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Chaitén bulletin no. 84 (16 March 2009) 19 March 2009

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Bulletin no. 84 on the eruption of Chaitén volcano, dated 16 March 2009, has been released by the Chilean state geological service SERNAGEOMIN. The original document (PDF) is available via the ‘informes’ page at OVDAS, and a shortened version can be found on the SERNAGEOMIN website. Translation as follows.

ERUPTION OF CHAITÉN VOLCANO
TECHNICAL BULLETIN NO. 84
10 TO 16 MARCH 2009
OVDAS-SERNAGEOMIN

1. Visual monitoring

During this period the dome complex has maintained constant growth in the central-south sector, where the central pinnacle and a significant part of Dome 1 are located. The images captured by the DGAC camera continue to show frequent gravitational collapses taking place on the unstable flanks, especially on the central pinnacle and on the southern slope of Dome 1. All the collapses observed generate block-and-ash flows of various dimensions (Fig. 1) and, with these, are channelled towards the headwaters of the tributary of the Chaitén river. The fine particulate material of pulverized rock of the block-and-ash flows rises with yhe continual emissions of volcanic gases and water vapour, forming light-brown coloured plumes, which have dispersed principally in the eastern and south-eastern direction, towards Futaleufú (Fig. 1).

Figure 1.
Fig. 1. Images captured by the DGAC camera on 11 and 14 March. Clearly visible is the particulate material produced by the frequent formation of block-and-ash flows by gravitational collapses of unstable areas of the dome complex, which rises along with volcanic gases and water vapour.

2. Seismic activity

The seismicity recorded by the Chaitén volcano monitoring network shows a slight decline in the number of HB-type [hybrid] earthquakes with an average of 1 earthquake every 4 hours, with a magnitude between 3.5 and 4.0 (Fig. 1). The occurrence of the earthquakes of smaller magnitude has been 1 to 2 earthquakes per hour.

The position of the HB earthquakes corresponds with the area south of and below the caldera of Chaitén volcano (Fig. 2), with depths which have remained around 5.0km. The Seismological Service of the University of Chile has continued to report earthquakes with depths between 1.0 and 17.0km. During this period VT-type [volcano-tectonic] earthquakes have practically disappeared.

Figure 2.
Fig. 2. Seismogram recorded by the STAB station, for 13 March between 12:00 and 23:59 GMT. The average occurrence of HB-type earthquakes with magnitudes between 3.5 and 4.0 is 1 earthquake every 4 hours (red elipses). [Click on picture for enlarged version.]

Figure 3.
Fig. 3. Position of earthquakes on 14 and 15 March. They are located to the south of the Chaitén volcano caldera (red symbols). [Click on picture for enlarged version.]

3. Conclusions and interpretation

During the last two weeks the seismicity of Chaitén volcano has been declining in the number of HB-type earthquakes of larger magnitude, that is to say, between 3.5 and 4.0. Furthermore, the VT-type earthquakes have all but disappeared at the STAB, PUMA and PILL stations. This situation, of low seismic activity, suggests that the dome complex will continue to grow, possibly at a lower rate of speed.

In relation to the possible scenarios, they remain as laid out in previous reports, that is to say, possible total or partial gravitational collapses of the remaining segment of Dome 1 towards the south; gravitational collapse of the central pinnacle (possibly towards the south-west); in the event of a major collapse during periods of intense rain, the block-and-ash flows may produce hot lahars in the direction of Chaitén. The large amount of accumulated volcanic material and tree-trunks in the valley of the Chaitén river could easily be dislodged during intense rain towards the town of Chaitén producing new lahars.

Taking into consideration the continuing seismic activity with HB-type earthquakes of up to 4.0 magnitude, the continuing growth of the dome complex and its frequent gravitational collapses at the southern section of Dome 1 and the central pinnacle, associated with the generation of pyroclastic flows (block-and-ash flows), in addition to the high probability of the occurrence of new lahars during 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