Chaitén update, 20 February 2009 20 February 2009Posted by admin in activity reports, Chaitén, Chile, eruptions, volcano monitoring.
Tags: Chaitén, Chile, South America, volcanic activity reports, volcanic eruptions, volcano monitoring
SERNAGEOMIN have a new and very lengthy Chaitén update available on their website: 20.02.2009 Sernageomin Informa sobre el Volcán Chaitén. Translation as follows (first part only below – click on ‘more’ to read the whole thing).
[N.B. BBC News have a short video from the overflight referred to in this report, available here.]
20.02.2009 SERNAGEOMIN report on Chaitén Volcano
Until approximately 10:28 yesterday the seismic activity had remained essentially similar to that registered on preceding days. From 10:28 a small swarm of earthquakes began of HB type, associated with a slightly energetic background tremor, which was related to the collapse of a significant sector of the southern slopes of the domes, lateral explosions and block-and-ash flows and pyroclastic flows, all directed towards the south, which reached the upper part of the valley of the Chaitén river up to 5km north of the city of the same name. In addition, it generated eruption columns which, around 14:00, reached approximately 8km in altitude, with a dispersal plume directed to the south-east, precipitating ash towards the town of Futaleufú.
In several previous reports the certain possibility of an occurrence of such collapses of the domes towards the south, with lateral explosions and the generation of block-and-ash flows and pyroclastic flows, has been highlighted. In accordance with the preceding, we reiterate what was stated in the special report issued yesterday, in the sense that it is not possible to rule out the occurrence of new collapses, lateral explosions and the generation of large pyroclastic flows that could affect the town of Chaitén and also towards other sectors of the volcano, in addition to the dispersal of ash which will affect areas situated, principally, in the south-east and north and south.
As was indicated in the special report issued during the afternoon of 19 February, according to telephone information from Chaitén, from Captain Villablanca of the police, at approximately 11:00 an explosion with emission of ashes was produced by the volcano. This eruptive activity was recorded by the DGAC camera at Chaitén airfield and Mr Dagoberto Guzmán. Moreover, the administrator of Pumalín Park reported that an ash cloud descended the Chaitén (or Blanco) river for approximately 5km towards Chaitén.
At approximately 14:00 on 19 February 2009 SERNAGEOMIN personnel, along with the Regional Intendente, the Presidential Delegate and the Regional Director of ONEMI, flew in a Chilean Air Force Twin Otter aircraft over the area of Chaitén volcano. At this time, most of the volcano was obscured by cloud cover, and it was only possible to make observations of the west flank of the volcano. Above the cloud level, at approximately 800m above sea level, the eruption column was observed climbing to between 6-8km, the upper portion formed of water vapour, and formed of water vapour and ash towards the base. Information from local inhabitants estimated that in the morning the column had reached between 15-20km in height and the dispersion plume was directed towards the south-east, extending to the area of Futaleufú. The part of the north flank which it was possible to observe presented an appearance similar to that of previous weeks, although the channel of the river Blanco or Rayas was notably darkened, probably because of the ash content in its waters. Towards the south flank, partial observations confirmed that part of the forest had been burned and large amount of water vapour emission were seen at points along the middle course of the Chaitén river. Given the limited visibility, the aircraft returned to the airfield on Talcán Island.
Figure 1. Plume consisting of water vapour and ash at 14:00, 19 February 2009 (photograph P. Duhart).
Later, from Talcán, an overflight took place in a Bolkow 44 helicoper of the Navy over the valley of the Chaitén river, which is on a direct route to the town of the same name. Thus, it was possible to observe the broad ridge produced by a significant partial collapse of the domes, which reached dimensions of 500 by 500 metres, estimated from its largest axis. This partial collapse occurred on the southern flank, and was connected to a lateral explosion and the generation of block-and-ash flows and pyroclastic flows which advanced 4-5km along the valley of the Chaitén river, destroying the native forest on the sides and bottom of the valley. Also, at the base of the valley of the Chaitén river, located at a distance of 3-4km from the town of the same name, five or six points with emission of gases and water vapour, with the appearance of fumaroles, were observed. This process can be interpreted, on a preliminary basis, as side effects of the pyroclastic flows, which when in contact with water generate a large amount of steam. Studies in progress will allow validation of these observations.
After landing in the town of Chaitén, observations from the vicinity of the río Blanco bridge towards the south flank of the volcano revealed the tremendously steep walls of much of the southern sector of the domes, facing the valley of the río Chaitén. It must be stressed that, given the weather conditions, these observatios reflect only partially awareness, at the moment, of occurrences in the mid-eastern and northern sectors of Chaitén volcano, where there are reports of collapses and pyroclastic flows that are not yet confirmed.
Figure 2. Ridge generated by partial collapse of the dome, viewed from west to east (photograph C. Brown).
Figure 3. Course of the Chaitén river valley showing towards the left emissions of water vapour (photograph C. Brown).
Figure 4. Detail of water vapour emissions in areas apparently not directly reached by the pyroclastic flows (photograph C. Brown).
On 17 February the Chaitén volcano seismic monitoring network registered 9 HB earthquakes with magnitudes between 3.5 and 3.9, which corresponds to comparable activity recorded in the preceding days. Similarly, on 18 February there were 13 HB earthquakes registered with magnitudes varying between 3.6 and 3.9.
From 10:28 on 19 February a slightly energetic background tremor and a swarm of HB earthquakes with magnitudes between 3.6 and 4.2 began, with depths between 3 and 5km, which were maintained until 13:46, when the seismic activity began to decline. This small swarm of HB earthquakes has been located, on a preliminary basis, immediately to the east of the volcano. In the preceding period, the collapse of the southern sector of the domes, lateral explosions and the subsequent pyroclastic flows and block-and-ash flows occurred.
Figure 5. Seismogram from the Santa Bárbara station for 19 February 2009 indicating the swarm of HB earthquakes and the energetic tremor occurring between 10:28 and 13:46.
In consequence, bearing in mind the persistence of the seismic activity associated with the eruption, which continues the growth of the dome and which has generated imporant collapses of the domes, lateral explosions, block-and-ash flows and pyroclastic flows, both towards the south and to the north of the volcano, SERNAGEOMIN maintains Volcanic Red Alert.
[End of SERNAGEOMIN bulletin.]
For all our Chaitén coverage: Chaitén « The Volcanism Blog.
Global Volcanism Program: Chaitén – summary information for Chaitén (1508-41)
SERNAGEOMIN – Servicio Nacional de Geología y Minería (Spanish)
Erupción del Volcán Chaitén – extensive coverage of the Chaitén eruption