Geoscience Reference
In-Depth Information
local earthquake activity. The Earth's ability to
transmit currents, or its
electric resistivity
, can fore-
shadow earthquake activity. Changes in resistivity are
monitored by sending electrical pulses through the
ground and measuring their strength some distance
away. Decreases in resistivity of 10-15 per cent have
been detected several months before major earth-
quakes, while stepped increases have been detected
several hours in advance, even for earthquakes some
distance from the monitoring site. Electrical currents,
termed
telluric
currents (or Earth currents), are also
continually flowing through the Earth's crust. Changes
in the order of 2 mV have been observed hours before
small earthquakes.
Many minerals such as quartz have
piezoelectric
properties. When stress is applied to rocks containing
these minerals an electrical current is generated.
Changes in the electrical potential between two rods
bored into the crust can be used to measure the mag-
nitude of stress building up along a fault line before an
earthquake. The best example of this technique is the
controversial VAN method - named after the initials of
Professors P. Varotsos, K. Alexopoulos and K. Nomicos
who developed it to forecast earthquakes in Greece.
The method allegedly predicts the magnitude and
epicenter of earthquakes, greater than M
s
5, within an
error of 0.7 units and 100 km respectively, several
hours to days in advance.
Groundwater fluctuations in wells can foreshadow
earthquakes with magnitudes greater than 5 on the
Richter scale, 0.5-10.0 days before the event. This
technique is used extensively now as a forecasting tool
in China. The longer the existence of anomalous water
levels, and the wider the anomaly, the larger the
impending earthquake. Changes in groundwater
may be related to tectonic strain induced by crustal
movements, or to the dilatancy phenomenon. A
feature associated with changes in groundwater level is
the change in the water's radon content. Radon, espe-
cially in mineral waters, will increase exponentially
years before an earthquake, and then decrease rapidly
to previous levels after the event. The increase in
radon may reflect increased water movement through
cracks opened by dilatancy, permitting more of this
isotope to be incorporated into groundwaters.
There are also natural indicators of impending
earthquakes. Catfish in Japan become very active
before earthquakes and even jump out of the water.
In addition, local fish catches around Japan increase
significantly just before earthquake activity. Ground
animals will leave tunnels, stabled animals will become
restless, and dogs will start barking hours before an
earthquake. Snakes, weasels, and worms deserted the
ancient city of Helice, Greece, days before it was com-
pletely destroyed by an earthquake in 373 BC. Before
the Lisbon earthquake of 1755, worms crawled from
the ground and covered the surface. The evacuation of
the city of Haicheng, China, in February 1975 - just
before it was destroyed by an earthquake - followed
observations of strange animal behavior: geese flew
into trees, pigs became aggressive, chickens refused to
enter coops, and rats appeared drunk. In the San Fran-
cisco zoo, animals have been found to group together
by species about half-an-hour before an earthquake.
Animal behavior is now being monitored daily in zoos
and marine lands around San Francisco to incorporate
animal behavior into an earthquake prediction system.
It is not exactly clear why animal behavior is
prognostic of earthquakes. A response to long wave
electromagnetic radiation has been given as one reason
for animal panic. Animals may also be supersensitive
to vibrations and
ultrasound
generated by small
earthquakes preceding the main event. However,
reptiles and birds that exhibit unusual behavior do not
hear ultrasound. In addition, ultrasound from deep
earthquakes is absorbed by rock, leaving only very low
frequency sound to reach the surface. In this case,
humans as well as animals should hear the sounds.
Another reason for odd animal behavior may be an
animal's extreme sensitivity to the smell of methane,
known to leak from the ground during tremors.
However, many burrowing animals that continually
tolerate natural methane as they dig also panic before
earthquakes. Finally, a barrage of
electrostatic
particles
emanating from the ground may precede tremors.
Animals with fur or feathers are very sensitive to
electrostatic charges, and the sudden increase in
quantity of such particles may simply irritate them to
the point of panic or flight. Whatever the cause, scien-
tists in China and other parts of the world now view the
monitoring of animal behavior as the best means of
forecasting the occurrence of major earthquakes.
Unusual weather changes may also be a harbinger
of impending disaster. Mists close to the ground, as
well as glowing skies, have been reported before
earthquakes. In some cases, the light appears to
emanate from the ground as a flash of flame. While this
luminosity has often been dismissed, there is a new
