Geoscience Reference
In-Depth Information
building of the Hoover Dam, which impounded Lake
Mead on the Colorado River. The area was not noted
for earthquake activity before the dam was constructed.
Since that time, however, over 1000 earthquakes
have been generated with enough strength to be felt by
local inhabitants. Since then, this effect has been noted
following the construction of 10-15 other reservoirs
worldwide including the Kariba Dam on the Zambezi
River in southern Africa and the Koyna Dam near
Mumbai, India. It should be noted that not all artificial
reservoirs have generated earthquakes, even when
they have been located in active seismic zones.
However, enough moderate earthquakes registering
5-6.5 on the Richter scale have been produced at
Koyna, India, to cause the loss of 177 lives. It appears
that reservoir water penetrates the underlying
bedrock, reducing effective frictional resistance along
fractures, and permitting slippage significant enough
to generate earthquakes. Mining, water and oil extrac-
tion, and waste-fluid disposal underground have also
resulted in local seismic activity. For example, waste-
water from chemical warfare manufacturing was
pumped underground at Denver, Colorado, between
1962 and 1965, resulting in measurable tremors.
However, the effects of these latter operations appear
minor compared to those produced so far by reservoir
construction. Finally, there is a statistical correlation
between the wettest time of year and eruptions of
Yasur, Mt Etna, Mt St Helens, and Soufrière on the
island of Montserrat. Rain immediately preceded
recent eruptions of the latter volcano on three differ-
ent occasions. Rain seeps into the volcano, and upon
contact with the magma chamber, turns to steam
causing a blast.
activity over decades or centuries, and clustering of
events.
Over geological time, periods of volcanism have not
occurred as random events. Over the past 250 million
years, volcanic activity appears to be enhanced every
33 million years with a minor peak occurring every
16.5 million years. The greatest peak in activity
occurred 60-65 million years ago, corresponding to
the formation of the Deccan Plateau in India, and
the Cretaceous mass extinction that wiped out the
dinosaurs. Periods of volcanic activity correlate with
other mass extinctions, major reversals in the Earth's
magnetic field, meteor or comet impact-cratering and
other terrestrial processes. The cause of these cata-
strophic events is related to the passage of the solar
system through the
galactic disc
. Our galaxy - the
Milky Way - is a flattened spiral of stars that, side-on,
looks like a plate. The solar system rotates relative to
the center of the galaxy, passing from one side of the
disc to the other. A complete circuit takes 33 million
years, such that the solar system passes through the
axis of the galaxy every 16.5 million years. At present,
the solar system is passing through the galactic disc and
volcanism has been increasing over the last 2 million
years.
Extended periods of volcanism represent a major
change in the
isothermal
, circulatory, and convective
behavior of the mantle. As a result, major plate move-
ments take place, leading to faster crustal spreading
rates, to broadening of oceanic ridges and to sea level
rises. More importantly, increased volcanic dust during
major periods of volcanic activity screens out enough
solar radiation, reducing
photosynthesis
and disturbing
the food web sufficiently, to cause the disappearance
of many land- and ocean-based species. Astronomical
periodicities at the galactic level are sufficient to account
for most periods of volcanism on the Earth, concomitant
with increased comet impacts,
magnetic reversals
,
climatic change and, ultimately, mass extinctions. Vol-
canism must be one of the most far-reaching and
profound processes affecting the Earth geologically.
Blong (1984) has summarized some interesting data
on the frequency of volcanic eruptions over the past
10 000 years. Over the last 500 years, individual volca-
noes have erupted at the median rate of once every
220 years. The number of eruptions per century, when
plotted on log probability paper (Figure 9.9), forms a
straight-line relationship similar to the frequency of
occurrence of most natural hazard events. About
PREDICTION OF EARTH-
QUAKES AND VOLCANOES
Clustering of volcanic and seismic
even
ts
Volcanoes
(Gribbin, 1978; Lamb, 1972, 1982; Blong, 1984; Pandey
& Negi, 1987)
The prediction of earthquakes and volcanoes can be
broken down into two time spans: long-term prediction
beyond a period of several years, and short-term pre-
diction from a couple of years to several days or hours
before the event. Long-term prediction of volcano and
earthquake activity involves the delineation of cycles in
