VOLCANIC HAZARDS 
D=V,*V, 4.1 (4.2) 
DESCRIPTION AND CHARACTERISTICS OF 
THE MAIN VOLCANIC HAZARDS 
An active volcano can produce different hazards as defined 
by the IAVCEI (1990). These can be subdivided into: 
Primary or direct hazards due to the direct impact of 
the eruption products. 
Secondary or indirect hazards due to secondary conse- 
quences of an eruption. 
4.2.1 Direct hazards 
One can distinguish four prmcipal types of direct volcanic 
1965). These are: lava flows; pyroclastic hazards (Holmes, 
flows; ash fulls and block falls; and gases: 
(a) Lava flows (Figure 4.2); 
(b) Pyrodastic flows such as pumice flow, nuke ardente, 
base surge, . .. (Figure 4.3); 
(c) Ash falls and block falls (Figures 4.4,4.5 and 4.6); 
fd) Gases (Figure 4.7). 
Indirect hazards 
Chapter 4 
4.2 
* 
(4.1) 
INTRODUCTZON TO VOLCANIC RISKS 
Every year several of the 550 historically active volcanoes on 
earth are restIess and could pose a threat to mankind; two 
recent examples are particularly relevant. On 19 September 
1994, the Vulcan and Tavurvur volcanoes in the Rabaul 
Caldera, Papua New Guinea, began to erupt. Monitoring of 
precursors and awareness of the population of the eruptions 
allowed the safe evacuation of 68 000 people. The economic 
damage due to ash fall was significant. On 18 July 1995, a 
steam blast explosion occurred on the dormant Soufrikre 
Hills volcano, Montserrat, West Indies. This event was fol- 
lowed by an ongoing activity h a t included a larger event on 
21 August 1995,which generated an ash-cloud that menaced 
5 000 out of 12 500 inhabitants the capital, Plyrnouth.About 
of the island were temporally evacuated from the southern 
risky area towards the centre and the north of the island. 
Since then, the volcanic activity progressively devebped to 
the point where it affected Plymouth on 6 August 1997, 
Eighty per cent of the buildings were either badly damaged 
or destroyed, but the previously evacuated population were 
safe, although 
north. These two cases demonstrate that with a good under- 
standing of the hazardous phenomenon, appropriate 
information, and awareness of the population and she 
authorities, it is possible in most cases to manage a difficult 
situation. This, of course, does not alieviate aIi personal suf- 
fering, but contributes to its reduction. 
Before 
for greater security, they were moved further 
enrering into a descriprion of volcanic hazards 
and the different ways in which they can be surveyed. it is 
important to present the way in which they are integrated 
into risk analyses (Tiedemann, 1992). This approach pro- 
vides the basis for developing sound mitigation measures. 
Figure 4.1 gives a global mew of the problem, whilst its dif- 
ferent aspects will be presented Iater in this chapter. 
Volcanic risk may be defined as: The possibility of loss 
of life and damage 10 properries and culrural heritage in an 
area exposed to the threat of a volcanic eruption. 
This definition can be summarized by the following 
formula (UNDRO, 1980): 
Risk* = f( hazard, vulnerability, value) 
The volcanic hazard, denoted H,, can also be written in 
the following form: 
H, 
4.2.2 
One can distinguish rhree main types of indirect volcanic 
hazards. These are lahars, landslides and tsunamis. The h s t 
two are often triggered by explosive eruptions and so volca- 
nologists tend to classify them as prlmary hazards, which is 
a matter of debate. 
(0) 
(b) 
(c) 
Tsunamis may be generated from volcanic activity when 
huge masses of water are suddenly displaced by an eruption 
or an associated landslide. The explosion of the Krakatoa 
volcano in 1883 provoked a tsunami that killed more than 
Lahars 
They correspond to a rapidly flowing sediment-laden mix- 
ture of rock debris and water. One can classify them 
according to their sediment content. Hyper-concentrated 
flows contain between 40 and 80 per cent by weight sedi- 
ment and debris flows more than 80 per cent (Fisher and 
Smith, 1991). One can categorize these flows as debris flow, 
mud flow and grandar flow (Figure 4.8). 
