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Chaitén update, 16 December 2008 16 December 2008

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The latest SERNAGEOMIN bulletin on Chaitén, no. 65, covering the period 9 to 15 December 2008, has been released. The complete original report as a PDF is available via Werner Luis’s Chaitén site, and a condensed version has been published on the SERNAGEOMIN site. The newest dome, ‘New Dome 2’, which has been growing very rapidly, is clearly a very unstable structure and is experiencing major gravitational collapses, producing lateral explosions and generating block and ash flows which are choking the Chaitén river and threatening the remains of Chaitén town with new inundations. There is also concern about the strength and stability of the south flank of the volcano’s original dome, which is bearing the weight of the new domes that have developed since May. A failure here would bring a large-scale collapse, swamping Chaitén river valley and town with vast quantities of ash and debris. In short, a very unstable and unpredictable situation. Translation as follows:

ERUPTION OF CHAITÉN VOLCANO
TECHNICAL BULLETIN NO. 65
9-15 DECEMBER 2008
OVDAS-SERNAGEOMIN

1. Visual monitoring

During this period and on the basis of observations made from the town of Chaitén through the DGAC camera, it is clear that the continuing growth of New Dome 2 on the north-eastern part of New Dome 1 has caused partial collapses of New Dome 2, generating block and ash flows. Additionally, the gaseous emissions have shown a predominance of water vapour, although at times there has been an increase in the concentration of volcanic gases, as well as changes in colour in the columns emitted from New Dome 2, depending on the concentration of particulate matter. The gas columns have reached a height of up to 2.0km above the dome complex and the prevailing winds have been from the west-north-west, after some days of the Viento Puelche [wind from the east] which dispersed the ash towards the west.

On 13 December between 17:15 and 17:40, and for the first time, it was possible to observe gravitational collapses of New Dome 2, with falls of rock southwards onto New Dome 1 and towards the tributary of the Chaitén river which rises in the caldera, as can be clearly seen in the sequence of images from the DGAC camera (Fig. 1).

Figure 1
Fig. 1. Sequence of images from Chaitén (DGAC camera) on 13 December, between 17:15 and 17:40, where the generation of block and ash flows towards the south-east and south can be clearly seen.

It has not been possible to see the generation of smaller lateral explosions, but it cannot be excluded that they have occurred, given the large quantity of particulate material which generates the block and ash flows. The prevailing wind from the north-west, with relatively high speeds (~50-70km/h), dispersed the plume towards the south-east in the direction of Futaleufú.

On 14 December between 13:15 and 14:04 the process was repeated with a slightly higher intensity as can be seen from the DGAC camera sequence (Fig. 2). Although the strong wind continued from the north-west, in the final image a faint cloud of ash can be seen moving against the wind, which would have been generated by a smaller lateral explosion directed towards this area (Fig. 2).

Figure 2
Fig. 2. Sequence of images from Chaitén (DGAC camera) on 14 December, between 13:05 and 14:04, in which the greater intensity of generation of block and ash flows towards the south can be seen. In the final image, a faint cloud of ash can be seen moving in a western direction (against the wind), the result of a smaller lateral explosion that may have occurred towards this area.

The occurrence of collapses of the summit of New Dome 2 towards the south, with rock falls on New Dome 1 and the generation of block and ash flows towards the tributary of the Chaitén river which rises in the caldera of the volcano, is causing significant depositions of volcanic materials of various sizes in the hydrographic basin of the Chaitén river. These materials are accumulating in the above-mentioned tributary and with the rains will be displaced towards Chaitén town, creating a dangerous new process for the town. There have not yet been lateral explosions towards the southern sector. However, the continuing gravitational collapses of New Dome 2 and the consequent block and ash flows towards the south could affect the valley of the Chaitén river, increasing danger in that area.

Finally, the stability of the south wall of the old dome, upon which New Domes 1 and 2 have developed, is an unknown quantity. The pressure exerted by the large volume of material accumulated in the new domes, added to the intense gas emissions from the south wall, have weakened this area and, with the continuation of eruptive activity, it could suffer an eventual collapse into the Chaitén river, accompanied by possible lateral explosions and greater gravitational landslides.

