Natural Hazards: Causes and Effects - Disaster Management Center ...

then recognized, but today an increase in the frequency of tremors is one of the most reliable
methods of volcanic prediction. A network of seismographs can not only detect tremors but also
determine the position and depth of their origin—which is important when tracing the movement
of magma up through the core of a volcano. Eruptions of Japan’s Asama volcano are predicted
on the frequency build-up of tremors within 1,000 meters (3,300 miles) laterally or vertically of
the vent. 14
Animal Behavior before Volcanic Eruptions
On several occasions animals on and near a volcano have been observed to be uneasy for
several days before an eruption. Thus, cattle started to move off the Arenal volcano two weeks
before the outbreak in 1968, although persons in the area felt the first earthquakes only two
days before the outbreak. About four days before the outbreak at Kilauea in 1955, dogs
became uneasy and started digging in the ground and sniffing in holes as though in pursuit of
some burrowing animal. Observers could detect no odor of gas or anything else to cause the
dogs’ uneasiness. However, a seismograph a few kilometers away was recording hundreds of
earthquakes too small for people to feel, and these may have been disturbing the dogs.
Disturbance of animals should be taken into account, along with any other available signs, in
attempting to forecast eruptions.
Instrumentation
The present tendency in observing volcanoes is toward elaborate and expensive
instrumentation. Where this is not economically feasible, much can still be done with simpler
means. Mechanical seismographs of moderate magnification are relatively inexpensive both in
capital and in manning costs. These may often be adequate. Simple water-tube tiltmeters also
are cheap; measuring the opening and closing of cracks in the summit region gives much the
same information as geodometer measurements. Fumarole temperatures can be measured by
an observer with an ordinary maximum thermometer. Telemetry saves footwork, but it is not
indispensable. 15
Evacuation
Timely evacuation from volcanic eruptions can eliminate the hazard to human life, even though
destruction of buildings may still have to be accepted as inevitable. The real danger is from the
more violent types of volcano in the less well-developed parts of the world where monitoring
cannot be comprehensive. In this situation danger zoning is probably the only available
defense—and even this is lamentably inadequate in many parts of the world. Programs of
hazard zoning on many volcanoes in populated areas could be a worthwhile planning aid to be
carried out well before the first rumblings of an impending eruption.
Mitigation
Certain characteristics of relatively thin and fluid basaltic lava flows make it possible to control
their spread and at least partially to direct their courses. Well-established pahoehoe flows are
fed by movement of the fluid lava through pipe-like lava tubes. If the tube can be clogged and
its roof broken open, it may be possible to cause the liquid lava to spill out high on the flank of
the volcano where it will do little or no harm. The reduction of the supply to the flow front stops
the lava’s advance toward some threatened area, such as a city. The most practical means of
breaking open the tube roof in many cases is by aerial bombardment or explosives. Clogging of
the tube is partly by debris from the roof, but violent stirring of the liquid in the tube by bursting
bombs may result in transforming it into more viscous lava, which moves through the tube less
readily and may even clog it completely.