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Chaitén bulletin no. 91 (5 May 2009) 9 May 2009

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Bulletin no. 91 on the eruption of Chaitén volcano, covering the period 29 April to 4 May 2009, is now available as a PDF via the informes page of the Observatorio Volcanológico de los Andes del Sur (OVDAS) website, and a shortened version has been published on the SERNAGEOMIN website. Translation of the full document as follows.

ERUPTION OF CHAITÉN VOLCANO
TECHNICAL BULLETIN NO. 91
29 APRIL TO 4 MAY 2009
OVDAS-SERNAGEOMIN

1. Visual monitoring

During this period the volcano has been observed during fieldwork carried out on 29 April, and also in partial form during an overflight carried out on 1 May.

The eruption continues with the consistent growth of the dome complex and their column of gas and ash, which has occasionally reached 2.0 km above the volcano (Fig. 1), with gravitational collapses generating block-and-ash flows (FBC; Fig. 2). The emission of gas and ash is concentrated principally in the centre of emission located in the south-east sector of the dome complex (Fig. 2).

On 1 May there was an overflight of the volcano in a Twin Otter aircraft of FACH [Chilean Air Force], co-ordinated for Mr Guillermo Nuñez, Director of ONEMI [state emergencies office] for Los Lagos Region. It was possible to observe that the central pinnacle has fractured into three main, very unstable, blocks and that the dome complex has continued with its rapid growth, expanding considerably (Fig. 3). This produces a major danger for the valley of the Chaitén river, as possible gravitational collapses in the southern sector may generate block-and-ash flows (FBC). The surface morphology of the dome complex is very irregular, with various pinnacles and notable heights (estimated as in the region of 100m).

Figure 1
Figures 1 [above] and 2 [below]. Views of Chaitén on 29 April, showing the gas and ash column with a predominance of water vapour from condensation which reached an altitude of 2.0 km (1). The complex of domes continue their growth with gravitational collapses generating block-and-ash flows (FBC) (2). Also shown is the active centre of emission in the south-east sector of the dome complex (2).
Figure 2

Figure 3
Figure 3. Aerial view of the dome complex, the volume and extent of which have grown significantly, and of the very fractured central pinnacle (courtesy Mr Javier Romero, Vialidad MOP, Puerto Montt).

2. Seismic monitoring

The seismicity registered by the monitoring network of Chaitén volcano this last week has remained relatively stable with an average of 12 to 14 earthquakes of HB [hybrid] type per day. After the slight decline in magnitudes on 25 and 26 April, there was again an increase after 1 May, nevertheless, the RSAM values have not exceeded 115,000 units and only four events have exceeded 100,000 units (Fig. 4). During the last few days there has again occurred a slight decline in the higher magnitudes, the values of which have varied between 3.5 and 4.0, registering a maximum of up to 4.2. With regard to the epicentres of the earthquakes, these have continued to be located on the edges of the caldera, with depths that are principally concentrated between 3 and 13 km.

Figure 4
Figure 4. RSAM graph showing the relative stability of the seismic energy from 15 April to 4 May 2009. [Click on image for enlarged version.]

3. Conclusions and interpretation

The eruptive activity continues with the growth of the dome complex, which has risen and expanded. Although the seismic activity occurring to date shows a slight decline in magnitudes, the highest reaching up to magnitude 4.2, the number of HB-type earthquakes has been maintained. In relation to the seismic energy released the levels have been maintained, a phenomenon which is directly related to the growth of the dome complex. In consequence, there is a continuing latent danger of the occurrence of possible explosions and block-and-ash flows (FBC), along with lahars, which may affect valleys adjacent to Chaitén volcano.

In consequence, given that the seismic activity remains elevated and, in accordance with this, the sustained growth of the dome complex, with the possibility of the generation of block-and-ash flows (FBC) that may affect the surrounding valleys, SERNAGEOMIN maintains Volcanic Red Alert.

[End of SERNAGEOMIN bulletin.]

For all our Chaitén coverage: Chaitén « The Volcanism Blog.

Information
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

The Volcanism Blog

Comments

1. Dan - 9 May 2009

I’d like to see a more zoomed out aerial view so we can get a better idea of what the entire caldera looks like. The last time we got one of those photos was after the big dome collapse a few months ago.

2. theroachman - 10 May 2009

The photo of the spine is awesome!

3. Fresh Bilge » Monster - 11 May 2009

[…] explosions, the volume of ejected material is becoming very impressive. In the latest bulletin, translated at The Volcanism Blog, a chart shows seismic activity remaining at the intense level observed since early April. 8:11 […]

4. Perry - 11 May 2009

That spine in fig 3 looks on a par with the spine that collapsed at MT. PELÉE in 1903.

I wonder what knowledge the few inhabitants who remain in Chaitén have of volcanoes? They are unlikely to have internet access to research the subject and assess the possible consequences of remaining in the town. It does not look good.

http://www.geology.sdsu.edu/how_volcanoes_work/Pelee.html


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