Landslides 
These may include slumps, slides, subsidence block falls, 
debris avalanches and gigantic subsidence of volcanic flanks 
or cones (Figure 4.9). 
Tsunamis 
people. The collapse of Mt Mayuyama in 1792 at the 
= f (E,P) 
with E being an event in terms of intensity or magnitude 
and P being the probability of occurrence of that type of 
event. The product of the vuherability, denoted V,, times 
the value of the property, denoted V,, is a measure of 
the economic damages that can occur and 1s given by the 
relation: 
* See the glossary for the different definitions 
34 000 
Unzen volcano in Japan generated a major tsunami that 
killed 15 000 peopIe (Figure 4.10). Comprehensive risk assessment for natural hazards 
(short term) 
(= Maxlrnurn 
ampiitude) 
Volcano monitoring 
characteristics of occurrence 
eruptive activity 
VOLCANIC HAZARD ZONATION: 
- classificarion of hazard parameters 
PREVISION 
(Medium and long term) 
VOLCANIC RISK ZONATION 
P R E V E N T I O N 
1 - Flow chart for the rnrtigarion of volcanic hazards and risk assessments Figure 4 
35 
Settientent 
OF SURROUNDINGS 
EVALUATION 
PREPAREDNESS 
- long-term planning for human settlements 
- preventive information for population 
- emergency management, evacuation plans 
- warning and alert instrucrions 36 
Figure 4.2 - Hawaiian rype (Type example, Hawaii, USA) 
Figure 4.4 - Plinian type (Refined ar Vesuvius, Italy) 
Figure 4.8 - Sketch ofa luhar 
(d) 
4.3 
Figure 4.6 - Strombolian type (Defined at Stromboli, Itah) 
Subsidence 
Others 
These are other notable indirect hazards such as acid rain and 
ash in the atmosphere (Tilling, 1989). Their consequences lead 
to property damage and the destruction of vegetation and 
pose a threat to airplane traffic. 
TECHNIQUES FOR VOLCANIC HAZARD 
ASSESSMENT 
Volcanic hazards may be ex-aluated through two main comple- 
mentary approaches, which iead to their prediction (Scarpa 
and TilIing, 1996): 
Chapter 4 - Volcanic hazards 
Martinique, French West Indies) 
Figure 4.3 - Pelean Tpe (Type example M t Pel&, 
Figure 4.7 - Sketch of gas emission 
Figure 4.9 - Sketch of a landslide 
4.10 - Skerch of a tsunami 
Figure 4.5 - Vzrlcanian type (Defined at Vulcano, Italy) 
SO2 Sulfur dioxide 
CO; Carbon dioxide 
Hydrofluoric acid, etc. 
f-- 
Fig. 
Medium- to long-term analysis; volcanic hazard 
mapping and modelling, volcanic hazard zoning. 
Short term; human surveillance and instrumental 
monitoring of the volcano. 
4.3.1 Medium- and long-term hazard assessment: 
zoning 
In most cases, one is able to characterize the overall activity 
of a volcano and its potential danger from field observations 
by mapping the various historical and prehistoric volcanic 
deposits. These deposits can, in turn, be interpreted in terms Coniprehenjivr risk assessment for natiirctl hazards 
of eruptive phenomena, usually by analogy with visually 
observed eruptions. It is then possible to evaluate 
characteristic parameters such as explnsivity, using the 
volcanic explosivity index (VEI) listed in Table 4.1 (Newhall 
and Self, 1982), intensity, magnitude and duration. This 
allows the reconstruction of events and their quantification 
in terms of, for example, plume elevation, volume of magma 
emitted and dispersion of the volcanic products. This 
activity is illustrated within Figure 4.1. 
Each parameter may then be sorted, classified and 
compared with a scale of values, permitting the zoning of each 
volcanic h;rzard. This can be drawn according to: the intensity, 
for example the thickness or extent of the hazard, such as 
shown in Figure 4.1 L; the frequency of occurrence; or their 
corn bin at ion. 
