The alert level 3 category of Mayon volcano in Albay province has now been downgraded by the Philippine Institute of Volcanology and Seismology to alert level 2 status due to declining level of volcanic activities.
In its latest volcanic bulletin, Phivolcs recorded a decrease in volcanic earthquakes, generated by magma extrusion, and degassing and occasional rock-fracturing beneath the edifice — from a monthly average of 11 events daily in November 2023 to nearly 0 event per day in the first week of December 2023.
It also noted that rockfall and pyroclastic density currents or PDCs significantly decreased from monthly averages of 122 to 87 events/day and 5 to 2 events/day, respectively, between October and November 2023 to virtually 0 event/day in the first week of December 2023.
Phivolcs said these decreases indicate that magma supply to the summit crater has significantly diminished, resulting in the cessation of lava effusion from the crater and lava collapse-driven rockfall and PDC activity.
It logged a lessening incandescence of the summit crater and a reduction of the 2023 lava flow deposits since the last week of November.
Phivolcs said the advancing front of overlapping lava flows on the Mi-isi, Bonga and Basud gullies have been stalled at 2.8 kilometers (km), 3.4 km and 1.1 km, respectively, from the summit crater from July to August.
Of these, the newest lava deposit was observed to have rested within 400 meters of the crater on the Bonga Gully since 23 November, with a total volume of erupted deposits remained at approximately 49 million m3 since then.
"The stabilization and diminishing incandescence of lava deposits at the summit and depositional gullies are consistent with the cessation of magma supply to the summit crater," Phivolcs said.
The state volcanology added that the "campaign microgravity measurements indicate positive (density increase) and negative (density decrease) anomalies on the lower and middle slopes of the volcano, respectively."
Overall, ground deformation and microgravity parameters indicate that pressurization of the Mayon edifice has abated in the past month, but that the edifice remains generally pressurized (inflated) due to magma intrusion that has been transpiring since the onset of unrest in June 2023.
Volcanic SO2 emission measured by campaign Flyspec measurement peaked at an average of 4,756 tonnes/day on 16 August 2023 and has since been decreasing and increasing in cycles of generally lessened peaks.
"The overall weakening of cyclic SO2 emission indicates that volcanic gas from lava exposed on the crater and slopes of Mayon and from magma stored within the volcano is diminishing, but that minor volumes of magma are still periodically intruded in storage beneath the edifice, consistent with ground deformation and microgravity data," Phivolcs said.
