On 1 September SERNAGEOMIN issued a new bulletin on the Chaitén eruption, Technical Bulletin No. 50, which is available in full (PDF) at Werner Luis’s site and which is also very briefly summarized in the latest public bulletin (which also has something to say about Llaima) at the SERNAGEOMIN site. The reports of increased seismicity are of interest, particularly the increasing energy of the hybrid earthquakes recorded over the last days of August, and the occurrence of LP (long period) quakes. SERNAGEOMIN’s suggestion is that the magma movement and associated rupturing of rocks, which has been happening at depth for some time, is now taking place in the upper levels of the volcanic system, near the surface, and that this may herald an increase in eruptive activity. Translation, with images, as follows.

CHAITÉN ERUPTION
TECHNICAL BULLETIN NO. 50
20-29 AUGUST 2008
OVDAS-SERNAGEOMIN

1. Visual observations

On 20 and 21 August it was not possible to observe the eruptive activity because of cloud. On 22 August the upper part of the eruptive column, above the low clouds, was observed from Queilen, with an altitude of less than 2.0km, of white colour, comparable to that of the previous days. On 23 and 24 August, eruptive activity continued with emission of gases and pyroclasts which, according to images obtained from the town of Chaitén, produced virtually no true column, but rather a small vigorous plume that dispersed towards the south-west and east. This phenomenon results from a low emission rate that hinders the development of a significant eruption column.

On 26 and 29 August, the bad weather again prevented the direct observation of eruptive activity from Queilen. During those days, images from the north DGAC camera at Chaitén airfield intermittently showed that the eruptive activity was being maintained with a column of a maximum height of less than 1.5 km and a plume dispersing mainly to the south-west or east (Fig. 1).

Figure 1. Picture taken by the DGAC camera during the morning of 27 August, showing the eruption column and plume dispersing towards the east.

2. Seismic activity

Between 20 and 21 August, through the PUMA and SANTA BARBARA telemetry stations, a slight increase in seismicity was detected, with a majority of HB-type and VT-type earthquakes. On 21 August a level of between 3 and 8 by earthquakes an hour was reached, mostly of HB-type with 1-3 of VT-type (Fig. 2). Among the VT-type earthquakes, the most notable was that registered at 10:38 (green circle in Fig. 2), which had a magnitude of 1.7M. According to the difference in arrival time of the ’s-p’ waves, the epicenters of these VT-type earthquakes are mainly in the sector south of the volcano.

Seismic activity was virtually absent on 22 August. That day, only rare and small HB-type earthquakes were recorded, which barely stood out from the background tremor. In addition, only one VT-type earthquake was registered, at 15:24, the magnitude of which was in the vicinity of 1.0M. Thus, seismic activity that day declined notably, although the eruptive activity remained similar to that of previous days.
 

Figure 2. Seismogram from the PUMA station Pumas for the afternoon of 21.08.08. The green circle highlights the VT-type earthquake at 10:38. The great majority of the other earthquakes are of HB-type.

Between 23 and 26 August, seismic activity increased again, with 3 to 6 HB-type low-energy earthquakes per hour, and VT-type earthquakes were again recorded (1 to 2 per hour) the magnitude of which did not exceeded 1.3M.

Between 27 and 29 August, seismic activity continued, showing a slight but steady increase. On both 28 and 29 August the increase mentioned was reflected mainly in an increase in amplitude of the HB-type earthquakes (increased seismic energy), which maintained relatively constant numbers (about 4 events per hour). For their part, the VT-type earthquakes showed a slight decline (one per hour). However, during these days, significant LP-type earthquakes were registered (about 5 per day), which are emphasized by their greater amplitude in the seismogram (Fig. 3).

Figure 3. Seismicity recorded by the PUMA station on 29 August, which emphasizes the presence of LP-type earthquakes (those of greater amplitude in the seismogram), combined with HB-type earthquakes.

3. Findings and interpretation

The last few days have seen a slight but steady increase in seismicity. Additionally, the past two days have seen the recording of an increase in energy of the HB-type earthquakes and the occurrence of LP-type earthquakes. The continued and increasing energy of the HB-type earthquakes is interpreted as indicative of the movement of magmatic fluids and the rupturing of rocks within the volcanic system. The occurrence in the past two days of LP-type earthquakes indicates that the movement also includes surface levels of the volcanic system. The increase in seismic activity, especially of HB-type earthquakes with the occurrence of LP-type earthquakes, could be expressed in the coming days in a partial increase of eruptive activity. Accordingly, SERNAGEOMIN maintains Volcanic Red Alert and vigilant monitoring of the evolution of seismic and eruptive activity.

[End of bulletin.]

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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