*As a genera! rule, the intensity of volcanic phenomena 
decreases with the distance from the eruptive centre - 
crater or fissure. Topographic or meteorological factors may 
the modify progression of the phenomenon, such as the 
diversion of lava flow by morphology. 
Basically the delineation of areas or zones, previously 
threatened by direct and indirect effects of volcanic eruptions, 
is the fundamental tool to estimate the potential danger from a 
future eruption. To fully use zoning maps, it is important to be 
familiar with the concept of zone boundaries and assessment 
scales. 
Zoning boundaries 
Boundaries are drawn on the volcanic zoning maps using 
expert judgement based OR physical conditions and the esti- 
mate of the nature (explosivity, intensity) of the future 
volcanic activity They are drawn to one of two scales relat- 
ed to the phenomenon’s intensity and frequency of 
occurrence, where frequency is estimated from data and 
geological surveys and antedotal evidence. 
General 
Table 4.1 - Volcanic Explosivity Index (after Newhall and Seg 1982) 
Cloud column Qualitative Classification 
(a) 
VEI description 
Non-explosive 
Small 
Moderate 
Mvderate Large 
Large 
Very large 
V 
Volume of 
tephra (m3) 
104 
7 06 
107 
108 
109 
1010 
7 0” 
1 0 ’ 2 
~~ ~ 
height (km) 
<o. 1 
0.1-1 
1 -5 
3-1 5 
10-25 
25 
, 
I 
. -. - 
(b) 
* 
- 
Zoningscales 
In vulcanological practice, each hazard may be zoned 
according to two assessment scales. The first is the 
frequency scale and is comprised of four levels. The levels 
are: 
annual frequency 
decennial frequency 
centennial frequency 
millennia1 frequency 
moderate intensity 
intensity 
- 
The second is the intensity scale which would cover 
aspects such as lava flow extension and thickness of cindcrs. 
This scale is also comprised of four levels. They are: 
very high intensity + total destruction of 
population, settlements, 
- high intensity 
* low 
The establishment of hazard maps is a fundamental 
requirement but not sufficient for appropriate mitigation 
action, as their information has to be interpreted in con- 
junction with the vulnerability of the exposed environment 
in a broad sense. 
exposed 
population 
+ damage for agricukure 
-+ abrasion, corrosion of 
machinery, tools, ... 
description 
Gentle. effusive 
v 
Explosive 4 
Hawaiian 
Vulcanian 
I 
Severe, 
violent, 
terrific 
j Cataclysmic, 
I paroxismal, 
colossal Ultra-plinian 
1 t 
Plinian 
I 
Y 
A 
i 
I 
~ + .____ 
+ permanent hazard 
+ very high hazard 
+ high hazard 
+ low hazard 
and vegetation 
+ settlement, buildings 
partial destroyed 
37 
+ important danger for the 
population 
partial damage of 
structures 
+ population partly 
-+ 
+ no real danger for 
Historic 
eruptions 
up to 1985 
487 
623 
3 176 
733 
119 
19 
5 
2 
0 38 
4.3.2 
[a) 
1.3 
Short-term hazard assessment: monitoring 
Geophysical and geochemical observations are used to deci- 
pher the ongoing dynamlcs of a volcanic system. The main 
goal is to develop an understanding sufficient to allow the 
forecasting of an eruption. To achieve this, a volcano must 
be monitored in different ways: 
In real- or near-real-time through physical or chemical 
sensors located on the volcano and connected to an 
observatory by telecommunication lines such as cable 
UHF or VHF telephone, cellular phones and standard 
radio via terresrrial or satellite links. 
in the field where operators regularly make direct 
observations and/or measurements. 
By laboratory analysis of freshly expelled volcanic 
material collected in the field. 
The different tools commonly empIoyed are documented 
by Ewert and Swanson (1992). They are described below. 
Seumic ncriviry 
Volcano-seismic observations allow a major contribution to 
be made to the prediction of a volcanic eruption. This 
domain is complex, not only because the seismic sources of 
the volcanoes imply the dynamics of fluids (liquids and 
gases) and of solids, but also because the wave propagation 
evolves in an heterogeneous and anisotropic medium that 
can be very absorbent. In addition, the topography can be 
very irregular. 
