Environmental Engineering Reference
In-Depth Information
3600 years ago and hence, lava flow hazard is low
but larger than in chile and the canaries.
Following the convention defined by the United
states Geological survey (UsGs), the level of haz-
ard for lava flows has been scaled from 1 to 9, being 1
the most hazardous (Mullineaux & Peterson).
The UsGs assigns a hazard level 7 to the area of
Mauna kea observatory, meaning that at least 20%
of this area has been covered by lavas in the last
observatory was set to 8, since the area covered by
lava flows in the last 10000 years is significantly
lower than 20%, Both the chilean sites and Roque
de los Muchachos observatory in the island of la
Palma have a hazard level of 9, since there have not
been volcanic activity in the last 60 ka.
explosivity index for eruptions in the island of la
Palma has been low (Vei < 3), although eruptive
columns up to 4 km high have been recorded in
historical times when the uprising magma encoun-
tered groundwater, producing phreatomagmatic
explosions (Romero-Ruiz 1991). in the case of the
island of Tenerife, explosive eruptions might occur
in the central part of the island, associated to el
Teide-Pico Viejo strato-volcano and its peripheral
vents. last explosive event took place 2 ka and
corresponded to the Vei = 4 sub-plinian eruption
of montana Blanca volcano (ablay et al. 1995).
Due to the complex nature of the central volcan-
ism in Tenerife, other types of explosive eruptions
have occurred and might occur in the future, namely
phreatomagmatic and/or violent strombolian erup-
tions. low intensity (Vei < 3) eruptions have
occurred in the vicinity of el Teide observatory, and
hence similar future eruptions should be considered
in the hazard analysis associated to tephra fall.
The explosivity index for hawaiian eruptions
has also been low (Vei < 3), but with chances of
phreatomagmatic eruptions like those reported
for the kilauea volcano in historical times, namely
600 years ago, in 1790 and May 1924 (McPhie
et al. 1990) and in prehistoric times (2700, 2100
and 1100 years ago). Moreover, other widespread
ash units point to larger phreatomagmatic erup-
tions at kilauea and Mauna kea, likely Vei = 3
(Mullineaux et al. 1987).
Unlike the hazard analysis for lava flows, there
is not established convention to define the haz-
ard due to ash/tephra fall. hence, a numerical
simulator of volcanic ashfall, TePhRa2 (connor
et al. 2001), was used to analyze the extent of ash
deposits at each site, considering the typical wind
conditions and the Vei of the volcanoes located
in the surroundings of the sites. in this sense, we
supposed two different eruptive events (Vei = 4
and 6) for the lascar volcano in chile, the closest
were also considered in Tenerife, a Vei = 4 erup-
tion in Montana Blanca and a Vei = 2 eruption on
the nW ridge. in the case of la Palma, two cases
were modeled, a Vei = 2 and a Vei = 3 eruptions
located in cumbre Vieja volcano. Finally, six events
were considered in the island of hawaii, namely
Vei = 2,3 eruptions at hualalai, Mauna loa and
kilauea volcanoes.
Teide observatory might be affected by deposition
of ash (exceeding 1 cm) from a Vei = 4 eruption
of el Teide-Pico Viejo complex due to the pre-
vailing wind conditions. a Montana Blanca type
eruption affecting the observatory would be fol-
lowed by years of contamination of the site by
reworked volcanic fine ash blown about by the
wind (Mullineaux et al. 1987). a Vei = 2 eruption
4.2
Volcanic ash hazard analysis
ash/tephra fall poses the widest-ranging direct
hazard from volcanic eruptions. Vast areas (10
4
to
10
5
km
2
) have been covered by more than 1 cm of
tephra during some large eruptions, whereas fine
ash can be carried out over areas of continental
size (Gardeweg 1996). Burial by tephra could col-
lapse roofs, break power and communication lines,
whereas suspension of fine-grained particles in
air affects visibility, could damage unprotected
machinery, cause short circuits in electrical facili-
ties and affect communications.
The spatial extend of tephra fall depends on two
factors, namely the strength and direction of the
wind and the explosivity of the eruption (height
of the eruptive column). Wind speeds at differ-
ent heights above the selected sites were collected
from the (nceP/ncaR) Reanalysis database of
the national center for environmental Prediction/
national center for atmospheric Research. The
second major factor affecting the extent of ashfall
is the explosivity of the eruption, expressed as the
Volcanic explosivity index (Vei). The explosivity
index for eruptions in the central andes that have
been measured in historical times ranges from the
Vei = 4 sub-plinian eruption of lascar volcano
in 1967 or 1993 (Gardeweg 1996) to the Vei = 6
eruption of huaynaputina volcano in February
1600, corresponding to the largest volcanic explo-
sion in south america in historic times. The
Table 2. level of hazard by lava flows that was calcu-
lated following the UsGs convention.
observatory
Zone
last eruption
Mauna kea
7
3.6 ka
Paranal and Ventarrones
9
>1 Ma
Roque de los Muchachos
9
400 ka
Teide
8
300 ka
