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

ocean becomes a devastating 30-meter-high wave moving at 50 kilometers per hour when it
reaches shore. (See chapter 3 on tsunamis.)
The risk of fire immediately after an earthquake is often high because of broken electrical lines
and gas mains. In recent years, officials in most of the world’s major cities have installed
devices that shut these services down automatically if an earthquake strikes. Yet the threat still
exists in many of the smaller cities and the squatter settlements of the larger cities where open
fires are used for cooking. 8
Finally, disturbance of the subsoil causes changes in the course of groundwater flows. This can
cause abrupt changes in the level of the water table and sudden drying up of surface springs. 9
Behavior of Buildings during Earthquakes
As the vibrations and waves continue to move through the earth, buildings on the earth’s
surface are set in motion. Each building responds differently, depending on its construction.
When the waves strike, the earth begins to move backward and forward along the same line.
The lower part of a building on the earth’s surface immediately moves with the earth. The upper
portion, however, initially remains at rest; thus the building is stretched out of shape. Gradually
the upper portion tries to catch up with the bottom, but as it does so, the earth moves in the
other direction, causing a “whiplash” effect, speeding up the top of the building and creating a
vibration known as resonance. The resonance can cause structural failure in itself, or adjacent
buildings having different response characteristics because different building materials can
vibrate out of phase and pound each other to pieces. The walls of buildings without adequate
lateral bracing frequently fall outward, leaving the upper floors or roof to collapse into the inside
of the structure. 10 (See Fig. 2.6)
Earthquake Forecasting
The study of regional seismicity and the outline of seismic zones make it possible, within the
framework of historical incidence and global tectonics, to predict the regions in which
earthquakes will occur; the real problem in prediction is to be able to specify in advance the
exact place, day (as precisely as possible) and magnitude of a future earthquake. Only recently
has a strictly scientific approach been applied to the short-term prediction of earthquakes. The
first successes now achieved by certain research workers give reason to hope that such
forecasting will be possible in the fairly near future. It would then be possible, thanks to two
kinds of prediction, to adopt a preventive strategy that might greatly reduce human and material
losses.
Long-term forecasting could be used:
• for determining the optimum structures of existing buildings;
• for encouraging local authorities to issue new regulations on building and land use and, in particular,
for improving the choice of sites for new human settlements;
• for launching campaigns to inform and educate the population on safety rules and general preventive
measures;
• for drawing up relief plans;
• for improving response capabilities.
Short-term predictions, on the other hand, would make it possible:
• to mobilize relief in the event of a disaster;
• to implement procedures for evacuating endangered buildings and dangerous areas (fire risk);
• to shut down certain dangerous industries (nuclear reactors, electric power stations, oil and gas
pipelines, etc.);
• to evacuate low-lying coastal areas liable to be swept by tsunamis