Report on Bulusan (Philippines) — April 1986
Scientific Event Alert Network Bulletin, vol. 11, no. 4 (April 1986)
Managing Editor: Lindsay McClelland.
Bulusan (Philippines) Seismic swarm in summit caldera
Please cite this report as:
Global Volcanism Program, 1986. Report on Bulusan (Philippines). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 11:4. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198604-273010.
12.769°N, 124.056°E; summit elev. 1535 m
All times are local (unless otherwise noted)
A seismic swarm began on 19 April at 2022 and lasted for ~ 10 hours. A total of 229 volcanic earthquakes were recorded by most of the five seismic monitoring stations. The initial phase was characterized by high-frequency volcanic earthquakes, gradually replaced by low-frequency volcanic earthquakes during the peak and latter part of the activity. Three of the events were felt, with epicenters initially located ~ 7.4 km SE (azimuth 134°) of the summit crater, within the caldera.
No other significant change was observed. Steam emission remained weak and hot spring temperatures remained normal. Local seismicity gradually declined to a low level, with only 1 high-frequency volcanic earthquake recorded on 22 April.
The last eruption of Bulusan, in June 1983, was not preceded by an increase in seismicity, but hot spring temperatures had increased several degrees. The April 1981 eruption, however, was preceded by an 8-day earthquake swarm. A seismic swarm following that eruption did not culminate in additional eruptive activity.
Geologic Background. 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: PHIVOLCS.