You wait ages for a Chaitén bulletin from SERNAGEOMIN, then two come along at once. Today we have Bulletin No. 56, covering 1-6 October (the original in PDF can be found on Werner Luis’s site), and a Special Report covering 6-9 October (also available at Werner Luis’s site: scroll down to ‘Viernes 10-10′). Translations as follows.

ERUPTION OF CHAITÉN VOLCANO
TECHNICAL BULLETIN NO. 56
1-6 OCTOBER 2008
OVDAS-SERNAGEOMIN

1. Visual monitoring

During the week, and in a similar manner to previous reports, the eruptive activity has manifested in two columns of low vigour, which have not exceeded 1.5km altitude, and which have presented a clear predominance of water vapour over ash-type pyroclastic materials.

2. Seismic activity

Data supplied by the seismic network confirmed that the seismic activity in the area of Chaitén continues without major variations. While the activity is focused, mainly, in HB-type earthquakes (2-5 per hour), the number of significant earthquakes, with magnitudes exceeding 3.0M has tended to decline in the last two days, demonstrating a gradual downwards tendency. In relation to their distributions, 10 earthquakes have been located (Fig. 1), eight HB-type (yellow markers) and two VT-type (blue markers), which are located within the caldera and on its southern flank. Thanks to a new station located on the north flank of the caldera (Fig. 1, station BONO) the depths previously calculated as being most probable have been corroborated, placing the majority of the hypocentres between 0 and 5km deep.

Figure 1. Localization of epicentres calculated for 26 September and 2 October 2008. The yellow pointers correspond to HB-type earthquakes, blue to VT-type earthquakes. The depths are focused between 0 and 5km below the caldera.

3. Conclusions and interpretation

Since 20 September the HB-type seismic activity has again increased, with some earthquakes exceeding magnitude 3.0M. The data obtained indicates that this seismic increase has been concentrated in relatively superficial levels of the system and, therefore, should be related to the extrusion of the active dome and not to a new deeper injection associated with a new pulse of magma. Data supplied by the new station named BONO confirms the first hypothesis, achieving a better determination of the hypocentres associated with the seismic activity, whose depths, in general do not exceed 5km. As mentioned in the preceding report, the number of earthquakes whose magnitudes exceed 3.0M has begun to decline, however there remains the possibility of a renewed increase or the generation of a phreatomagmatic explosion at the level of the dome, as occurred at the end of July 2008. Finally, with the persistence of a marked variability in seismic activity, SERNAGEOMIN maintains Volcanic Alert Level Red.

[End of Technical Bulletin No. 56.]

SERNAGEOMIN SPECIAL REPORT ON THE ERUPTIVE ACTIVITY OF CHAITÉN VOLCANO, 6-9 OCTOBER 2008

1. Visual monitoring

On Monday 6 October both eruption columns were slightly thinner and less vigorous than on previous days, with a mainly white colouring that showed low concentrations of gas and a predominance of water vapour over gases, and with altitudes not exceeding 1.5km above the dome and then turning towards the south.

On Tuesday the eruptive activity increased slightly, with somewhat wider columns, although their colouring and altitudes were similar to those of the previous day. On both Monday 6 and Tuesday 7 October it was possible to identify points of increased eruptive activity that formed small mushrooms, which could be related to small explosions. The eruptive condition showed no major changes on 8 and 9 October.

2. Seismic monitoring

During the last four days the presence has been maintained of 3-5 HB-type earthquakes per hour (70 to 120 per day), of which only a few (6-7 per day) exceeded magnitude 3.0M. The earthquakes of greater magnitude show a tendency to decline in number and the VT-type earthquakes remain uncommon and of low magnitude.

3. Conclusions and interpretation

The eruptive activity includes periods dominated, principally, by degassing alternating with periods of relatively significant activity, which are associated with small explosions in the upper levels of the system.

During the last two weeks both the number and magnitude of the HB-type earthquakes, located mainly in the southern flank of the dome at depths not exceeding 5km and related to the growth of the dome, have remained relatively high.

SERNAGEOMIN maintains Volcanic Red Alert.

[End of SERNAGEOMIN Special Report.]

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

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