Volcano-seismic monitoring records events in an 
analogue or digital manner. In a first step they are 
interpreted in terms of source location, of strength 
(magnitude) and of frequency of occurrence. The location 
and the magnitude are determined with standard computer 
programs such as Hypo71 (Lee and Lahr, 1975) or SEISAN 
(Havskov, 1995). In a complementary step, one tries to infer 
what had initially generated the vibration, which is termed 
the source type (tectonic, internal fluid movements, 
explosions, lava or debris flow, rock avalanches). To facilitate 
this, one can use a classification based on the shape 
(amplitude and duration) and frequency content. It is 
possible to distinguish within the seismic activity, transitory 
signals, the standard earthquake and/or nearly stationary 
ones referred to as tremors. 
The observed events may have frequencies usually 
ranging from 0.1 to 20 Hz. In the classification of the events 
based on their frequency content, two broad classes are 
defined based on the source processes (see for example 
Chouet, 1996). The first class has earthquakes with high 
frequencies, and the second has those with low frequencies 
long period, LP). Those in the first class are (often called 
produced by a rupture of brittle rocks and are known as 
tectonic events. If they are directly associated with the 
dynamics of the volcano, they are defined as volcano- 
tectonic events. The low frequency, volcanic earthquakes 
and tremors are associated with fluid movement and range 
in vahe between 1 to 5 Hz. Observations show that many of 
these earthquakes are generally quasi-monochromatic (e.g., 
Hz) and often display a high frequency onset. Chouet 
(1996) also demonstrates the close similarity in the sources 
of tow frequency events and tremors. 
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 p a d 
(microradian) Depending on the type of volcano, an 
increase of 0.2 prad may be a precursor for a summit 
eruption, such as occurred in Sakurajima, Japan in 1986. In 
general, variarions of the order of 10 vrad indicare a future 
eruption (e.g., Etna, Italy, 1989), while values as large as 100 
p a d 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 a 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 mon- 
itoring was extremely useful in the case of predicting the 
St Helens eruption. 
Gas mission 
(b) 
Mount 
(c) 
Several gases are involved in eruptive processes such as 
HZS, HC1 and HF, while H20 vapour, CO, and SO, 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 anaIyse 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 ef 
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 
t/d. An increase in emission helps to forecast an 
eruption. In contrast, a decrease could mean the end of a 
major activity, or it could also be the sign of the formation 
of a ceiling in the volcanic conduit system leading to an 
explosion. 
A11 gases are not volcanic in origin, but can nevertheless 
be valuable indicators Such is the case of the Radon, a 
5 000 
radioactive gas, generated by Uranium or Thorium disinte- 
gration, which is freed at depth 
related to the ascent of magma. 
Gas monitoring has limited value by itself, and it should 
be considered as an additionat tool to seismic and ground 
deformation monitoring. SomeIimes, it IS helpful in 
recognition of a very early stage of a forthcoming eruption, 
and it could help to evaluate the end stage of a volcanic 
crisis. 
Chapter 4 - Volcanic hazards 
The careful analysis of the changes in seismic activity 
and specially the recognition of the occurrence of low fre- 
quency swarms allow warnings to be given. An example is 
the case of tephra eruptions, such as the eruption of 
Redoubt Volcano. Alaska (1989- 1990). 
by fissuration of rocks 
the Comprehensive risk assessment for natural hazards 
Other geophysicuf methods 
Injection of magma into the upper crust results in changes 
of density and/or volume (Ryrner, 1994) and also to a loss of 
magnetization (Zlotnicki and Le Mouel, 19881, 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 nl., 
1994). These approaches are of interest in better under- 
standing of the volcanic processes, but at this stage, they 
remain experimental and are generally not routinely used in 
operations. 
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 sig- 
nals 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 vol- 
canoes. The infrared band from LANDSAT is capable of 
seeing the “hot spot” on a volcano and, therefore, is of assis- 
tance 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, i994). 
The use of radar equipment such as Synthetic Aperture 
Radar (SAR) on satellites such as ERS-I 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 exarn- 
ple of such an application for the Etna volcano in Italy. 
