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Chaitén bulletin no. 81 (27 February 2009) 3 March 2009

Posted by admin in activity reports, Chaitén, Chile, eruptions.
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With the recent upsurge in activity at Chaitén volcano, SERNAGEOMIN has been issuing bulletins on an almost daily basis. All Chaitén reports are archived (as PDF files) on the ‘informes’ page at the website of the Observatorio Volcanológico de los Andes del Sur (OVDAS). The latest to be made available is bulletin no. 81, which reports activity for 27 February 2009. The original document (PDF) is available from the OVDAS website. Translation as follows.

ERUPTION OF CHAITÉN VOLCANO
TECHNICAL BULLETIN NO. 81
27 FEBRUARY 2009
OVDAS-SERNAGEOMIN

Visual monitoring

During an overflight co-ordinated with the Regional Office of ONEMI, in which the National Director of SERNAGEOMIN took part, it was confirmed that the south sector of dome 1 continues to be active, with relatively rapid growth and an increase in fracturing and instability of its flanks, with numerous collapse fans of rock and ash flows on the south slope (Fig. 1). During observations by SERNAGEOMIN personnel in the area, it was estimated that relatively significant collapses of the central spine (needle or pinnacle) and/or from the south sector of dome 1, both of which are growing, occur in the range of 4-5 per day, making the emissions much wider and irregular and changing their colouring to clear brown, because of their greater ash content (for example, at approximately 18:20 today, Fig. 2). Meanwhile minor collapses are observed periodically from the spine towards the interior of the 19 February collapse scar. There is a continual growth of the volume of material in the fan located in the south-eastern sector of the ring that surrounds the domes, which is formed by deposits of rock and ash flows associated with partial collapses of the central spine.

Thermal measurements

Preliminary reports and results confirm that the central spine has temperatures varying between 400ºC and 200ºC. In addition there are north-south [thermal] alignments on the south slope of dome 1, and in this same dome a zone has been detected with temperatures of up to 300ºC, located immediately to the east of the central spine. Finally, in the centre of dome 2 an area with temperatures of up to 270ºC has been observed, and the steam fumaroles in this dome vary between 130ºC and 180ºC. The foregoing confirms the data provided in the bulletin of 25 February for this dome complex and the observations reported in the bulletin before that. Also, it confirms the data contained in the bulletin of 25 February regarding the temperatures in the waters and the deposits in the middle course of the rió Chaitén.

Seismic activity

During the day, the seismicity has maintained the characteristics of the preceding days, with 2-3 HB-type earthquakes per hour with magnitudes of between 3.2 and 3.6, at relatively shallow depths.

Figure 1.
Figure 1. Detail of the south sector of dome 1, in which the steep, unstable sides can be seen, strongly fractured and with rock and ash flows on the southern slope caused by partial collapses. In the foreground can be seen part of the apex of the fan of rock and ash deposits which is continually augmented by partial collapses of the steep sides of the western part of the central spine (needle or pinnacle). Photograph P. Duhart.

Figure 2.
Figure 2. Partial collapse, of relatively small dimensions compared with that of 19 February 2009, of part of the central spine or of the southern slope of dome 1, generating an irregular cloud of ash and gas which turns to a clear brown colouring and appears to move towards the upper course of the drainage channel of the ring that surrounds the domes. Photograph by C. Gallegos, taken at 18:30 on 27 February from Chaitén.

Conclusions and interpretation

The growth and the partial collapses of the central spine and the south sector of dome 1 are continuing. The principal danger is focused in the growth and probable collapse of significant portions of the central spine and of the south sector of dome 1, along with the generation of renewed lahars during intense and prolonged rains, which will transport the volcanic deposits in the fan of accumulated rock and ash flows in the ring that surrounds the domes and in the middle course of the Chaitén (or Blanco) river, towards the town of Chaitén. The seismic activity continues without significant variations compared to that reported in the previous bulletin.

Taking into account the registering of earthquakes at relatively superficial levels and at depth, the continuing probability of the occurrence of further large collapses of significant parts of dome 1 and the spine, with associated explosions and pyroclastic and ash flows, along with the probability of the occurrence of renewed lahars during periods of prolonged and 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
Erupción del Volcán Chaitén – extensive coverage of the Chaitén eruption
Chaitén bulletins archive – at OVDAS, Observatorio Volcanológico de los Andes del Sur

The Volcanism Blog

Comments

1. Juhani - 3 March 2009

My comment is not really related to the bulletin, but I’ve tried to find an explanation to the term “HB-type” earthquake. Any good to link to explain this and obviously to the definitions of earthquake types.

2. volcanism - 3 March 2009

I should have explained this, my apologies. ‘HB’ is an abbreviation for ‘hybrid’. Of the two basic types of volcanic earthquake, ‘Volcanic’ or ‘volcanic-tectonic’ (VT) earthquakes are intense, high-frequency events associated with rock fracture, ‘Long-period’ (LP) earthquakes are low frequency and less intense events associated with fluid movements. Hybrid (HB) events combine characteristics of both kinds, and are particularly associated with shallow activity at lava domes.


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