Comprehensive Risk Assessment for Natural Hazards - Planat

Comprehensive risk assessment for natural hazards
these earthquakes are generally quasi-monochromatic (e.g.,
1.3 Hz) and often display a high frequency onset. Chouet
(1996) also demonstrates the close similarity in the sources
of low frequency events and tremors.
The careful analysis of the changes in seismic activity
and specially the recognition of the occurrence of low frequency
swarms allow warnings to be given. An example is
the case of tephra eruptions, such as the eruption of
Redoubt Volcano, Alaska (1989-1990).
(b) Ground deformation
With the growth of a magma chamber by injection of
magma and/or with the modification of its pressure by
internal degassing, the surface of a volcano can be
deformed (Dvorak and Dzurizin, 1997). In terms of tilting,
changes can be detected with a sensitivity of 0.01 μrad
(microradian). Depending on the type of volcano, an
increase of 0.2 μrad may be a precursor for a summit
eruption, such as occurred in Sakurajima, Japan in 1986. In
general, variations of the order of 10 μrad indicate a future
eruption (e.g., Etna, Italy, 1989), while values as large as 100
μrad or more have been observed in Kilauea, Hawaii. A
variety of topographic equipment is used in deformation
monitoring, such as theodolites, electronic distance meters
(EDM) and electronic tiltmeters. The recent development
and accuracy of Global Positioning System (GPS) satellites
make these very convenient tools to measure the inflation
or deflation rate. Laser distance meters, which do not need
a mirror, are also very useful for estimating the growth of a
dome. Deformation monitoring combined with seismic
monitoring was extremely useful in the case of predicting
the Mount St Helens eruption.
(c) Gas emission
Several gases are involved in eruptive processes such as
H 2 S, HCl and HF, while H 2 O vapour, CO 2 and SO 2 are the
predominant magmatic gases. The monitoring of these
different gases is the most effective diagnostic precursor of
magmatic involvement. There are a large variety of
methods to analyse these gases. One of the standard
approaches is to measure remotely their flux. For example,
the amount of emitted sulfur dioxide is measured with the
help of a correlation spectrometer “COSPEC” (Stoiber et
al., 1983). It compares the quantity of solar ultra-violet
radiation absorbed by this gas with an internal standard.
The values are expressed in metric tons per day. Current
values depend on the volcano and vary between 100 to
5 000 t/d. An increase in emission helps to forecast an
eruption.In contrast,a decrease could mean the end ofa
major activity, or it could also be the sign of the formation
of a ceiling in the volcanic conduit system leading to an
explosion.
All gases are not volcanic in origin, but can nevertheless
be valuable indicators. Such is the case of the Radon, a
radioactive gas, generated by Uranium or Thorium disintegration,
which is freed at depth by fissuration of rocks
related to the ascent of magma.
Gas monitoring has limited value by itself, and it should
be considered as an additional tool to seismic and ground
deformation monitoring. Sometimes, it is helpful in the
recognition of a very early stage of a forthcoming eruption,
and it could help to evaluate the end stage of a volcanic crisis.
(d) Other geophysical methods
Injection of magma into the upper crust results in changes
of density and/or volume (Rymer, 1994) and also to a loss of
magnetization (Zlotnicki and Le Mouel, 1988), which affect
the local gravity and magnetic fields. Movements of fluids
charged with ions will generate natural electrical currents
named spontaneous polarization (SP). Therefore, changes in
electrical fields are an additional indicator for monitoring
volcanic activity (Zlotnicki et al., 1994). Approaches based
on these other geophysical methods are of interest in better
understanding of the volcanic processes, but at this stage,
they remain experimental and are generally not routinely
used in operations.
(e) Remote sensing
Remote sensing can be divided in two branches. The passive
one, which is purely observational, concerns the natural
radiation of the earth, while the active one, which sends signals
down to the earth, detects the reflected radiation.
The observational satellites such as LANDSAT’s
Thematic Mapper (USA) and SPOT (France) produce data
that are useful for the surveillance of poorly accessible volcanoes.
The infrared band from LANDSAT is capable of
seeing the “hot spot” on a volcano and, therefore, is of assistance
in helping to detect the beginning of an eruption.
These satellites, including those with lesser resolution (e.g.,
METEOSAT), are very important after the eruption to track
the movement and dispersion of the volcanic plume
(Francis, 1994).
The use of radar equipment such as Synthetic Aperture
Radar (SAR) on satellites such as ERS-1 or RADARSAT is a
promising tool for the surveillance of volcanoes in any
weather, day and night. It also allows radar interferometry
analysis that gives information on the dynamic deformation
of the volcano surface. Lanari et al. (1998) provide an
example of such an application for the Etna volcano in Italy.
4.4 DATA REQUIREMENTS AND SOURCES
4.4.1 Data sources
Standard observational data related to volcanic activities are
usually sent to the Scientific Event Alert Network
(Smithsonian Institution/SEAN, 1989) of the National
Museum of Natural History of the Smithsonian Institution
in Washington. A monthly bulletin is available. These data
can also be accessed by Internet. Recently, many research
groups on volcanoes and observatories are placing their data
on the World Wide Web (http://www.nmnh.si.edu/gvp).
4.4.2 Monitoring — Data management
Rapid progress in electronics, especially in microprocessing,
has contributed most to the improvement of digital data
acquisition systems. The data are stored on various media,
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