Alert Level Raised To Maximum 4 At Sinabung Volcano In Sumatra, Indonesia

Red Alert Issued

The alert level of the volcano was again raised from “siaga” to “awas”, the highest on the Indonesian 1-4 scale. The reason is a currently elevated risk of possibly larger pyroclastic flows, that could be triggered by collapse of the increasing volume of the viscous lava lobe emplaced on the upper SE flank. 
 
The alert level of the volcano had been at 4 during late Nov 2013 until April 2014,- the first and so far most violent phase of pyroclastic flow generation after the lava had spilled over the summit crater and built a large lobe that reached the base of the cone.
The lava lobe on Sinabung’s upper SE flank on 3 June 2015 (Beidar Sinabung / facebook) 
Since then, for more than a year now, lava effusion has continued at mild, more or less constant rate. Tremor data suggest that the supply rate of magma to the volcano’s summit has been more or less constant during the past year.
 
This activity produced successive lava lobes on the upper flank, all on top and/or at the upper sides of the main flow emplaced during the first months of the eruption. Associated avalanches and pyroclastic flows had mostly been smaller in comparison, although a few larger ones occurred as well. 
 
 
Rock avalanche on the ENE flank (Mbah Lëwå / facebook)
 
However, the number of rock avalanches and pyroclastic flows has not been constant over time. It shows an amazing cyclic pattern at a rhythm of approx. 4-5 weeks for each. This corresponds to alternating phases of slow growth of a lobe, during which relatively few rockfalls occur, and a following period when the lobe is being destroyed by more frequent rockfalls / pyroclastic flows of various sizes.
 
Different to the previous episodes, the current lobe which appeared in late April has now become significantly bigger than its predecessors. Therefore, it could in turn also produce much larger pyroclastic flows if it collapses. Its volume has been estimated to be approx. 3 million cubic meters, about twice as the previous lobe that formed during late March-April.
 
The current lobe has even become large enough to overspill to the ENE side where a number of rock avalanches and smaller pyroclastic flows have started to occur, in an area that has not been touched for months.
 
Authorities started to evacuate more than 1,800 people from several villages (Jeraya, Mardinding, Sukanalu, Sigarang-fierce, Kutagugung, Lau Kawar) and the exclusion zone was increased to 7 km distance from the summit in the SE sector of the volcano.
Number of rockfalls / pyroclastic flows since 2015 (PVMBG)
Courtesy of volcanodiscovery.com

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