(d) 
(e) 
4.4 DATA REQUIREMENTS AND SOURCES 
4.4.1 Data sources 
Standard observational data reIated to volcanic activities are 
usually sent to the Scientific Event Alert Network (SEAN, 
1989) of the National Museum of Natural History of the 
Srnithsonian Institution in Washington. A monthly bulletin 
is available. These data can also be accessed by Internet. 
Recently, many research groups on volcanoes and observa- 
tories 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 microprocess- 
ing, has contributed most to the improvement of digital data 
acquisition systems. The data are stored on various media, 
such as high-density magnetic tapes, magneto-aptica1 disks 
and CD ROMs. The largest database is for the volcano- 
seismic observations. To assist in the database management, 
39 
a program named BOB has been developed at the Cascades 
Volcano Observatory. It allows for the treatment of multi- 
methods observational data and to provide graphical 
displays of the data (Murray, 1990a and 1990b). 
Technologies for monitoring and evaluating volcanoes is 
available through STEND (1996). 
4.5 
4.5.1 
* 
* 
PRACTICAL ASPECTS OF APPLYING THE 
TECHNIQUES 
The following is largely inspired by a publication “Reducing 
volcanic disasters in the 1990’s”from the IAVCEI (1990). 
Practice of hazard zoning mapping 
To estabfish volcanic hazard maps, the minimum required 
materials are: 
Topographic base maps preferably to the 150 000 scale 
or more, including updated information on roads, 
schools and hospitals. 
Air photo coverage, 
Geologic maps. 
Stratigraphic studies. 
With these, maps are drawn by hand or using comput- 
erized systems such as Geographic Information System 
(GIS) (Bonham-Carter, 1994). This allows easy updating 
and the addition of newly available, relevant data. 
on sources of data, assumptions, condi- 
The volcanic hazard maps must include: 
Primary hazards potential (including a brief description 
of each, with its range of speed and travel distance). 
Secondary hazards potential (as above). 
Areas likely to be affected (by each hazard or by combi- 
nations of hazards). 
Information 
tions under which the map will apply, date of 
preparation and expected period of applicability. 
Fumarolic 
4.5.2 Practice of monitoring 
km from the vent and, if possible, on the 
A minimum degree of continuous surveillance is required to 
detect volcanic unrest, It may inctude the following ob- 
servat ions: 
Visual: frequent observations ( M y , weekly or monthly), 
in consultation with local residents, Includes local 
reporting of felt earthquakes. 
Seismic: continuous operation of at least one seis- 
mometer (this equipment should be located not more 
than 5 
bedrock). For crude earthquake location, a minimum 
of three stations is necessary). 
Ground deformation: at GPS benchmarks, a minimum 
of two remeasurements per year initially; thereafter at a 
frequency commensurate with observed changes. 
gases, hot spring temperature and simple 
chemical analysis, a minimum of two remeasurements 
per year initially; thereafter at a frequency commensu- 
rate with observed changes. 40 Chapter 4 - Vokanic hazards 
Crisis monitoring necessitates a dense seismic network 
and an expanded geodetic monitoring activity with appro- 
priate te1emetry to a safely located observatory. 
