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Chaitén bulletin no. 88 (14 April 2009) 18 April 2009

Posted by admin in activity reports, Chaitén, Chile, eruptions.
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Bulletin no. 88 on the eruption of Chaitén volcano, dated 14 April 2009, is available in original form as a PDF via the ‘informes’ page at OVDAS. The most significant news is the continuing and sustained increase in seismicity. Translation as follows.

7 TO 14 APRIL 2009

1. Visual monitoring

During this period the volcano has remained obscured and only occasionally has it been possible to observe the dome complex and its column of gases and ash. In general, the base of the column of gas and ash has appeared wider than in previous periods, although its height has not exceeded 1.5 km above the domes (Figs. 1 and 2). Moreover, during the night, the Canal 13 cameras have revealed great incandescence on the dome complex, which has correlated with the increase in the thickness of the base of the column (Figs. 3 and 4).

Figs. 1 and 2
Figs. 1 and 2. Views from the DGAC camera in Chaitén on 11 and 13 April, showing that the base of the column of gas and ash is much wider at the base and its height does not exceed 1.5 km.

Figs. 3 and 4
Figs. 3 and 4. Views from the Canal 13 camera in Chaitén on the night of 13 April, where incandescence throughout the whole dome complex and glowing emissions of gas and ash can be seen.

2. Seismic activity

The seismicity recorded by the network of stations at Chaitén volcano shows a continuous and sustained increase, that is reflected both in the increased number of HB-type [hybrid] earthquakes and their magnitudes, reaching, in some cases, up to 4.5. The number of greater magnitude earthquakes has risen from a previous daily average of 10 to 16 (Fig. 5), showing a clear increase in seismicity.

The epicentres show an increase in activity in the west and north-west flanks of the volcano, with depths that are principally concentrated between 1 and 10 kilometres (Fig. 6).

Fig. 5
Fig. 5. Seismograms for the STAB station for 4 April (left) and 12 April (right). A comparison clearly shows the increase both in the number of earthquakes and in their magnitudes, reaching up to magnitude 4.5. [Click on picture for an enlarged image.]

Fig. 6
Fig. 6. Epicentres of the earthquakes registered on 11 and 12 April 2009. [Click on picture for an enlarged image.]

In relation to the seismic energy released, expressed in RSAM units,*, these show a gradual increase from 8 April which becomes more marked from the afternoon of 11 April. In effect, before 8 April the average of the HB earthquakes was 20000 units RSAM and currently the average reaches up to 70000 units. This notable increase is directly related to the increase in the magnitudes of the HB-type earthquakes. Some of these have been perceptible to inhabitants of the area during the night, such as that occurring at 23:20 on 11 April.

Fig. 7
Fig. 7. RSAM graph which shows the increase in the seismic energy released from 8 April onwards, values which are reflections of an increase in the magnitudes of the HB-type earthquakes registered by the STAB station, located in the Santa Barbara area. [Click on picture for an enlarged image.]

3. Conclusions and interpretation

The seimic activity has shown a continuous and sustained increase, which has been registered from 8 April onwards, with earthquakes which have reached magnitude 4.5. Consistent with this increase in seismicity, the seismic energy released, also, has increased notably, phenomena which are possibly related to the much accelerated growth in the dome complex (much wider column and incandesence observed). This may cause eventual explosions and block-and-ash flows, which may affect the valleys adjacent to Chaitén volcano, with a danger of the generation of lahars.

In consequence, given that seismic activity and, probably, the growth of the dome complex has increased since 11 April, SERNAGEOMIN maintains Volcanic Red Alert.

* Note: the RSAM values (‘Real Time Seismic Amplitude Measure’) are determined by the average of the seismic amplitudes measured per minute by the seismograph.

[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

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