2. Seismic activity

Seismic activity remains stable as reported in the previous Technical Bulletin No. 64. HB-type earthquakes are continuing to be registered, with an average of 2-6 per hour and with magnitudes of 1.0 to 3.8. Among these, 1-2 earthquakes per hour stand out with greater magnitudes, varying between 3.0 and 3.8. With regard to the VT-type earthquakes, these have shown a slight increase, with an average of 5-7 events per day, although of small magnitude, remaining between 1.0 and 1.4. In relation to their locations, the arrival times of the p and s waves recorded by the STAB station remain between 2.1 and 2.4 seconds, indicating that the epicentres are still located under the growing dome and in the vicinity of the caldera of Chaitén volcano.

3. Conclusion and interpretation

The increase in seismicity recorded at the beginning of December, which related to both the growth and the number of HB-type earthquakes, was connected with an increase in the rate of emission of lava. In effect, New Dome 2 has experienced a remarkable growth with the formation of pinnacles at its summit and gravitational collapses through the instability of its flanks, which have generated rockfalls and block and ash flows. This process can be associated with a major movement of magmatic fluids in the principal conduit, fracturing surrounding rocks in its ascent and creating, with the greater rate of emission of viscous lava, the growth of New Dome 2.

Finally, the gravitational collapses through instability of the summit of New Dome 2, generating block and ash flows, have not been registered by the seismic telemetry network, as they are very superficial processes.

As has been noted in previous bulletins, this situation may continue for an indefinite time, so the growth of the domes, their gravitational collapses, the eventual generation of lateral explosions, with variations in the height of the eruption column, will continue until the eruption of Chaitén volcano finally comes to an end. New Dome 2 maintains its rapid growth and could reach even greater dimensions, and may also form new domes. In consequence, the caldera may be filled and begin to generate rockfalls outside itself. The block and ash flows have already begun to fall towards the south, threatening the basin of the Chaitén river with new accumulations of volcanic materials that may be displaced by the rains towards Chaitén town. The eventual occurrence of lateral explosions towards the south, with the instability and collapse of the old dome, are processes that cannot be ruled out while the eruption continues.

Considering that the eruption is continuing with the formation and growth of new domes, accompanied by the generation of block and ash flows towards the south, SERNAGEOMIN maintains Volcanic Red Alert.

[End of bulletin.]

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

Information
Global Volcanism Program: Chaitén – summary information for Chaitén (1508-41)
ONEMI, Oficina Nacional de Emergencia – Chilean government emergencies office (Spanish)
SERNAGEOMIN – Servicio Nacional de Geología y Minería (Spanish)
Erupción del Volcán Chaitén – extensive coverage of the Chaitén eruption

The Volcanism Blog

Comments

1. george wohanka - 20 December 2008

Chaiten is to the South at about 48.0 but the Longitude is similar. The number of moderate quakes at the same location is interesting. Any volcanic activity near the quakes
DATE LAT LON MAG DEPTH km REGION

19-DEC-2008 19:25:57 -32.48 -71.63 4.9 45.8 NEAR COAST OF CENTRAL CHILE
19-DEC-2008 16:48:52 -32.54 -71.73 5.0 35.0 NEAR COAST OF CENTRAL CHILE
19-DEC-2008 13:57:31 -32.51 -71.60 5.1 29.7 NEAR COAST OF CENTRAL CHILE
19-DEC-2008 09:36:06 -32.44 -71.71 5.5 27.5 NEAR COAST OF CENTRAL CHILE
19-DEC-2008 07:30:10 -32.51 -71.74 5.2 33.2 NEAR COAST OF CENTRAL CHILE
19-DEC-2008 03:04:18 -32.46 -71.90 4.1 40.7 NEAR COAST OF CENTRAL CHILE
18-DEC-2008 21:50:28 -32.48 -71.67 5.8 10.6 NEAR COAST OF CENTRAL CHILE
18-DEC-2008 21:19:31 -32.34 -71.44 6.3 35.0 NEAR COAST OF CENTRAL CHILE


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