Canal Volcan´s timelapse webcam capture of the eruption on 17 September, propelling an ash cloud up 6 km.
Thousand of villagers arount Mount Sinabung had to evacuate on September 15, 2013, when the volcano erupted the second time within 3 years after a long period of dormancy. Several people reportedly died of ash inhalation. This BBC gallery illustrates how returning evacuees were confronted by the new explosion on September 17, driving thousands more out of their homes. The Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, Center for Volcanology and Geological Hazard Mitigation) raised the alert level on 15 September 2013 from level II to III and ordered evacuation (in Indonesian) in a 3 km exclusion zone. Several seismic and GPS stations deployed during the crisis in 2010 monitor volcanic activity and PVMBG reported repeated fluctuations from 2012 to 2013 which stabilized from July to September 2013. There were several hundred volcanic earthquakes in August and September prior to the first eruption. Darwin VAAC released a volcanic ash advisory on 17 September reporting the ash cloud rising to FL200 (20000 ft = 6100 m).
Mount Sinabung (Indonesian Gunung Sinabung) is a 2460-m-high andesitic-to-dacitic stratovolcano with many lava flows on its flanks. It lies 25 km NW of Lake_Toba which formed in a supervolcanic eruption 69,000 to 77,000 years ago. The youngest crater is at the southern end of four overlapping summit vents along a N-S line. Apart from an unconfirmed eruption in 1881 and some solfataric activity in 1912 the last explosive eruption happened during August-September 2010 shooting up 5 km above the summit. In the years prior to that eruption an InSAR time-series revealed 2.1 cm inflation per year which only partially reversed after 2010 until the end of the measuring period 2011.
Sumatra has a volcanic arc above a NE-dipping subduction zone. In contrast to Bali/Java, the Indian oceanic plate plunges under the Sundaland continental margin at a rather shallow angle, resulting in only 100 km depth from the volcanic centers to the potential magma source. Thus, Sumatra does not behave like a typical island arc subduction zone. Sumatra´s volcanism dates back at least to the late Permian starting with porphyritic basic lavas. Late Mesozoic volcanic rocks distributed along the Great Sumatran fault, a strike-slip fault which splits Sumatra by it´s entire length, include include ophiolite-related spilites (resulting from alteration of oceanic basalt), andesites and basalts. Quaternary volcanoes are more acidic, thus more explosive, mainly calc-alkaline andesites, dacites and rhyolites with few basalts, including the >2000 km3 Pleistocene rhyolitic Toba Tuffs. It is not clear whether recent volcanism in Sumatra relates to extensive fault movements, to the peculiar structure of the fore-arc or is not related to subduction at all. More details on the complicated geology of Sumatra can be found in this thesis (abstract).
Dfm found information on the 2010 eruptions of Sinabung here and in more detail here. Time series plots of the seismicity and cumulative seismic energy release during 2010 show no sharp increases during the three eruptions, suggesting that they were rather small. A 2D projection of a 3D plot of volcanic earthquakes may indicate a presumable magma storage volume at shallow depth. Hendrasto et al. 2012 describe two concentrations of the epicenters along a northeast-southwest lineament, near an elongated sinistral fault zone between Sinabung and neighbouring Sibayak, both being back-arc volcanoes. Sutawidjaja et al. 2013 classify the 2010 eruption as phreatic event and vent clearing after a 1200 years long dormancy.
Fortunately there is a webcam (Badan Geologi, http://storage.vsi.esdm.go.id/monitoring/CCTV/Sinabung/Latest/cam_1.jpg):
Granyia overlayed two screenshots taken at dusk and at night showing incandescence on the flank several hundred meters below the summit:
Additional links and images (found by cbus and Milord P):
Up-to date information and ash trajectory by BMKG: http://www.bmkg.go.id/BMKG_Pusat/Klimatologi/Informasi_Gunung_Api.bmkg