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Chaitén update, 24 February 2009 24 February 2009

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Bulletin no. 76 on the eruption of Chaitén volcano, covering the period 19-20 February 2009 and thus covering the dome collapse of 19 February, 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), and has also been published in slightly edited form on the SERNAGEOMIN website. Translation as follows (click on ‘more’ for the whole document).

19 TO 20 FEBRUARY 2009

Visual observation

Today, Friday 20 February, in coordination with the Intendente Regional, the Presidential Delegate and the National Director and Regional Director of ONEMI, among others, an overflight of the area of Chaitén volcano was carried out [by SERNAGEOMIN] in a Twin Otter aircraft of the Chilean Air Force. Because of the conditions of low cloud it was not possible to observe the eruptive activity, the dispersal of the plume, nor the development of the collapsed sector of the dome.

The overlight revealed that there are continuing emissions of water vapour along the course and banks of the upper sector of the río Chaitén (Blanco), which are interepreted, on a preliminary basis, as a result of the contact between water in the river with hot volcanic deposits produced by the pyroclastic flows and the rocks and ashes originating from the collapse and lateral explosion from the south of the dome which occurred on the morning of Thursday 19 [February].

Using photographs obtained in the overflights of yesterday, Thursday, and observations made during the overflight of today, Friday, it was estimated that the pyroclastic flows and rock-and-ash flows associated with the collapse and lateral explosion of the dome involved a quantity of approximately 30 million tonnes of material being removed its southern slope (10 million cubic metres). This material is dispersed in the form of sediments and volcanic deposits on the floor of the upper part of the valley and on the lower slopes of the sides of the upper course of the río Chaitén (Blanco). The preceding increased, considerably, the quantity of volcanic material available in the headwaters of the river and, again, increased the danger of the generation of continuous or sporadic lahars, encouraged by the intense and prolonged rains, which continue to affect the town of Chaitén long-term, especially in the winter.

Seismic activity

From 13:46 on 19 February the seismic activity registered by the five seismological stations installed at Chaitén volcano showed a decline, returning to the conditions maintained until the day before and during 18 February and in the early hours of 19 February, that is to say a record of HB-type earthquakes at a rate of 1-2 per hour on average, with magnitudes between 3.3 and 3.7, with relatively energetic background tremor (Fig. 1) and depths, at preliminary estimates, of between 2 and 15km, a condition that continued until 17:00 today (Friday 20 February) at the time of writing.

Figure 1.

Figure 1. Seismogram from the Santa Bárbara seismological station, from 08:30 yesterday [19 February 2009] until 13:30 today, 20 February 2009. The times are in GMT, to find local time subtract 3 hours. The yellow highlights mark the time period in which the collapse of the southern sector of the slopes of the dome occurred. After 13:45 the seismic activity returns to levels comparable with those recorded in the hours and days preceding the collapse, that is to say 1-2 HB-type earthquakes per hour (24-60 earthquakes per day).

Conclusions and interpretation

After the collapse of 19 February 2009, seismic activity has returned to levels normally experienced during the days before the collapse, including magnitude and frequency [of earthquakes]. The collapse directed towards the south propelled a considerable volume of volcanic material towards the headwaters of the río Chaitén, which is now available to be transported downstream during intense and prolonged rainfall, generating lahars similar to those which have affected the town of Chaitén before.

Moreover, the occurrence of new collapses, lateral explosions and the generation of pyroclastic flows and major rock-and-ash flows directed towards the town of Chaitén and also to the north, east and west of the volcano cannot be ruled out.

Consequently, bearing in mind the characteristics of the evolution of the current eruptive phase, with the potential for the generation of further collapses of the domes, accompanied by their respective pyroclastic flows and block-and-ash flows, with the subsequent growth of the column and the dispersion plume, SERNAGEOMIN maintains Volcanic Red Alert.

[End of SERNAGEOMIN bulletin.]

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

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



1. Alan Sullivan - 24 February 2009

Interesting seismic trace. The cluster of strong quakes came after the initial collapse. This suggests that release of pressure did play a role in events that day. The whole system responded.

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