PHIVOLCS lowered the Alert Level for Mayon to 2 (on a 0-5 scale) at 0800 on 8 December due to a continuing decline in activity during the previous several weeks. Volcanic earthquakes indicating lava extrusion, degassing, and occasional fracturing beneath the edifice decreased from an average of 11 events per day in November to almost zero events per day in the first week of December. Similarly, the average number of daily rockfalls and pyroclastic density currents (PDCs, or pyroclastic flows) decreased from monthly averages of 122 to 87 and 5 to 2 events per day, respectively, from October to November; there were almost zero events per day in the first week of December. Incandescence at the summit crater and from lava flows had declined starting in the last week of November. The length of the lava flows in the Mi-Isi (S), Bonga (SE), and Basud (E) drainages had remained at 2.8 km, 3.4 km, and 1.1 km, respectively, since July and August; the newest lava deposits observed since 23 November were in the Bonga within 400 m of the summit. Sulfur dioxide gas emissions peaked at 4,756 tonnes per day (t/d) on 16 August and had been variable but declining overall since; a monthly average of 1,417 t/d in November decreased to an average of 1,095 t/d in December. Electronic Distance Measuring (EDM), continuous GPS, and electronic tilt monitoring data showed continuing deformation. Deflation was detected in GPS data beginning around August-September and in tilt data starting in November. Net inflation in the longer-term was indicated in electronic tilt data as far back as June and in EDM data starting in February. Residents were reminded to stay away from the 6-km-radius Permanent Danger Zone (PDZ) and pilots were advised to avoid flying close to the summit.

Geological Summary. Symmetrical Mayon, which rises above the Albay Gulf NW of Legazpi City, is the most active volcano of the Philippines. The steep upper slopes are capped by a small summit crater. Recorded eruptions since 1616 CE range from Strombolian to basaltic Plinian, with cyclical activity beginning with basaltic eruptions, followed by longer periods of andesitic lava flows. Eruptions occur predominately from the central conduit and have also produced lava flows that travel far down the flanks. Pyroclastic density currents and mudflows have commonly swept down many of the approximately 40 ravines that radiate from the summit and have often damaged populated lowland areas. A violent eruption in 1814 killed more than 1,200 people and devastated several towns.

Source: Philippine Institute of Volcanology and Seismology (PHIVOLCS)