4.6 
of the Mount Figure 4.11 - Map of the 1991 eruption 
Pinatubo volcano, Philipprnes, exhibiting ushfull, pyroclustic 
flow and lahar (mudflow) deposits (PHIVOLCS, 1992) 
MUDFLOW 
MHigh risk 
Valtey-now in which m e 
cmM be a hlgh degree of danger 
fm relatndy small ~HII frequent 
mudflows and flccds 
a Moderate nsk 
by a 
Valley-fkm ares whrh m l d be 
wvered 
the EleEtron mudfbw. in valley$ In 
as large as 
wnicn mudflom Mve been rehrnely 
frequent 
Low Risk 
Valley-W areas which Gwld be 
covered by a mudflou, as large as Ihe 
Eleetron nmw. n wlleys ~n Htrich 
mudflows have been relattvelv infre 
qwnt and areas in tlle dher valleys 
which might be subject to flooding 
whrd.l 
PRESENTATION OF HAZARD AND RISK 
ASSESSMENT MAPS 
hIanyexamp1e.s of volcanic hazard maps (e.g.,volcanic hazard 
map from the Mount Pinatubo in the Philippines, 1991, see 
Figure 4.1 1) have been published (Crandell eral, 1984). They 
are very useful if provided in an accessible and appropriate 
form, in a timely manner, to the public administration and to 
thc inhabitants of the concerned area. The Nevado del Ruiz 
volcanic crisis of 1985, which started in December 1984, 
provides an ilhstration of a process that resulted in extreme 
loss of life. The provisional hazard map was only presented 
and available on 7 October 1985,a few weeks before the major 
event which occurred on 13 November 1985. It caused a 
partial melting of the ice cap, giving rise to disastrous lahars 
that wiped out the locality of Arrnero killing more than 22 OOO 
people. The unfortunate consequences of this natural hazard 
should have been averted. Voight (1990) provides a detailed 
description of the breakdown of administrative processes 
leading to these catastrophic consequences 
hazurd 
Figure 4.12 - Example of u volcanic risk map of Mount 
Rainier map from Crandell, 1973 bused on u 
(from NRC, 1994) 
\ 
TEPHRA 
a H i g h risk 
Moderate nsk 
whzh lherc could be a bw 
of cangec to pcperty 
Areas IP +ich there could k a -1g" 
degree of danger for human [Me and 
property from asphyxlatlon. falling rock 
fragment and accumulaimn of twhra 
in sQAreas 
degree of danger to human lrfe and a 
W r a ! e degree 
from falling rock fragments and 
actumulatm of lephra 
LOW Risk 
Areas in 
of tephra, bu! 
there could be a bw nsk 
of danger to property from accurnubtion 
isk lo nrtuaKy m direct u 
~---~I.-- ...... I..... ..... ... ..... .... . . ......... ...... -./ -___ - human lrle .- 
- 
Table 4.2 - Threrrts to ly5e 
- - 
volcanic activity (after 
TEPHRA, 1995) 
Hozord 
Lava flows 
Tephra falls 
Pyroclastic flows 
and debris 
avalanches 
Gases and acid 
rains 
Lahars 
Alert stages -Alert Table 4.3 
based on the monitoring 
and the action to be 
Comprehensive risk assessrnrnt fcr natural hazards 
aridproperty based on 
I 
lnlerpretation - violent 
High 
a period of 
Years or months 
Months or weeks 
Weeks or days 
Days or hours 
taken by the public 
administration and its 
Action by Public 
eruption possible within Administration and by its 
emergency committee 
Inform all responsible 
officials. Review and 
update emergency plans 
Local to regional 
- - 
emergency committee (afier 
UNIIRO-UNESCO, 1985) 
Threat to life 
low 
CeneraHy low, 
except close to vent 
Extremely high 
In general 
Moderate 
low 
Phenomena 
observed 
Abnormal local seismic 
activity; some ground 
deformations; fumarole 
temperature increases 
Significant increase in 
local seismicity, rate of 
deformation, etc. 
Dramatic increase in 
above anomalies, locally 
felt earthquakes, mild 
eruptive activity 
Protracted seismic 
tremor, increase of 
eruptive activity 
1994; 
Access to the hazard map is not sufficient in itself as it only 
shows which area may be affected. It is also necessary to know 
the risk. It is, therefore, important to assess the environmenta1, 
structural and societal vulnerabdities (Blaikies et al., 
Blong, 1984; Cannon, 1994) in respect to the various volcanic 
phenomena previously described. In Table 4.2, TEPHRA 
(1995) enumerates in a simple manner the effects of vokanic 
activity on life and property. 
This combined assessrncnt of hazards and vulnerabili- 
ties permits the realization of risk maps (e.g. volcanic risk 
map of Mount Rainier, Washingon, USA, 1993, is shown in 
Figure 4 12) that are an important tool in defining an appro- 
priate srraregy for mitigation acnons. 
stuge 
It (Yellow) 
HI (Orange) 
1V (Red) 
4.7 RELATED MITIGATION SCHEME 
As previously mentioned the knowledge of volcanic hazard 
may be approached through two complementary avenues. 
