Philippines: Mayon Volcano Bulletin 20

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Philippine Institute of Volcanology and Seismology

02 January 2010
8:00 AM

The seismic monitoring network around Mayon Volcano (13.2576 N, 123.6856 E) detected 13 volcanic earthquakes and 68 rockfall events related to the detachment of lava fragments at the volcano's upper slopes during the past observation period. Emission of weak volume of white steam at the summit crater was observed during cloud breaks yesterday. Sulfur Dioxide (SO2) emission rate was measured yesterday morning at an average value of 2,621 tonnes/day.

The latest activity of Mayon still indicates that its overall state of unrest remains relatively high. However, this phase of unrest, characterized by moderate seismicity, high volcanic gas outputs and continuing glow of the summit are processes normally associated with very gradual return to the repose period. The volcanic system is expected to continue producing earthquakes and to vent a large amount of gases because fresh magma still resides along the whole length of the volcanic pipe and near the summit.

From 28 December to present, a declining trend in Mayon volcano's activity was noted as reflected by the following observations:

1. No ash ejections were observed since 29 December. Steam emission was most of the time weak and white in color indicating considerable decrease in energy and absence of ash.

2. Majority of the type of earthquakes that were recorded during the past days were associated with rockfalls and rolling down of fragments from the lava deposits along Bonga gully and the advancing lava front.

3. Measured SO2 levels have also showed a decreasing trend from a maximum of 8,993 tons per day to 2,621 tons per day. The still high concentration of SO2 gas emission suggests that there is residual magma degassing at shallow depth.

In view of the above observations, PHIVOLCS-DOST is lowering the alert status of Mayon from Alert Level 4 to Alert Level 3 to reflect the overall gradual decrease of activity. Alert Level 3 means that there is less probability of a hazardous explosive eruption. However, the lowering of the alert level from 4 to 3 should not be interpreted that the unrest of the volcano has ceased. If there is resurgence in the volcano's activity and the potential for explosive eruptions is perceived to be forthcoming, the alert level may be raised back to 4 but if there is noticeable downward trend in the monitored parameters, then the alert will be further lowered to Alert Level 2.

PHIVOLCS-DOST reminds the public that at Alert Level 3 sudden explosions may occur due to localized pockets of gas within the magmatic system. The effects of these explosions are expected to be contained within the 6-kilometer radius Permanent Danger Zone (PDZ) around the volcano and within the 7-kilometer radius EDZ in the southeast quadrant. PHIVOLCS-DOST recommends that the 6-km radius PDZ around the volcano and the 7-km EDZ on the southeast flank of the volcano should be free from human activity because of sudden explosions that may generate hazardous volcanic flows. People residing close to these danger areas are also advised to observe precautions associated with post-eruption activity, such as rockfalls, pyroclastic flows, and ash fallout which can also occur anytime due to instabilities of lava deposited on steep slopes. Active river channels and those perennially identified as lahar prone in the southern sector should also be avoided especially during bad weather conditions or when there is heavy and prolonged rainfall. Civil aviation authorities must advise pilots to avoid flying close to the volcano's summit as ejected ash and volcanic fragments from sudden explosions may pose hazards to aircrafts. PHIVOLCS-DOST is closely monitoring Mayon Volcano's activity and any new significant development will be immediately posted to all concerned.