Understanding Earthquakes and Tsunamis: Risks and Prevention
Understanding Earthquakes and Tsunamis
Tsunamis and earthquakes are natural catastrophes, unlike technological disasters caused by human activities, such as the Bhopal gas leak that caused a toxic cloud.
Natural Risk Factors
Natural risk is the likelihood of damage occurring to the population of an area, driven by natural events. A natural phenomenon is not a risk in itself but becomes a threat to humans.
The value reaching the hazard depends on:
- Hazards that can potentially cause an event and its frequency.
- Exposure of goods and people that may be affected.
- Vulnerability to being damaged.
The risk reaches a magnitude that also depends on human decisions that determine where the population is located. Two earthquakes of the same magnitude would cause more damage in a populated place. Also, better-constructed buildings will be less vulnerable to catastrophic effects.
To reduce risk, predictions are made based on statistical data that indicate the frequency of the phenomenon. Measures of prevention are proposed, such as adopting measures necessary for the event to have minimal effects.
Earthquake Risk
Earthquakes are ground vibrations caused by the abrupt release of accumulated energy in rocks under stress. They originate when great masses of rock fracture or break at faults. The fractures in which the earthquake originates is the seismic focus, where vibrations or seismic waves are transmitted in all directions. This focus is placed inside the Earth, up to 700km deep. The point on the surface nearest to the focus is the epicenter.
Measuring Earthquakes
Instruments called seismographs are used to draw graphs of an earthquake, called seismograms. Magnitude, the amount of energy released, is measured with the Richter scale, in which each level corresponds to 32 times the energy released by the previous level (e.g., the higher earthquake in Chile in 1960 measured 9.5).
There are hundreds of thousands of earthquakes per day throughout the world, although most are of less magnitude. Less hazardous earthquakes usually last from 1 to 3 minutes.
The most seismically active places are located on boundaries between lithospheric plates. Seismic risk is assessed in light of:
- Whether the area is on a plate boundary.
- Local geological features.
- Seismic history of the zone.
Earthquake Prevention
Seismic effects can be reduced by taking action:
- Develop risk maps to establish preventive rules.
- Implement standards of earthquake-resistant construction.
- Limit building height and make structures tough and elastic.
Tsunamis
A tsunami is a huge set of waves caused by the abrupt rising of a large mass of water at the bottom of the sea. They are often due to earthquakes with an epicenter beneath the sea, although sometimes they are generated by submarine volcanic explosions or meteorite impacts.
Origin of Tsunamis:
- Subduction Zone: An oceanic lithospheric plate is introduced into the Earth. The upper end of the plate gets stuck, is washed out, and accumulates tension.
- Unlocking of Tension: The abrupt release of tension generated by the earthquake causes the sea floor to rise (rebound). As a result, a large volume of ocean water is displaced upwards.
- Wave Propagation: Generated waves propagate in all directions at up to 700km/h. The volume of water is enormous.
- Shallowing Waters: As the wave nears the shore, friction with the bottom slows the wave, producing the “crowding effect.”
- Inundation: The rising wave enters the land.