SERNAGEOMIN have issued their Bulletin No. 49 on the Chaitén eruption, dated 19 August, which can be found (as a PDF) at Werner Luis’s Chaitén site. The document runs to three pages and includes a number of illustrative figures. The overall picture is one of a steady-state low-level eruption with variable but basically low seismicity. The changes in seismic behaviour at Chaitén over recent days do not appear to correlate with any changes in the volcano’s eruptive behaviour: increases in seismicity did not relate to increases in eruptive activity, and recent decreases have not corresponded to declines in activity. SERNAGEOMIN are keeping a close watch on developments are are maintaining Volcanic Red Alert, as before.

To read the whole document in translation, complete with figures, click on ‘more’.

ERUPTION OF CHAITÉN VOLCANO
FORTY-NINTH TECHNICAL BULLETIN
16-19 AUGUST 2008
OVDAS-SERNAGEOMIN

1. Visual observations

During the morning of 16 August, the eruption column could not be clearly observed from Queilen. Only at noon was it possible to see, among the clouds, grey-coloured emissions, without being able to estimate the height of the column or the direction of dispersal of the plume. On 17 August continuation of adverse weather conditions prevented direct observation of the volcano. On 18 and 19 August several images of the volcano were obtained, through the cameras at Chaitén airfield and from Queilen (Figs. 1 and 2). In these images a column not exceeding 2km in height could be seen, with a plume reaching a slightly higher altitude and dispersing towards the east. According to estimates made from Queilen, the dispersion plume would have reached 3km in altitude, also being visible from Puerto Montt.

Fig. 1. Images captured on 18 August (left) and 19 August (right) by the northern camera located at Chaitén airfield.

Fig. 2. Photograph taken from the town of Queilen on 19 August at approximately 14:00, where a wide eruption column was observed, whitish and no more than 2.0km in height.

2. Seismic activity

Saturday 16 August was marked by an absence of significant earthquakes. Only five small earthquakes were recorded, three HB-type and two VT-type, both around magnitude 1.2M. Seismic activity on 17 August remained low, with four earthquakes, three HB-type and 1 VT-type of magnitude 1.2M, the latter recorded at 04:27 (Fig. 3). On 18 August the volcano continued with low seismicity, with five small HB-type events and three VT-type, with magnitudes below 1.5M.

Fig. 3. Sismogram recorded by PUMA station on 17 August indicating the four earthquakes detected on that day.

On 19 August a slight upsurge in seismicity was recorded, marked by a sudden increase in the number of HB-type earthquakes, with two to three low magnitude earthquakes per hour (Fig. 4).

Fig. 4. Seismogram from PUMA station for 19 August on which a significant number of HB-type earthquakes are recorded.

3. Conclusions and interpretation

During the last few days seismic activity has remained relatively low and eruptive activity has continued without major variations (column moderately vigorous, with emission of pyroclasts, gases and water vapour, about 2.0km in height). Thus, the low seismicity has not accompanied significant decreases in eruptive activity in the same way in which, during the month of July, the increase in seismicity was not associated with evident increases in the energy of the eruption.

Considering the changes in seismic behaviour, which do not correlate with significant variations in eruptive activity; the permanent presence of HB-type earthquakes combined with VT-type earthquakes, interpreted as indicative of the movement of magmatic fluids and the rupture of rocks in the interior of the volcanic system, SERNAGEOMIN maintains Volcanic Red Alert and monitoring of the evolution of eruptive and seismic activity.

[Report ends.]

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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