Geoscience Reference
In-Depth Information
theory that suggests charge separation occurs deep in
the Earth's crust under the pressures that build up
before earthquakes. Igneous rocks contain minerals
with paired oxygen atoms. Under stress, these break,
with a negative ion remaining trapped in the crystal
lattice of the mineral and the positive charge flowing to
the Earth's surface. The positive charge can combine
with an electron to give off infra-red light. Prior to the
devastating 26 January 2001 earthquake in Gujarat,
India, increased infra-red emissions were detected by
satellite around the quake's epicenter. Otherwise, the
charge builds up to 400 kV over a short distance at
which point it ionizes air causing a luminous plasma -
known as earthquake lights - to form. Alternatively,
laboratory studies have shown that rock crushed under
pressure gives off luminescence.
The prediction of earthquakes is still not an exact
science. China, because it faces such large death tolls
from earthquakes, has developed the most advanced
predictive techniques. Five major earthquakes struck
China in the early- to mid-1970s. All but one was pre-
dicted early enough to permit orderly evacuation of
people from buildings with minimal death or injury.
The exception, the Tangshan earthquake of 28 July
1976, took the lives of 250 000 people - the highest
number of lives lost in an earthquake in two centuries.
While the early warning signs of that earthquake were
observed, they were too faint to arouse concern.
two. The movement of a single grain may trigger
another grain to move or the collapse of half the sand
pile - or anything in between. This is characteristic of
earthquakes, which are often preceded by swarms of
smaller earthquakes, but you can never predict which
one will trigger the 'big' one.
Most of the short-term precursors have only been
identified as such following an earthquake. These
alleged signals vary greatly from earthquake to
earthquake and rarely can be detected as a regional
phenomenon surrounding the epicenter of an earth-
quake. Crucially, the track record at predicting
earthquakes has failed dismally. Even the success of
predictions for the 1975 Haicheng earthquake can be
challenged. Official records now indicate that 1328
were killed and 16 980 injured. For every precursor
event linked to an earthquake, there are hundreds that
have produced false alarms. For example, the crust
around Palmdale in Los Angeles swelled rapidly by as
much as 20 cm in 3 years in the 1970s. After the expen-
diture of millions of dollars on monitoring, the bulge
was found to be an artefact of measurement error. The
area is still awaiting an earthquake. The VAN method
using geo-electric activity has also failed at predicting
earthquakes in Greece, because the signals being
measured are indiscernible from background noise.
Each false prediction is just another statistical 'nail in
the coffin' of earthquake prediction.
The seismic gap concept is flawed. Many earth-
quakes occur in swarms with the leading earthquake
not necessarily being the largest one. Earthquakes
should be viewed as chaotic geophysical phenomena,
generated by a spectrum of seismic waves with varying
amplitudes and periods. If this is the case, then
earthquakes behave as white noise. One of the aspects
of such systems is that earthquakes recur at the same
location rather than in areas that are more quiescent.
For example, earthquake activity along the western
North American Plate boundary increased in the years
leading up to the San Francisco earthquake of 1906.
A predominance of magnitude 5 earthquakes in a
region usually indicates, several months in advance,
the imminent occurrence of a much larger earthquake.
These smaller earthquakes often ring the future
epicenter, forming what is known as a Mogi doughnut ,
named after the discoverer of the effect.
Some of these fine details about the spatial structure
of earthquake swarms are being teased out. Earth-
quakes are produced by changes in stress at an
Randomness versus clustering
(Lighthill, 1996; Bak, 1997; Geller et al., 1997;
Stein, 2003)
Foolproof prediction of large earthquakes is still not
possible. The consensus today is that earthquakes
occur randomly. If the cumulative frequencies of
earthquakes are plotted against magnitude for various
depths through the Earth's crust, the resulting curves
have a similar shape following a power law. This is
characteristic of systems that are self-organized and
contain critical thresholds. For example, if you build
up a pile of dry sand, it will reach a critical slope angle
where failure will occur. The scale of this failure is
random, from individual grains to massive sections of
the sand pile. Massive failure occurs much less fre-
quently than does the movement of individual grains.
If the cumulative frequency of failure is plotted against
volume, the resulting curve fits a power curve. More
importantly, small events often trigger much larger
ones, but there is no direct relationship between the
 
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