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Pinatubo Volcano Alert Signals PDF Print
Wednesday, 10 August 2011 17:25




No alert

Background, quiet

No eruption in foreseeable future


Low level seismic, fumarolic, other unrest.

Magmatic, tectonic or hydrothermal disturbance; no eruption imminent.


Low to moderate level of seismic, other unrest  with positive evidence for involvement of magma

Probable magmatic intrusion; could eventually lead to an eruption.


Relatively high and increasing unrest, including numerous b-type earthquakes, accelerating ground deformation, increased vigor of fumaroles, gas emission

Increasing likelihood of an eruption, possibly witin days to week.


Intense unrest, including harmonic tremor and/or may “long period” (=low frequency) earthquakes or quiet lava emissions and/or dome growth and/or small explosions

Magma close to or at earth’s surface.  Hazardous explosive eruption likely, possible within hours to days


Hazardous explosive eruption in progress, with pyroclastic flows and/or eruption column rising at least  6 km or 20,000 feet above sea level

Explosive  eruption in progress.  Hazards in valleys and downwind


In order to protect against “lull before the storm” phenomena, alert levels will be maintained for the following periods AFTER activity decreases to the next lower level:

From level 5 to level 4:

Wait 12 hours after level 5 activity stops

From level 4 to level 3 to 2:

Wait 2 weeks after activity drops below level 4

From level 3 to level 2:

Wait 2 weeks after activity drops below level 3


Ashfall will occur from secondary explosions for several years after the 1991 Plinian (calderagenic) eruption, whenever rainfall and lahars come in contact with still hot-hot 1991 pyroclastic flow deposits.  These secondary explosions will occur regardless of alert level.