This report summarizes activity at Bulusan from March 2011 through May 2016. Eruptive activity that began on 6 November 2010 with ash explosions was continuing in late March 2011 (BGVN 36:03), and ended on 16 May 2011. The volcano was quiet until renewed activity during 1 May-17 July 2015. Another brief eruption was noted on 22 February 2016. Activity typically includes phreatic explosions from the summit crater and flank vents, ash-and-steam plumes, and minor ashfall in surrounding villages. Information was provided by the Philippine Institute of Volcanology and Seismology (PHIVOLCS) and the Tokyo Volcanic Ash Advisory Center (VAAC).

Activity during March-May 2011. Following lahars in late February 2011, the number of daily volcanic earthquakes dropped to six or fewer in the first week of March. This small daily number of events continued through early May, except for nine earthquakes reported on 26 April.

During March through early May 2011, weak steaming from the SE and NW vents was only intermittently observed when cloud cover did not obscure the view. Steam emissions seen in late April from the NW and SE vents reached an approximate height of 50 m before drifting WSW. During six days in March and eight days in April the Manilla airport reported to the Tokyo Volcanic Ash Advisory Center (VAAC) that volcanic ash had been observed, but the VAAC could not identify ash in satellite data.

A phreatic eruption occurred at the cloud-covered summit crater of Bulusan on 13 May 2011 at 0025 local time. Rumbling sounds from the explosion were reported up to 5 km away from the volcano. The explosion earthquake associated with this event lasted for 10 minutes. The seismic network had detected only 10 earthquakes in the preceding 24 hours. Field investigations conducted immediately after the explosion confirmed the presence of thin (0.5-2.5 mm) ash deposits reaching approximately 9 km away from the crater to the NW and SW. Eight barangays in the municipality of Juban NW of the volcano (Caladgaw, Rangas, Puting- Sapa, Sangkayon, Bacolod, Buraburan, Gurayan, and Aniog) and four in the municipality of Irosin SSW of the volcano (Cogon, Bolos, Umagom, and Gulang-gulang) reported light ashfall after the event. Eleven volcanic earthquakes were recorded in the 24 hours after the explosion, but only four more in each of the next 24-hour periods.

A significant increase in the seismic activity occurred during the morning of 16 May when 80 volcanic earthquakes were detected. This number increased to 144 volcanic earthquakes for the next 24-hour period. Only weak to moderate steaming from the active vents was reported following the explosion through 16 May. After this, the frequency of volcanic earthquakes declined to baseline levels of 0-2 events per day.

Short-lived swarms of 9-21 events were reported on 25 May, 10 August, 11 September, and 20-26 December 2011, and 22 January 2012. In April 2012 ground deformation surveys indicated that the edifice had deflated since late November 2011. This overall decline in activity led PHIVOLCS to lower the hazard status in April 2012 from Alert Level 1 to Alert Level 0 (on a 0-5 level scale), meaning no eruption was foreseen for the immediate future.

Activity during May-July 2015. Bulusan remained quiet following the 13 May 2011 explosion until 1 May 2015, except for a single episode of increased seismicity (13 volcanic earthquakes over 24 hours) and minor inflation in mid-July 2014. A series of explosions during 1 May-17 July 2015 originated from the NW vent, generally sending ash plumes to heights of 150-250 m above the summit, with larger plumes rising 1-1.5 km on 16 and 19 June.

A five-minute-long steam-and-ash explosion erupted from the cloud-covered summit in the morning of 1 May 2015 and was followed by strong to moderate steaming from the NW vent that reached 200 m high and drifted WNW. The explosion distributed traces of ash in the areas W and WNW of the volcano, affecting the barangays of Bolos, Cogon, Gulang-Gulang, Sangkayon, Tinampo, and Umagom up to 7 km to the SW, and barangay Putting Sapa up to 12 km to the NW. Only five volcanic earthquakes were detected during the week prior to the eruption by the seismic network, but 62 were recorded in the eight hours following the explosion. A second 3.5-minute-long steam-driven explosion occurred on 6 May at 2146 that sent an ash plume approximately 250 m high before it drifted W. Traces of ash were observed about 8 km SW in Sitio Tulay, Cogon, Monbon, and Tinampo.

