Geoscience Reference
In-Depth Information
The heightened volcanic activity of the last four
centuries indicates that the Earth may be witnessing a
period of increased volcanism not seen for 10 000 years.
to 1000 km away from earthquake epicenters. The
causes are due to static stress changes near the epicen-
ter of an earthquake, stress changes associated with the
slow relaxation of the lower crust, or dynamic stresses
triggered by the earthquake. Static stress changes are
caused by changes in pressure in a magma body lying
close to an earthquake. The change may squeeze the
magma upward like toothpaste from a toothpaste tube
or enhance melting adjacent to the magma. More
likely, the stress change may trigger the formation of
bubbles in the magma and the freeing of conduits
linking the magma chamber to the surface. These
processes probably caused Mt Fuji in Japan to erupt in
1707, one month after an 8.2 magnitude earthquake;
and Mt Pinatubo to erupt in 1991 following a 7.7 mag-
nitude earthquake 100 km away. It is also possible
that inflation of the magma chamber may trigger
earthquakes that positively feed back on the inflation
process. This pairing of events within ten years of each
other seems to be characteristic of major eruptions
of Vesuvius. Earthquake swarms over periods of
7-50 years are also related to viscous relaxation of the
lower crust. After an earthquake, relaxation of shear
stresses in the upper 15-20 km of crust triggers a
concomitant relaxation in the underlying plastic crust.
This effect induces volcanic eruptions either immedi-
ately or 30-35 years afterwards up to 1000 km from
the earthquake epicenter.
The most likely process causing volcanic activity
after earthquakes takes place in the magma. Seismic
waves can dislodge bubbles held by surface tension on
the floor and walls of a magma chamber. The same
seismic waves can also increase bubble size. During
the dilating phase of a seismic wave, bubbles grow as
gas escapes under lower pressure from the magma.
During compression, the gases dissolve back into the
magma. However, periods of dilation are longer than
those of compression. Hence, as the seismic waves
pass through magma chambers, the bubbles grow in
size. Within a few hours, the 28 June 1992 Landers,
California, earthquake triggered 14 other earthquakes
up to 1200 km away. Many of these earthquakes
occurred in the southern part of the Long Valley
Caldera where it is estimated that the Landers earth-
quake increased pressure by 7-10 MPa. Finally,
earthquakes may dislodge crystals that have precipi-
tated out of the magma body. Because these crystals
are denser than the magma, they settle downward,
forcing gas-rich magma upward. This latter magma is
Earthquakes (Whittow, 1980)
There is some evidence to suggest that regional earth-
quake activity is clustered. Figure 9.11 plots the mag-
nitude of earthquakes based on the Richter scale in
Japan since 684, the Mediterranean for the period
since 1900, and the United States since 1857. Japan,
which is seismically very active, has experienced a
series of high-magnitude earthquakes centered on the
eighth, sixteenth and twentieth centuries. In the nine-
teenth century, earthquakes appeared to cluster every
50 years. The most seismic period in the record
occurred after the Second World War, but the most
destructive earthquake in Japan in recent times
occurred in 1923 at Tokyo. The Kobe earthquake of
17 January 1995 may simply have been part of this
enhanced twentieth century activity. In the Mediter-
ranean, earthquakes occur more regularly (Figure
9.11b). Since 1920, there have been four clusters of
earthquakes, centered around 1930, 1941, 1953-1956,
and 1977. Except for Turkey, the Mediterranean
region has been quiescent in the latter part of the
twentieth century. In North America, clustering of
earthquakes has also occurred along the western
continental margin. The most intense phase of seismic
events has occurred since 1980. Other periods of
clustering occurred around 1906 - just before the
San Francisco earthquake - 1930, 1951, and 1964.
However, the clustering is not nearly so pronounced as
in the Mediterranean region. If clustering of earth-
quakes is real, then volcanic eruptions and earthquake
activity should also be coherent because they have the
same underlying geophysical causes.
Interaction between earthquakes and volcanoes
(Hill et al., 2002)
There is a high occurrence of volcano activity immedi-
ately after earthquakes greater than 6.5 in magnitude.
For example on 29 November 1975, Kilauea volcano
erupted 30 minutes after a 7.5 magnitude earthquake
on its southern flank. Cordon Caulle volcano in central
Chile erupted two days after the great 9.5 M w magni-
tude Chilean earthquake of 22 May 1960. While these
eruptions occur within days of an earthquake, the
effect may extend to hundreds of years afterwards up
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