at least 5 km away from the summit, as far as 13 km from the summer on the SE flanks, and 500 m from Kobokan drainages within 17 km of the summit, along with other drainages originating on Semeru, including the Bang, Kembar, and Sat, due to lahar, avalanche, and pyroclastic flow hazards.

Eruptive activity continued during 10-13 December. At 0653 on 10 December a dense white-to-gray ash plume rose about 700 m above the summit and drifted N. White-and-gray ash plume were visible at 0652 on 11 December and 0727 on 12 December rising at least 500 m and drifting S and SW, respectively.

Geological Summary. Semeru, the highest volcano on Java, and one of its most active, lies at the southern end of a volcanic massif extending north to the Tengger caldera. The steep-sided volcano, also referred to as Mahameru (Great Mountain), rises above coastal plains to the south. Gunung Semeru was constructed south of the overlapping Ajek-ajek and Jambangan calderas. A line of lake-filled maars was constructed along a N-S trend cutting through the summit, and cinder cones and lava domes occupy the eastern and NE flanks. Summit topography is complicated by the shifting of craters from NW to SE. Frequent 19th and 20th century eruptions were dominated by small-to-moderate explosions from the summit crater, with occasional lava flows and larger explosive eruptions accompanied by pyroclastic flows that have reached the lower flanks of the volcano.

Sources: Badan Nacional Penanggulangan Bencana (BNPB), Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM)