CVGHM reported that activity at Merapi declined during 1-3 December. Seismic data showed a decrease in the number of earthquakes as well as amplitude of the events. Deformation measurements did not show any significant changes. Although fog often prevented visual observations or views through webcams, gas plumes were seen rising 500 m above the crater and drifting W. Sulfur dioxide plumes were no longer detected in satellite imagery. CVGHM noted that lahar deposits were seen in multiple drainages and that several bridges had been recently damaged by lahar activity. On 4 December, the Alert Level was lowered to 3 (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)