Environmental Engineering Reference
In-Depth Information
volcanic/tectonic regions, seismic activity strongly
depends on the geological setting and thus, signifi-
cant differences in seismicity are expected. This is
clearly illustrated in the distribution of epicenters
recorded by the national earthquake information
center (neic),
http://neic.usgs.gov
, for the period
1973 to 2008 (
Fig. 1
).
seismic hazard for the canary islands is slightly
higher than that reported by the GshaP in the basis
of recent results. in this sense, recent analysis of his-
torical records revealed that earthquakes with inten-
sities of Viii Msk took place during the dikefed
volcanic eruptions in cumbre Vieja, la Palma and
the eruptions of 1704-1705 closer to Mount izaña
in Tenerife (Romero-Ruiz 1991). also, observa-
tional seismicity from volcanic rift zones worldwide
suggests the maximum magnitudes of dike-induced
earthquakes are M = 3.8 ± 0.8 (1996). The focal
mechanism of the M = 5.2 earthquake recorded in
1989 May 9 (the largest earthquake registered in the
canaries) and the aftershock distribution helped to
identify a submarine fault parallel to the eastern
coast of Tenerife (González de Vallejo et al. 2006).
hence, the activity of this fault and the effect of
volcanic activity should be included in the hazard
calculation, increasing the PGa from 0.15 m/s
2
to
0.56 m/s
2
(0.06 g) for eastern Tenerife (including el
Teide observatory) and 0.5 m/s
2
(0.05 g) for the rest
of the islands. in any case, following the conven-
tion of the GshaP, seismic hazard at the canary
islands remains at the lowest level.
The high rate of earthquake occurrence, coupled
with the potential for large events, places hawaii
among the areas of highest seismic hazard (
Figs. 1
and
2
). Three types of seismicity are found (Wyss
1988). on one hand, particularly on the island of
hawaii, earthquakes are the result of the crustal
stresses imparted by the volcanic activity, in particu-
lar in kilauea and Mauna loa volcanoes. secondly,
the largest earthquakes in hawaii have occurred
beneath the flanks of the kilauea, Mauna loa, and
hualalai volcanoes. The flanks of these volcanoes
adjust to the intrusions of magma into their adjacent
rift zones by storing compressive stresses and occa-
sionally releasing it in crustal earthquakes. largest
earthquakes recorded in hawaiian history (the 1975
M = 7.2 kalapana earthquake beneath kilauea's
south flank, and the 1868 M = 7.9 Great kau earth-
quake beneath Mauna loa's southeast flank) are
examples of these crustal earthquakes. The third type
of earthquakes corresponds to deeper mantle earth-
quakes at approximately 30-40 km depth resulting
from flexural fracture of the underlying lithosphere
in long-term geologic response to the load of the
island mass. examples of these mantle earthquakes
are the 2006 october 15 kiholo Bay earthquake
and the 1973 M = 6.2 honomu earthquake (on the
northeast coast of hawaii island).
seismic hazard is very high on the chilean sites
due to the intense seismic activity. seismicity results
from the release of stresses generated by the
subduction of the oceanic nazca plate beneath
the south american plate. The focal mechanisms
of these earthquakes indicate subduction-related
thrusting, likely on the interface between these two
plates, that is located between 35 to 50 kilometres
beneath the observatory sites.
4
Volcanic haZaRD analYsis
Two different volcanic phenomena have been con-
sidered in the analysis of the effect of near future
eruptions on astronomical sites, namely ashfall and
lava flows, since they are the most likely volcanic
hazards affecting all sites, either at short (years) or
medium (decades) terms.
4.1
Lava flow hazard analysis
in the case of the chilean sites of Paranal and cerro
Ventarrones, the closest volcanoes of the central
andes are at least 200 km distant, being significa-
tive longer than the typical length of lava flows in
the region (30 km). Moreover, volcanic activity at
the observatory sites ceased more than 20 million
years ago. hence, hazard due to lava flows is negli-
gible at Paranal and cerro Ventarrones.
The canarian site of Roque de los Muchachos
is emplaced atop the extinct Taburiente volcano,
being approximately 1000 metres above and more
than 15 kilometres distant from Montaña Que-
mada, the closest eruptive vent of cumbre Vieja
volcano (Romero-Ruiz 1991). in this sense, hazard
due to lava flows is also negligible at Roque de los
Muchachos observatory. el Teide observatory is
emplaced atop Mount izaña, where volcanic activ-
ity ceased more than 300 ka. however, the observa-
tory is closer to el Teide-Pico Viejo complex, where
there has been an intense volcanic activity in the last
150 ka, whereas several effusive basaltic eruptions
took place about 35 ka and in the XViii century
within 2 and 10 kilometres from the observatory,
protect the observatory from lava flows originated
within the caldera area, while local topography and
the altitude of the observatory makes the hazard to
lava flows from the ne ridge system negligible.
There is not lava flow hazard at Mauna kea
observatory due to the eruptive activity of other
volcanoes such as Mauna loa, kilauea or hualalai
volcanoes, as the result of the topographic protec-
tion given by the altitude the observatory is located,
above 4100 metres above sea level. however, Mauna
kea is still an active volcano which last erupted
