Volcanic and seismic activity were relatively high at Semeru during 17-30 December. During the report period the most notable seismically recorded events were 1,085 explosions, 49 lava avalanches, 6 pyroclastic flows, and 3 floods/lahars. Explosions sent ash plumes to 400 m above Jonggring Seloko crater. On 25 December a pyroclastic flow traveled 2.5 km into the Besuk Kembar River. On 29 December during 1700-2015, a lahar traveled along the Besuk Kembar River relatively close to Supit village. Early that morning the residents of Supit were evacuated. On 30 December pyroclastic flows traveled 2 km toward the Besuk Kembar River at 0720, and at 1000 one traveled 4 km toward Supit village. The Alert Level at Semeru remained at 2 (on a scale of 1-4).

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.

Source: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM)