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Chaitén update, 27 February 2009 27 February 2009

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Bulletin no. 78 on the eruption of Chaitén volcano, covering the period 20-24 February 2009, has been released by the Chilean geological service SERNAGEOMIN. It is available from the OVDAS website (PDF) and has also been published on the SERNAGEOMIN website. Beware of the latter link: because huge images have been squeezed into small spaces on the page (forcing 3264 pixels width into 350 pixels) rather than properly resized the whole thing is very slow and unresponsive. Anyway, translation as follows (click on ‘more’ for the whole document).

ERUPTION OF CHAITÉN VOLCANO
TECHNICAL BULLETIN NO. 78
20-24 FEBRUARY 2009
OVDAS-SERNAGEOMIN

Visual monitoring

An overflight carried out today, 24 February, in a UH-1H helicopter of the Chilean Air Force, in conjunction with the Presidential Delegate and representatives of the DOH [Dirección de Obras Hidráulicas – Hydraulic Works Directorate] and Vialidad [Dirección de Vialidad – Roads Directorate] allowed the verification of the current state of eruptive activity, the morphology of the domes and the dimensions and precise location of the area affected by the collapse that occurred last Thursday (19.02.09).

Also observed was the distribution of the pyroclastic flows and the block-and-ash deposits, originating with the collapse of the domes, across the headwaters of the río Chaitén and in the depression (basal ring) which surrounds the domes (Fig. 1).

The escarpment or scar of the collapse is oriented in an approximately south-western direction, with a greatest length of approximately 500m (in the south-western direction) and a maximum height of the of the escarpment slopes of 200m. The area of collapse is located in the south-western quadrant of the dome complex and affects, approximately, some 10% of the total volume. The collapse escarpment surrounds the south and west of the spine or pinnacle which has grown in the central sector of the dome complex. The majority of the material removed is from Dome 1, the development of which began in May 2008 (Figs. 2 and 3).

The northern sector of the dome shows occasional eruptive activity, which is concentrated to the immediate north of the pinnacle, at the pinnacle, and at a remnant of dome 1 located to the south of the collapse escarpment (Figs. 2 and 3). In the northern sector, within dome 2, eruptive activity was observed in two sectors and was manifested in the emission of water vapour with occasional ash content (Fig. 2). The majority of the eruptive activity was concentrated on the spine or pinnacle, with constant emissions of brown-coloured ash and water vapour. The southern remnant of dome 1 intermittently emits ash and water vapour of similar colouring to that at the pinnacle, in addition to producing sporadic explosions. Together, they form four eruption columns of gas and ash, which exceed 1.5km in height and which during small collapses of the steep slopes of the pinnacle tend to form one wide and irregular column (Fig. 2).

On a periodic basis, collapses or small landslides are generated from the growing spine and in the southern wall of the remnant of dome 1 located in the south sector (Fig. 1). Probably, one of these landslides may correspond to that described yesterday (23/02), after 20:40, by the inhabitants of Chaitén and those that occurred today (24/02) at 14:30 and 18:40.

Figure 1.
Figure 1. General view towards the north-east which shows the dome complex, the area of collapse, the collapse deposits in the form of a fan in the bottom of the basal ring and the upper course of the drainage channel. It is also possible to see a block-and-ash deposit on the southern slope of dome 1, immediately to the north of the drainage channel of the basal ring (photograph P. Duhart).

Figure 2.
Figure 2. View towards the east of the dome complex and the four emission columns of gas and ash. On occasions, the fumarolic activity in the southern remnant of dome 1 happens to conform to the real emission column (photograph P. Duhart).

No damming was seen along the course of the río Chaitén, nor in the drainage channel of the basal ring of the domes (Figs. 1 and 4), nor in the latter. Pyroclastic flows and rocks and ash were deposited, and they burned, but did not raze, the forest on the slopes of the middle part of the channel, the drainage and the bottom of the río Chaitén valley.

The south-western sector of the basal ring contains a significant volume of materials from the major collapse, which fill this area in the form of a fan (Figs. 1 and 2). This fan does not show any processes of erosion and constitutes an area where material from subsequent collapses has accumulated (Fig. 2).

Figure 3.
Figure 3. Detail of the collapse escarpment which surrounds the central pinnacle or spine, which continues to grow constantly. View towards the north-east (Photograph P. Duhart).

Figure 4.
Figure 4. General view of the middle course of the río Chaitén and its drainage (photograph P. Duhart).

Seismic activity

Between Friday 20 February and Monday 23 February (until 08:00) the seismicity recorded at Chaitén volcano was characterized by the presence of HB-type earthquakes, without the presence of tremor (Figs. 5 and 6). The largest earthquakes were located beneath the volcano and immediately to the east, and occurred at the rate of 1-2 per hour, with magnitudes estimated as between 3.2 and 4.8. Some of these larger earthquakes were perceived by people in Chaitén. The depths estimated for the 16 larger earthquakes that occurred between 20 and 24 February, published by the Institute of Seismology of the University of Chile, ranged from 0.3 to 20km, with the majority of these located at between 10 and 20km depth. The depth calculations carried out by OVDAS personnel for some of these same earthquakes, in general terms and in a preliminary manner, coincide with the depths published by the Institute of Seismology.

The activity of Friday 20 February corresponded to HB-type earthquakes, including up to 2 major earthquakes per hour, with magnitudes between 3.0 and 4.8. Of all the larger earthquakes registered during this day, only 3 had magnitudes of over 3.9. The activity of Saturday 21 February consisted again of HB-type earthquakes, but the number and magnitude of larger quakes was reduced, with a range of between 3.2 and 3.6. This decline in the number and magnitude of larger earthquakes was maintained on 22 February, when an average of 1 large earthquakes per hour was recorded, a trend that remained up to 20:00 on 24 February.

Figure 5.
Figure 5. Seismogram from Santa Bárbara station from 07:00 on 20 February until 07:00 on 21 February 2009, highlighting the occurrence of the larger HB-type earthquakes.

Figure 6.
Figure 6. Seismogram from Santa Bárbara station from 19:00 on 21 February until 19:00 on 22 February 2009, again highlighting the occurrence of larger HB-type earthquakes.

Conclusions and interpretation

The overflight carried out on 24 February allowed the verification of the exact position of the area of the dome collapse that occurred on Thursday 19 February, and confirmed that a large quantity of volcanic material is accumulated at the base of the domes and along the course of the río Chaitén and is available to be transported downstream in the form of lahars during periods of intense and prolonged rains. In addition, the occurrence of minor collapses was noted, which occur frequently in the southern part of the domes affected by the major collapse and at the pinnacle.

The seismic activity recorded during the period of this bulletin is comparable to the activity recorded in the days and weeks previous to the collapse and lateral explosion of 19 February. It is possible to associate the seismic activity with the growth of the pinnacle structure and with the remnant of dome 1, but the presence of localized activity at greater depth cannot be ruled out.

Bearing in mind the recording of continual seismic activity, which means that the probability of new major collapses occurring is maintained, with explosions and pyroclastic flows and associated ash, in addition to the generation of lahars during prolonged periods of intense rainfall, 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