Chaitén bulletin no. 82 (28 February to 1 March 2009)

Bulletin no. 82 on the eruption of Chaitén volcano, covering the period 28 February to 1 March 2009 , has been released by the Chilean geological service SERNAGEOMIN. It is available from the OVDAS website (PDF) and from Werner Luis’s Chaitén site (PDF). Translation as follows.

ERUPTION OF CHAITÉN VOLCANO
TECHNICAL BULLETIN NO. 82
28 FEBRUARY TO 3 MARCH 2009
OVDAS-SERNAGEOMIN

1. Visual monitoring

Through ground and aerial observations from 28 February to 3 March it has been possible to assess the evolution of the Chaitén volcano dome complex, its large central pinnacle and the volcanic gases with a predominance of water vapour that it continually emits (Fig. 1).

Figure 1. View of the large central pinnacle and the southern slope of a segment of Dome 1 from Chaitén on 1 March. The steep and unstable sides can be clearly seen. Photograph C. Gallegos.

During this period it has been possible to observe that the growth of the dome complex continues, particularly in the central-southern sector, where the central pinnacle and a substantial remaining portion of Dome 1 are located. This can clearly be inferred from the frequent collapses that occur on the unstable slopes, particularly in the central pinnacle and in the southern wall of Dome 1 (Figs. 2 and 3). All the gravitational collapses have produced block-and-ash flows and the majority of these have added to the large depositional cone in the south-west sector of the domes and, in part, have been channelled towards the headwaters of the tributary of the río Chaitén. The fine particulate material of pulverized rock (the so-called ‘ash’) of the block-and-ash flows has risen with the volcanic gases and water vapour, forming plumes of light brown colouring, which have tended to disperse in a south-east direction towards Futaleufú. During the night of 1-2 March, some fine ash material was also deposited in Santa Bárbara and Chaitén.

Fig. 2. Gravitational collapse in the south wall of the remnant of Dome 1 on 1 March. These collapses, generating pyroclastic flows (of block-and-ash type), reveal both the growth of this area and the great instability of the south wall at the headwaters of the tributary of the río Chaitén. Photograph C. Gallegos.

Fig. 3. Another view of a somewhat larger gravitational collapse in the south slope of the remnant of Dome 1 on 2 March. The pyroclastic flows of block-and-ash flow type accumulated in the headwaters of the tributary of the río Chaitén. Photograph C. Gallegos.

On 3 March during an overflight co-ordinated with the Regional Office of ONEMI it was possible to verify that the deposits of the block-and-ash flow that descended through the río Chaitén on 19 February still maintaind high temperatures in some areas, with emanations of water vapour and even burning of trees where they had come into contact with lava blocks from the domes, with high temperatures (over 300ºC). In addition it was observed that a gas column had condensed to form a dense steam column of up to 1km altitude (due to high atmospheric humidity) with a dispersion plume towards the south that contained traces of fine particulate material of light brown colouring.

During this period, the collapses have occurred at a frequency of 2-3 per hour, of varying magnitudes. In the afternoon of 3 March they took place every 40 minutes.

2. Seismic activity

The data provided by the Chaitén volcano monitoring network shows an average of 2-3 HB-type [hybrid] earthquakes per hour, with magnitudes of between 1.3 and 3.6. Among these, some earthquakes of greater magnitudes stand out (between 3.2 and 3.6), which occurred at a rate of 1 earthquake per hour, on average, on 28 February. This later declined to 1 earthquake every 2 hours in the first days of March.

In relation to the VT-type [volcano-tectonic] earthquakes, these have occurred in a very isolated form and with magnitudes not exceeding 0.8.

In preliminary form, the HB-type earthquakes have continued to be located principally in the southern area of the Chaitén caldera, with a depth calculated at between 5 and 10km. On the other hand, the Seismological Service of the University of Chile has reported much deeper earthquakes, located between 17 and 24km.

Figure 1 [should be 4]. Seismogram recorded by the station STAB [Santa Bárbara] for 1 March between 12:00 and 23:59 GMT. The HB-type earthquakes of greater magnitude decline to a rate of 1 earthquake every 2 hours (red ellipse).

3. Conclusions and interpretation

The seismicity of Chaitén volcano has declined slightly in the number of larger HB-type earthquakes, that is to say, with magnitudes of between 3.2 and 3.6, in comparison to previous days. The earthquakes that have been recorded are principally of HB-type, which indicates the continuing supply of magma to the dome complex.

Some processes that may occur in view of the current situation of the volcano:

• Total or partial gravitational collapse of the remaining segment of Dome 1 towards the south.
• Gravitational collapse of the central pinnacle (possibly towards the south-west).
• If major collapses take place during periods of intense rain, the block-and-ash flows will produced hot lahars in the direction of Chaitén.
• The large quantity of loose volcanic material and tree trunks in the valley of the Chaitén river may easily be dislodged towards the town of Chaitén during intense rain, producing further lahars.

Taking into account the continuation of seismic activity with HB-type earthquakes of up to magnitude 3.6, that the growth of the dome complex and the occurrence of collapses of part of Dome 1 and of the central pinnacle continues, associated with explosions and pyroclastic flows (block-and-ash flows), along with the high probability of the occurrence of further lahars during periods of intense rain, 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