Geoscience Reference
In-Depth Information
epicenter. This stress is made up of two components,
one parallel to the plane of a fault, and the other per-
pendicular to it. If shear stress along the fault exceeds
frictional resistance, or the stress pressing against the
fault from the sides is relaxed, the crust on both sides
of the fault will slip, producing an earthquake. The
total stress field is known as Coulomb stress. During a
major earthquake, Coulomb stress can alter by less
than 3 bars. The stress that appears at the epicenter of
an earthquake on a fault line simply shifts elsewhere
along the fault because stress must be conserved. The
best example of this phenomenon occurred in Turkey.
Here, the 17 August 1999 earthquake at Izmit that
took 25 000 lives was the twelfth to occur along
the North Anatolian fault since 1939. Each event
simply shifted the stress along the fault, triggering an
earthquake at another location a few years later. The
Izmit earthquake reduced stress along 50 km of fault
line, shifting it to Düzce 100 km to the east, and
towards Istanbul in the west. A magnitude 7.1 earth-
quake subsequently affected Düzce in November
1999. Fortunately, this stress shift was monitored and
publicized. Engineers pressured authorities in Düzce
to close schools two months beforehand. Many of these
schools were destroyed in the November earthquake.
The stress build-up towards Istanbul remains and
experts predict that the annual probability of an
earthquake in the capital has now risen from
1.9 per cent to 4.2 per cent. The probability of an
earthquake occurring in this city before 2030 has risen
from 48 per cent to 62 per cent.
Stress mapping has also been used to explain the
pattern of earthquakes that followed the 28 June 1992
Landers earthquake. Records show that there is a
67 per cent chance of another large earthquake along
the San Andreas Fault within a day of a 7.3 magnitude
earthquake. The Landers earthquake was of this
magnitude, and within three hours, a 6.5 magnitude
earthquake occurred at Big Bear, 40 km to the south-
west. Coulomb stress had not only shifted along
the San Andreas Fault but also laterally onto a parallel
fault line. Seven years later, in 1999, a 7.1 magnitude
earthquake occurred at Hector Mine, 40 km to the
north of Landers. Swarms of minor earthquakes that
followed the Landers earthquake also occupied areas
of higher stress. Worldwide, 61 per cent of aftershocks
greater than magnitude 5 - that have occurred within
250 km of the epicenter of a magnitude 7 or greater
earthquake - correspond to areas where Coulomb
stress has increased. Seismicity never completely shuts
down around the center of an earthquake. Earthquake
activity simply increases away from the epicenter. This
may explain why the number of earthquakes in the
San Francisco area decreased after 1906 relative to the
preceding 60 years. Earthquake activity has simply
shifted elsewhere along the San Andreas Fault. The
Loma Prieta earthquake of 1989 may have been
the beginning of a return to the previous rate of activity
in the San Francisco Bay area.
Pred iction of volcanoes
(Decker, 1976; Tazieff & Sabroux 1983; Smith, 1985)
While the first outburst of volcanic activity can now be
predicted, it is at present almost impossible to predict
the direction or intensity of activity that follows. To
date, only a handful of eruptions have been forecast,
the earliest being the renewed activity of Kilauea,
Hawaii, in November 1959. Most of these predictions
so far have been for eruptions that involved fluid
magma and did not threaten life. Volcanic eruptions
that consist of viscous magmas, or that become explo-
sive, still cannot be predicted. For example, while the
renewed activity of Nevado del Ruiz, Colombia, in
November 1985, was noted, the timing of the final
eruption could not be predicted, and 20 000 people
lost their lives in the ensuing heated mudflows. There
have also been some notable and costly false alarms for
these types of volcanoes. On 12 April 1976, the resi-
dents of Guadeloupe in the West Indies were told
to evacuate because of an imminent eruption of
La Soufrière volcano. Over 75 000 people heeded the
warning and were evacuated. They waited for 15 weeks
until the volcano finally produced a small, very
harmless eruption on 8 July. It cost $US500 million to
maintain the evacuation and brought economic ruin to
many. Such predictions, while soundly based, only tend
to weaken the credibility of subsequent warnings.
The techniques for predicting volcanic eruptions or
activity are just as varied as, but more technical than,
those for predicting earthquakes. Precursors for volca-
noes group into the following four categories: land
deformation, seismic activity, geomagnetic and geo-
electric effects, and gases. Ground deformations around
volcanoes are due to the subterranean movements of
molten magma. These movements can be vertical,
lateral, or oblique. They manifest themselves at the
surface by tilting of the Earth's surface, which can be
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