Page 199 - Fundamentals of Physical Volcanology

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176 CHAPTER 11

Table 11.2 Volcano alert systems.

Color coded alert levels Numerical alert levels
Color Implication Indicative phenomena Volcano status

Green No eruption anticipated. Volcano is 0 Typical background surface Usual dormant or
in quiet, “dormant” state activity; seismicity deformation quiescent state
and heat flow at low levels
Yellow An eruption is possible in the next 1 Apparent seismic, geodetic, Initial signs of possible
few weeks and may occur with little thermal, or other unrest unrest. No eruption
or no additional warning. Small indicators threat
earthquakes and/or increased levels
of volcanic gas emissions have been
detected locally
Orange An explosive eruption is possible 2 Increase in number or intensity Confirmation of volcano
within a few days and may occur with of unrest indicators (seismicity, unrest. Eruption threat
little or no warning. Ash plume(s) are deformation, heat flow, etc.)
not expected to reach > 9 km above
sea level. Increased numbers of
local earthquakes are evident.
Nonexplosive extrusion of a lava
dome or lava flows may be occurring
Red A major explosive eruption is 3 Minor steam eruptions. High or Minor eruptions started.
expected within 24 hours. Large ash increasing trends in indicators of Real possibility of
plume(s) are expected to reach at unrest, significant effects on hazardous eruptions
least 9 km above sea level. Strong volcano, possibly beyond
earthquake activity is detected even
at distant monitoring stations. An
explosive eruption may be in progress
4 Eruption of new magma. Hazardous local
Sustained high levels of unrest eruption in progress.
indicators, significant effects Large-scale eruption
beyond volcano now possible
5 Destruction with major damage Large hazardous
volcanic eruption
beyond active volcano.
Significant risk over wider areas in progress

sufficiently repeatable pattern of activity that any- starts, and various other actions can be taken dur-
thing other than a statistical forecast can be given. ing the eruption. Of course, monitoring the vol-
For many volcanoes this gives rise to levels of alert, cano, in the various ways described in the previous
either color coded or numerical, as shown in section, is itself one form of pre-eruption precau-
Table 11.2. tion. If sufficiently early and sufficiently reliable
warnings of an impending eruption can be given,
the local population can be evacuated safely. It
11.5 Hazard mitigation has to be said, however, that the word “reliable” is
important here: there are a number of examples
Reducing the severity of the outcome of an erup- of evacuations after which the expected eruption
tion can be approached in two ways. Certain kinds did not occur, and the social and economic damage
of precautions can be taken before an eruption done to the evacuated people was very great.