These are through: 
Zoning maps for the iriediurn to long-term honzon, 
and 
Short-term volcano monitortng. 
All the field observations are used to model the future 
behaviour of the volcano The ongoing monitoring and 
these models will be integrated for the forecasting and the 
management of a crisis. In most cases, warnings may be 
issued to the population, as listed in Table 4.3. Consequently, 
tion from risk zones 
Threat to properties 
Extremely high 
Variable, depends on 
thickness 
Extremely high 
Moderate 
~ 
41 
~- 
. . Areas affected 
Local 
Local to regional 
Local to regional 
Local to regional 
. - _ _ _ - ~ 
Check readiness of per- 
sonnel and equipment 
for possible evacuation. 
Check stocks of rnateri- 
als and relief supplies 
Public announcement of 
possible emergency and 
measures taken to deal 
with it. Mobilization of 
personnel and equip- 
ment from possible 
evacuation. Temporary 
protective measures 
against ash fall 
Evacuation of popula- -GOVERNMENT AUTHORITIES VOLCANOLOGISTS 
42 
P 
R 
E 
C 
5 
R 
S 
I 
I 
- - 
lnwab return d part 
or whde 01 populabon 
V 
A 
C 
E 
U 
A 
T 
0 
N 
I 
R 
N 
* 
f 
E 
U - - - - 
lives are raved and, in some cases, the economical damages 
are limited to the properties of the community. 
Mitigation includes the following major activities 
(WNDRO, 1991): 
Long term planning of human settlements. 
Building codes. - Preventive information for the population. 
Emergency managcment and evacuatlon plans. 
Warnings and alerts. 
The flow chart of Figure 4.13 outlining volcanic cmcr- 
gency planning can be of help. 
Based on this knowledge and evaluation of volcanic 
hazard and vulnerability, one may build a strategy and a policy 
_- Chapter 4 - Volcanic hazards 
1 
Frgure 4.13 - Volcanic 
emergency planning 
c - - 
, 
1. 
many uncertainties in projecting location. 
2. Structural vulnerability reduction through construc- 
- - 
V 
A 
A 
T 
C 
0 
E 
U 
I 
4 
R 
E 
- _ - N 
T 
U 
N 
4 
tion code (rules): 
--- 
(UNDRO-UNESCO, 1985) 
of mitigation through planning, as shown in Figure 4.13 and 
security measures, as fisted in Table 4.3. Options for volcanic 
risk reduction maybe pursued in three directions: 
Hazard modification, valid only for lava flows. 
- diverting (cg., Etna, 1992-1993); 
- channeling; 
- cooling (e.g , Heimaey, 1973). 
These are possible but difficult to apply, as there are 
appropriate roof slope; ash- fall protection; 
- use of inflammable materials; Comprehensive risk 
3. 
by the population. 
assessment for natural haznrds 
- burying vital infrastructures such as energy and 
(Heken e f al., 1995); communication networks 
- damming; lahar protection (e.g.,Pinatubo in 1991). 
These are possible to achieve and are important for vital 
during the crisis and for rehabilitation after the ftinctions 
crisis. 
Population vulnerability reduction through changing 
the hnctional characteristics of settlements: 
- regulation and land-use planning in exposed 
areas, depending on the type of volcanic hazards. 
These have a direct bearing on risk reduction. 
Last but not least, mitigation also includes a good level 
of preparedness taking into account education and effective 
media communications. It is important to recognize the 
complexity of the problem. Successful mitigation can only 
result from multi- and transdisciplinary activities. Another 
major factor €or success is the acceptance of the approacbes 
GLOSSARY OF TERMS 
Definitkons taken from IDNDR- DHA, 1992 
Debitions taken from R W. Decker and B.B. Decker, 1992 
Aerosol:** A suspension of fine liquid or solid particles in air 
Ashflow:* Pyroclastic flow including a Liquid phase and a solid 
j7ow:* A high-density mud flow with abundant 
volcano:** A volcano that is not expected to erupt 
again; a dead volcano. 
