In a special report posted on 27 December PVMBG reported that seismicity at Marapi increased during 25-26 December, characterized by an increase in the number of deep volcanic earthquakes, tornillo-type events, and shallow volcanic earthquakes associated with hydrothermal activity. Surficial activity showed no changes; diffuse white plumes rose as high as 100 m above the summit during the previous eight months. The Alert Level remained at 2 (on a scale of 1-4).

Geological Summary. Gunung Marapi, not to be confused with the better-known Merapi volcano on Java, is Sumatra's most active volcano. This massive complex stratovolcano rises 2,000 m above the Bukittinggi Plain in the Padang Highlands. A broad summit contains multiple partially overlapping summit craters constructed within the small 1.4-km-wide Bancah caldera. The summit craters are located along an ENE-WSW line, with volcanism migrating to the west. More than 50 eruptions, typically consisting of small-to-moderate explosive activity, have been recorded since the end of the 18th century; no lava flows outside the summit craters have been reported in historical time.

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