As a result of these two explosions, PHIVOLCS raised the hazard status from Alert Level 0 to 1. Between 6 May and 16 June the volcano was quiet with very few volcanic earthquakes and weak emissions of steam from the NW and SW vents that drifted SW or WNW. Precise Leveling surveys conducted in early May indicated very slight inflation relative to March surveys.

Activity resumed with two steam-driven explosions on 16 June at 1102 and 1120. The first event lasted approximately 10 minutes and produced a 1 km high steam-and-ash plume which drifted WSW. Rumbling from the explosion was heard by residents of Cogon on the WSW slopes of the volcano. The second explosion lasted for about one minute. A two-minute explosion was reported during 18-19 June, but dense clouds obscured visual observations of the summit. A seven-minute-long phreatic explosion occurred at 1455 on 19 June, and produced a 1.5-km high ash plume that drifted WSW. A low-level ash cloud on the upper NW flank, possibly from a short pyroclastic flow, was also observed. Minor amounts of ash fell in the neighborhoods of Bacolod, Buraburan, Mapili, Puting Sapa, and Juban 11 km to the NW. The eruption was followed by a voluminous steam emission, rising 250 m before drifting SW. A smaller explosion on 21 June lasting 111 seconds generated a 150 m high steam-and-ash plume that drifted E. Trace amounts of ash fell up to 8 km east of the volcano in San Jose, San Francisco, Bulusan Proper, Sapngan, San Rafael, and Dapdap. In late June, weak to moderate steam plumes up to 100 m high were observed drifting from the WNW and SW vents. Precise leveling surveys in late June 2015 indicated very slight inflation of the volcano's edifice relative to May 2015.

PHIVOLCS reported that at 1310 on 17 July a phreatic eruption from the active vent on its upper NW slopes was detected by the seismic network for 11 minutes and generated an ash plume that rose 200 m above the vent that drifted WNW before drifting further N and NE. Ashfall was reported in areas up to 18 km to the N and NE in the Sorsogon Province, including Inlagadian (municipality of Casiguran), Tigkiw, Tugawe, Nazareno, Bugtong and Rizal (municipality of Gubat), and Fabrica, San Isidro, Sta. Cruz, and San Ramon (municipality of Barcelona). The rumbling from the eruptions was audible up to 6 km from the summit.

Activity during July 2015-February 2016. Following the 17 July 2015 explosive activity, seismic activity decreased to background levels of 0-5 daily events with intermittent spikes a few times a month to a maximum of 24 earthquakes. Precise Leveling surveys and continuous GPS data between August 2015 and December 2015 indicated slight inflationary changes of the volcano's edifice during that time.

On 21 February 2016 the seismic network recorded 15 earthquakes, followed by two minor eruptions at 1501 on 22 February that generated a short steam-and-ash column approximately 500 m high from the WNW fissure vent. The eruption signal consisted of two earthquake events, a rock fracturing event, recorded as a high-frequency earthquake, followed by an explosion-type (low frequency) earthquake with a total duration of just over four minutes. Explosions were accompanied by a rumbling sound heard at Puting Sapa, Juban and Bolos, 12 km NW. Traces of volcanic ash were reported in barangays Putting Sapa, Sagkayon and Caladgao in Juban (12 km NW), and barangay Bolos in Irosin (8 km SW).

Precise leveling survey results from 23-28 February 2016 indicated slight deflationary changes of the edifice relative to December 2015, consistent with ground deformation measurements from continuous GPS data through February 2016.

Geological Summary. Luzon's southernmost volcano, Bulusan, was constructed along the rim of the 11-km-diameter dacitic-to-rhyolitic Irosin caldera, which was formed about 36,000 years ago. It lies at the SE end of the Bicol volcanic arc occupying the peninsula of the same name that forms the elongated SE tip of Luzon. A broad, flat moat is located below the topographically prominent SW rim of Irosin caldera; the NE rim is buried by the andesitic complex. Bulusan is flanked by several other large intracaldera lava domes and cones, including the prominent Mount Jormajan lava dome on the SW flank and Sharp Peak to the NE. The summit is unvegetated and contains a 300-m-wide, 50-m-deep crater. Three small craters are located on the SE flank. Many moderate explosive eruptions have been recorded since the mid-19th century.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/); Tokyo Volcanic Ash Advisory Center (VAAC), Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/).