Statement or statistical estimate of occurrence of a 
magma is stored. 
4.8 
* 
** 
phase composed mainly of ashes. 
Bomb:* See ejecta. 
Debris 
coarse-grained materials such as rocks, tree trunks, etc. 
Dome:* Lava which is too viscous to flow laterally and there- 
fore forms a dome above the eruptingvent. 
Dormant volcano:’* A volcano that is not currently erupting 
but is considered likely to do so in the future. 
Ejects:* Material ejected from a volcano, including large frag- 
ment (bombs), cindery material (scoria),pebbles (lapilli) 
and fine particles (ash). 
Explosiw iridex:* Percentage of pyroclastic ejecta among the 
total product of a volcanic eruption. 
Exrinct 
Forecast:* 
future event. This term is used with different meanings in 
different disciplines, as well as prediction. 
GIs: A geographic information system for managing spatial 
data in the form of maps, digitd images and tables of 
geographically located data items such as the results of 
hazards survey 
Huzarci.* A threatening event, or the probability of occurrence 
of a potentially damaging phenomenon within a given 
time period and area. 
Lahar* A term originating in Indonesia, desigiiating a debris 
Sow over the flank of a volcano. 
Lrwn ,flow.* Molten rock which flows down-slope from a 
volcanic vent, typically moving between a few metres to 
several tens of lulometres per hour. 
Mngma:‘ The molten matter including liquid rock and gas 
under pressure which may emerge from a volcanic vent. 
Magma Chcitnbcr:*‘ An underground reservoir in which 
43 
Mean return period:* The average tune between occurrence of 
a particular hazardous event. 
Mirigation:* Measures taken in advance of a disaster aimed at 
decreasing or eIiminating its impact on society and envi- 
ronment. 
Monitoring‘ System that permits the continuous observation, 
measurement and a valuation of the progress of a process 
or phenomenon with a view to talung corrective 
measures. 
Mud flow* The down-slope transfer of fine earth material 
mixed with water. 
Nuie ardentc* A classical expression for “Pyroclastic flow”. 
Precursor* Phenomena indicating a probabIe occurrence of 
an earthquake or a volcanic eruption. 
Predzction:* A statement of the expected time, place and 
magnitude ofa future event (for volcanic eruptions). 
Prevention:* Encompasses activities designed to provide 
permanent protection from disasters. It includes engi- 
neering and other physical protective measures, and also 
legislative rnezures controlling land use and urban plan- 
ning. 
Pyroclastic flow‘ High-density flow of solid volcanic frag- 
ments suspended in gas which flow downdope from a 
volcanic vent (at speed up to 200 h / h ) which may also 
develop from partial collapse of a vertical eruption cone, 
subdivided according to fragment composition and 
nature of flowage into: ash flow, glowing avalanche, (“nu& 
time:** The interval between eruptions on an active 
ardente”), pumice flow. 
Repose 
volcano. 
RIS~:* Expected losses (of lives, persons injured, property 
damaged, and economic activity disrupted) due to a 
particular hazard for a given area and reference period. 
Based on mathematical calculations, risk is the product of 
Tremor. 
hazard and vulnerability. 
Senmicity* The distribution of earthquake in space and time. 
hurmon~c:** Volcanic tremor that has a steady 
frequency and amplitude. 
Tremor, volcanic:** A continuous vibration of the ground, 
detectable by seismographs, that is associated with 
volcanic eruption and other subsurface volcanic activity. 
Viscosity.** A measure of resistance to flow in a liquid. 
fi‘olcanic eruption:‘ The discharge (aerially explosive) oE frag- 
mentary ejecta,lava and gd7fs from a volcanic vent 
vO/CQRO:* The mountain formed by local accumulation of 
volcanic materials around an eruptingvent. 
Vuherabhty:* Degree of loss (from O to 100 per cent) result- 
ing from a potentially damaging phenomenon. 
Zonation:* In general it is the subdivision of a geographical 
entity (country, region, etc.) into homogenous sectors 
with respect to certain criteria (for example, intensity of 
the hazard, degree of risk, same overall protection against 
a given hazard, etc.). 
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