PHIVOLCS reported that the eruption at Mayon continued during 11-18 July, with slow lava effusion from the summit crater feeding lava flows on the S and SE flanks. The length of the lava flow in the Mi-Isi (S) drainage remained at 2.8 km and the flow in the Bonga (SE) drainage advanced to 2.4 km by 18 July. The lava flow in the Basud drainage on the E flank was 600 m long. Collapses at the lava dome and from the lava flows produced incandescent rockfalls and pyroclastic density currents (PDCs, or pyroclastic flows) that descended the Mi-Isi and Bonga drainages; material also traveled as far as 4 km down the Basud drainage. Steam-and-gas plumes rose 200-750 m above the summit and drifted WSW, W, and ESE during 11-14 July; emissions were not reported on the other days of the week. Sulfur dioxide emissions were reported on most days and showed an increasing trend, averaging between 1,128 and 2,989 tonnes per day, with the highest value recorded on 15 July. Each day seismic stations recorded 150-423 rockfall events and 3-10 PDC events (from dome and lava-front collapses). There were 3-39 volcanic earthquakes recorded on most days, though 184 and 267 were recorded during 16-17 July and 17-18 July, respectively. At 2300 on 16 July the seismic network began recording a continuous series of weak low-frequency volcanic earthquakes (LFVQs). Some of the events were detected by infrasound sensors and produced audible rumbling sounds during the evening of 17 July and the morning of 18 July. By 1530 on 18 July a total of 573 LFVQs had been detected. Most of the events originated from a shallow source and were associated with a rapid release of volcanic gases, though visually there were no changes in effusive activity. The Disaster Response Operations Monitoring and Information Center (DROMIC) reported that as of 1800 on 17 July there were 5,801 families, or 20,257 individuals, that were either in evacuation shelters or staying in other types of alternative housing. The Alert Level remained at 3 (on a 0-5 scale) and residents were reminded to stay away from the 6-km-radius Permanent Danger Zone (PDZ). PHIVOLCS recommended that civil aviation authorities advise pilots 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 term andesitic lava flows. Eruptions occur predominately from the central conduit and have also produced lava flows that travel far down the flanks. Pyroclastic flows 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.

Sources: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Disaster Response Operations Monitoring and Information Center (DROMIC)