Visual observations during 17-20 December revealed that 109 lava avalanches traveled predominately toward the upstream portion of the Sat River, and to a lesser extent to the Lamat and Senowo rivers. The maximum run-out distance was ~2.5 km. A pyroclastic flow on 18 December traveled ~1 km to the upstream portion of the Bebeng River. During the report period, seismicity was dominated by avalanche earthquakes that were similar in frequency and magnitude to those of previous weeks. Merapi remained at Alert Level 2 (on a scale of 1-4).

Geological Summary. Merapi, one of Indonesia's most active volcanoes, lies in one of the world's most densely populated areas and dominates the landscape immediately north of the major city of Yogyakarta. It is the youngest and southernmost of a volcanic chain extending NNW to Ungaran volcano. Growth of Old Merapi during the Pleistocene ended with major edifice collapse perhaps about 2,000 years ago, leaving a large arcuate scarp cutting the eroded older Batulawang volcano. Subsequent growth of the steep-sided Young Merapi edifice, its upper part unvegetated due to frequent activity, began SW of the earlier collapse scarp. Pyroclastic flows and lahars accompanying growth and collapse of the steep-sided active summit lava dome have devastated cultivated lands on the western-to-southern flanks and caused many fatalities.

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