Geoscience Reference
In-Depth Information
than around the Pacific Rim. Here, the repeat time
of an earthquake becomes much larger. This has
significance for areas in the middle of plates that have
experienced isolated earthquakes in historical times.
For instance, the New Madrid earthquakes in the
Mississippi Valley in 1811-1812, and the Charleston,
South Carolina, earthquake in 1886 appear to be
anomalous (Figure 9.2). The concept of earthquake
repeat-cycles would indicate that these regions are
highly unlikely to experience a similar intensity
earthquake for hundreds, if not thousands, of years.
There are also active fault lines where little or no major
earthquake activity has been recorded historically.
For instance, the Wasatch fault, which passes through
Salt Lake City, appears to be more active than others in
the region. Yet, historically it has never experienced
a major earthquake. For these types of faults, major
earthquakes may occur at 500-1000-year frequencies.
natural phenomena. Land deformation studies are
based on the concept that the strain energy building up
in the Earth's crust will evince itself in minor lateral
or vertical distortions at the Earth's surface. Much of
the Earth's surface has been surveyed with bench-
marks or triangulation stations. In some countries,
these stations have been accurately surveyed to form
an extensive triangulation network. By resurveying
stations within this network every decade, it is possible
to detect gross movement, ranging from centimeters to
meters, in the Earth's crust. It is also possible to relate
these stations to tide gauges, to look for land move-
ments relative to sea level in coastal areas. On a smaller
scale, it is now possible with infra-red and laser survey
equipment to mount survey stations on opposite sides
of active fault zones and to measure earth movements
as small as 1 mm over distances of several kilometers.
This has been carried out along key sections of the
San Andreas Fault. Points showing rapid changes in
earth movement along the fault may signal future
earthquake activity nearby. Crustal deformations can
also be measured using tiltmeters and strain gauges.
Increases in strain signal earthquake activity within
months, while rapid increases in tilting of the Earth's
crust (called
tilt steps
) give warnings of activity within
hours.
On a smaller timescale,
foreshocks
or
microseisms
also indicate an impending earthquake. However, the
nature of the warning is paradoxical and depends upon
the area. Some earthquake zones give foreshocks days
to hours before the main shock, while others evince a
decrease in microseismic activity before the shock.
Changes in seismic wave velocities through rock can
also be used to foreshadow an earthquake. Strain
changes the velocity at which a seismic wave will travel
through the ground. By measuring the velocities of
human-induced shocks across known earthquake
zones, it is possible to forecast major earthquakes years
in advance. Decreases in strain of up to 20 per cent
have been measured for compression-type waves
before earthquakes.
There are also geomagnetic and geo-electric pre-
cursors. It has already been pointed out that the
Earth's geomagnetic activity varies with the sunspot
cycle, a relationship that may be linked to earthquake
activity. Geomagnetic activity also varies spatially and
temporally over the surface of the Earth. Anomalous
changes in geomagnetic activity in the order of 4-20
gammas have been measured up to ten years before
Short-term prediction of seismic activity
(Rikitake, 1976; Ward, 1976; Simpson & Richards, 1981;
Kisslinger & Rikitake, 1985; Lighthill, 1996)
Of all natural hazards, earthquakes generate the largest
and longest range of associated phenomena that can be
used to foreshadow the timing of the impending event.
Research into earthquake prediction has been carried
out since the late nineteenth century. In 1891, the
Japanese government set up an earthquake investiga-
tion committee, which collected historical records of
earthquakes in Japan as far back as 416 AD. In 1925,
the Earthquake Research Institute was established at
Tokyo University to give earthquake research a scien-
tific impetus. Following Japan's economic recovery
in the 1950s, earthquake prediction was intensified. In
China, where historical records go back 3000 years, a
major effort at identifying earthquake precursors was
initiated after 1950. On 5 February 1975, the city of
Haicheng became the first major inhabited area to
be evacuated before the occurrence of a significantly
large earthquake. In the months leading up to the
earthquake, ground tilting was noticed together with
unusual animal behavior and water spurting from the
ground. The city was evacuated 12 hours before
the earthquake struck. The earthquake measured 7.2
on the Richter scale and, despite the destruction of
90 per cent of the city, only a few lives were lost.
Precursors of earthquakes group into the following
five categories: land deformation, seismic activity, geo-
magnetic and
geo-electric activity
, ground water, and
