Philippines: Mayon volcano update 17 Mar 2000

The seismic monitoring network around the volcano detected thirteen (13) low-frequency volcanic earthquakes and eight (8) episodes of short duration tremors for the past observation period. Ground deformation measurements using the Electronic Distance Meter (EDM) line in Buang showed no significant change from its present deflated state. The Sulfur Dioxide (SO2) emission rate increased to 9,000 tonnes/day (t/d) as compared with yesterday's reading of 7,000 t/d. Steam emission from the summit area was generally moderate in volume and crater glow was observed during the night.

The day to day changes occurring at Mayon since the last major eruption of 01 March strongly indicates that the overall state of unrest remains high. However, this phase of unrest, characterized by moderate seismicity, high volcanic gas outputs and continuing glow of the summit and new lava deposits are processes normally associated with very gradual return to the repose. 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. The latter is the main reason why crater glow continues because magma remains very hot for a long time. But as observed since 01 March, the seismicity levels, SO2 emision rates and ground deformation do not exhibit the characteristic accelerations that are analogous to pre-eruption conditions. To the contrary, the ground deformation, either determined in the form of precise geodetic measurements or visual observations of summit activity (e.g. bulging of the crater floor) indicate that no active magma intrusion is evident. PHIVOLCS also notes that there has been no anomalous quiescence in the behaviors of the volcano, in the sense that sudden calmness could precede a major eruption. This condition is not a requirement as a prelude to an eruption but is a significant concern when it occurs very shortly after major eruptive activity. One cause for concern is a pressure buildup in the magma system that is temporarily sealed as a result of its previous eruptions. PHIVOLCS reiterates, however, that this condition is not indicated by any of the observations.

In view of these considerations, PHIVOLCS is lowering the alert status of Mayon from Alert Level 4 to Alert Level 3 to reflect the overall gradual decrease of activity since the 01 March 2000 eruption. Alert Level 3 means that there is less probability of a hazardous explosive eruption. However, PHIVOLCS reminds the public that sudden explosions may occur due to localized pockets of gas within the magmatic systems. The effects of these explosions are expected to be contained within the 6-kilometer radius Permanent Danger Zone around the volcano and within the 7-kilometer radius Extended Danger Zone in the southeast quadrant. People residing close to these danger areas are also advised to observed precautions associated with post-eruption activity, such as against secondary pyroclastic flows, debris flow, rockfalls and ash fallout which can also occur anytime due to instabilities of volcanic material deposited on steep grounds.

Alert Level 3 still means that the danger zones should remain off-limits to everybody including trekkers, tourists and other visitors. It should be realized that, independent of alert status, a large proportion of pyroclastic deposits is still susceptible to erosion and subsequent remobilization as lahars or debris flows. The areas in which potentially destructive volcanic flows may occur are lowlands fronting the Mabinit and Buyuan-Padang river channels in Legazpi City; Miisi in Daraga; Tumpa, Anoling and Quirangay channels in Camalig; Maninila and Upper Nabonton channels in Guinobatan; Basud-Lidong channels in Sto. Domingo, Bulawan channel in Malilipot, San Vicente and Buang channels in Tabaco and Upper Nasisi in Ligao. Residents adjacent to these channels are advised to be alert and ready to evacuate should there be intense and prolonged rainfall and/or large magnitude earthquakes which could dislodge and transport deposits to the lower slopes.

European Volcanological Society
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