well as
variations in the local pull of gravity and other physical properties.
Systematic accelerations in the rates of such precursory behaviour
may allow scientists to define predictive windows that allow
the timing of a forthcoming eruption to be constrained. Brief
explanations of four of the principal monitoring methods are
provided below.
Associated terms:
Seismicity
Seismic crises
Tectonic earthquake
Magmatic earthquake
Tremor
Ground shaking
Seismometer
Seismogram
The cracking caused by rising magma triggers earthquakes more
frequently than is normal when a volcano is quiet. Such periods
of elevated earthquake activity are called seismic
crises. In the build-up to an eruption, most earthquakes
can only be detected by instruments, but some may be strong
enough to be felt by animals or humans. Earthquake activity
is monitored using a seismometer,
which records the resulting ground shaking electronically or
as a printed record on a seismogram.
Seismic crises occur before and during eruptions. They can also,
however, occur without a following eruption. Swarms of earthquakes
occurred on Montserrat during the 1930s and 1960s but no eruptions
followed.
Monitoring scientists can distinguish different types of earthquake
by their signatures. For example, tectonic
earthquakes are caused by magma fracturing rock, while
magmatic quakes and tremor
result from the vibration of fractures as magma and gas pass
through them.
Although generally weak, volcanic earthquakes may sometimes
be strong enough to damage buildings and open cracks in the
ground.
Sometimes the terms seismicity and seismic crises are used in
relation to events detected by instruments, while earthquake
is reserved for ground movements strong enough to be felt by
humans. |
Associated terms:
Ground swelling
Surface deformation
Tiltmeter
Extensiometer EDM GPS SAR
As magma approaches the surface it has to make space for itself.
This has the effect of causing the surface to bulge upwards,
although usually the swelling is so small that it can only
be detected by instruments. Typically, the degree of swelling
will range from a few centimetres to tens of centimetres,
covering an area of several to several hundred square kilometers.
Occasionally, swelling may be great enough, close to the volcano,
to cause damage to buildings.
Several instruments are available for monitoring the various
aspects of ground swelling. Some are ground based or airborne,
while others are housed in orbiting satellites. These include:
Tiltmeters, typically
lodged in a borehole, measure tilting of the ground surface.
Electronic Distance Meters
(EDM) record changes in the distances between known positions
on the volcano.
Global Positioning System
(GPS) uses receivers that detect radio signals from satellites
to measure the relative positions of known points on the ground.
Extensiometers measure
directly any stretching of the ground surface.
Synthetic Aperture Radar
(SAR) reveals patterns of surface deformation by comparing
sequences of aerial or satellite radar images of the volcano.
Fig 9. GPS monitoring (Montserrat)
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