Natural Disasters: Tsunamis, Earthquakes and Volcanoes Explained

Natural and Technological Catastrophes

Natural disasters such as tsunamis and earthquakes are catastrophic events that originate from natural phenomena, unlike technological catastrophes caused by human activities, accidents, etc. An example of a technological catastrophe is the Bhopal gas disaster, which caused the first major toxic release of its kind.

A natural phenomenon is not in itself a risk. It becomes a risk when it is likely to produce harm to the population in an area. The value of the risk depends on three factors:

• Hazard: the potential for the event to occur and its frequency.
• Exposure: the people and goods that may be affected.
• Vulnerability: the degree to which a community is damaged.

The magnitude of a risk also depends on human decisions, such as where the population settles. Thus, two earthquakes of the same magnitude can cause very different damage depending on the town where they occur, and on whether buildings were constructed to withstand them. Reduced vulnerability lessens the catastrophic effects.

Statistical predictions, based on data about how frequently a phenomenon occurs, indicate what can happen. They allow prevention measures to be adopted in order to minimize its effects.

Earthquakes

Earthquakes are sudden vibrations of the ground produced by the release of energy accumulated in rocks under stress. When the rocks fracture, they are displaced along faults. The point of origin inside the Earth is the seismic focus, and seismic waves travel from the focus in all directions. The point on the surface directly above the focus is the epicenter. Earthquakes can occur at depths of up to 700 km.

Earthquakes are measured with instruments called seismographs, which draw graphs known as seismograms. The magnitude of an earthquake is a measure of the energy released, expressed on the Richter scale, where each degree corresponds to about 32 times the energy of the previous one. The largest recorded earthquake was 9.5 in 1960.

Around 100,000 earthquakes occur worldwide each day, most of them of small magnitude. Those that are felt are usually frequent and last from 1 to 3 seconds to a minute. Cities located in areas of seismic activity are at risk.

Plate Tectonics: The Lithosphere Creates and Destroys

Hot materials in the Earth's interior move, causing the lithospheric plates to shift. Magma rises at oceanic ridges, where new lithosphere is created and the plates move apart, while at subduction zones, plates converge and one descends beneath the other, destroying lithosphere. Subduction zones are the regions of greatest seismicity. At transform faults, plates slide horizontally past one another without creating or destroying lithosphere, producing earthquakes.

Whether a location is at a plate boundary or not, the local geological characteristics of the area determine its seismic risk.

Earthquake Prevention

Seismic prevention is based on studying the history of earthquakes in a region, which may make it possible to forecast events several months ahead. Preventive measures include mapping risk zones, establishing rules for earthquake-resistant construction, limiting building heights, and using elastic and resistant materials.

Tsunamis

A tsunami is an enormous set of waves caused by the sudden displacement of a large volume of water, usually in the ocean. Earthquakes with their epicenter under the sea are the most common cause, although tsunamis can also be generated by underwater volcanic explosions or by landslides and meteorite impacts.

How a Tsunami Forms

1. A subduction zone causes one plate to slide beneath another. The overriding plate becomes locked and is dragged down, accumulating tension.
2. When the locked plate suddenly releases that tension, a large earthquake occurs and the seafloor is displaced upward or downward.
3. This displaces an enormous volume of water, generating waves that propagate in all directions, sometimes across distances of 700 km.
4. As the waves approach the shore, friction with the seabed slows them down, their height increases, and the "stacking effect" causes the water to build up.
5. The wave eventually collapses and makes landfall, flooding coastal areas and buildings.

The risk from tsunamis is low in many places, but when they occur they can be devastating. Scientists stress the need for a warning network to help save lives.

Volcanoes

Volcanic activity is an important natural hazard. There are two main types of eruption:

• Effusive: slightly viscous lava flows, gases escape easily, and there are few explosions and pyroclasts.
• Explosive: very viscous lava blocks the conduits, gases accumulate and cause explosive eruptions that eject pyroclasts into the volcanic area.

The danger of volcanoes comes from toxic gas emissions, incandescent clouds (pyroclastic flows), explosions, and lava flows.

Signs of an Eruption

• Small local earthquakes caused by the movement of magma.
• Changes in the inclination of the ground.
• Changes in temperature and water levels in wells.
• An increase in gas emissions.

Monitoring systems are essential for detecting these signs and warning the population when an eruption is likely.