PHILVOLCS reported increased seismic activity during 22-25 Dec preceded the event which seems to be the result of destabilization of the shallow hydrothermal system. GPS data also indicate slight inflation of the volcanic edifice, indicating internal pressurization (likely caused by an underlying magma body). [moins]

FVFE01 at 07:22 UTC, 29/12/16 from RJTD [moins]

FVFE01 at 07:52 UTC, 23/10/16 from RJTD [moins]

FVFE01 at 22:12 UTC, 18/10/16 from RJTD [moins]

The new emissions came from the new vent on the southeast slope observed last week. The resulting ash plume rose approx. 1000 m and caused light ash falls. According to PHILVOLCS, the activity is driven entirely by steam and does not involve fresh magma.
Alert level of the volcano remains at 1 with an exclusion zone of 4 km radius around the summit crater. [moins]

FVFE01 at 00:58 UTC, 17/10/16 from RJTD [moins]

Bad weather prevented direct observation of the eruption, but ash falls were recorded in several neighboring communities (Rizal, Buenavista Gubat Proper, Tugawe, Tigkiw, Ariman, Bintuco, Bulacao, Naagtan).
PHILVOLCS reported that new fissures, which formed in late September on the southeast side of the summit cone, have recently become more active and produce strong degassing and occasional ash venting. Apparently, this activity has further increased since yesterday's explosion: this indicates that the internal hydrothermal system is now more pressurized and more unstable than before. [moins]

FVFE01 at 09:21 UTC, 16/09/16 from RJTD [moins]

(Smithsonian / USGS Weekly Volcanic Activity Report 20-26 July 2016) [moins]

According to PHILVOLCS, the eruption preceded by a seismic swarm with more than 100 small earthquakes and lasted 5 minutes. The eruption itself was only steam-driven - no magma itself has reached the surface,- and caused by explosion of over-pressurized and over-heated fluids from the volcano's strongly developed hydrothermal system.
A similar explosion had occurred on 21 February. Following it, scientists monitoring the volcano have been seeing an inflation of the edifice (swelling of the ground) since May, probably caused by magma intruding at depth. This would have been providing new energy (heat) to the system and cause increasing instability. [moins]

The eruption was likely phreatic or hydrothermal, i.e. the result of phreatic activity (overheated, pressurized water in the upper conduit area flashing to steam with no fresh magma involved, but this can only be determined if an analysis of the ash is done.
The eruption came with little warning, although PHILVOLCS mentions that increased seismicity was recorded on 20-21 February (something that in itself is not very unusual or sufficient to predict eruptions). The explosion itself was immediately preceded by a rock fracturing earthquake, which reflected the moment rock layers inside the volcano ruptured to release accumulated pressure. [moins]