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Translations of Encyclopedia about Geology

 

Occurrence and Prediction

Earthquakes originate always in the interior of the earth. As was mentioned before, the starting point of an earthquake is the hypocentre, while the point vertically above the hypocentre on the earth's surface is the epicentre.

Viewed globally, the distribution of earthquakes is very uneven. More than half of all earthquakes occurs around the Pacific Ocean, 30 percent in the tropical sea regions. Many earthquake hypocentres are found also in the region of the Mid-Ocean Ridge.

In Europe, shocks occurs mainly in the Alpine region countries, in the Balkans, and on the Apennine islands. Slight shocks are recorded also in certain regions of Germany, for example, in the Black Forest (Schwarzwald), the upper Rhine basin, the Lower Rhine region, and in the Schwabian Jura.

Old continental cores and ocean floors are not primary seismically active regions, the young mountain ranges and steep slopes of coastal lines are. We distinguish between flat plains and basins. When an earthquake hypocentre is located 70 kilometres below the surface, in the case of basins this figure is 700 kilometres into the earth's interior. Basins occur mainly in the Pacific region, while in the region of the Mediterranean they are rather scarce.

The seismographs register almost one million shocks every year, the majority of which is not noticed by general population. Roughly 100 earthquakes reach an intensity between 6 and 7 on the Richter scale.

An earthquake measuring over 8 on Richter scale (M) occurs once every 5 to 10 years. Fortunately, most earthquakes are smaller. These damage buildings near the epicentre. When an earthquake over 8 on Richter scale occurs, however, there is total destruction and many people are killed.

Earthquakes may cause numerous damage and destruction. Ocean earthquakes near the coast often produce gigantic flood tides called tsunami. The seismic waves travel at great speed across the ocean without losing its intensity. When they near a coast, they produce waves of up to 20 metres, which devastate the coastal regions.

An earthquake may set off an avalanche. When gas lines and high-voltage electric wires are destroyed, this often leads to dangerous fires, which magnifies the range of the catastrophe. For example, in 1923, out of a total of 99.000 victims, 38.000 were killed by fire.

Underground motion during an earthquake is the most damaging. As a consequence, entire buildings and other structures may collapse. Roads and bridges are demolished, because strong seismic waves may cause certain types of soil to behave as liquids, and as such these sweep away everything in their path. In the close proximity to the epicentre the movements of the ground may be so strong, that objects fly in the air.

What can be done to minimize damages caused by an earthquake? Tsunamis pose a great danger for the Pacific coast; they occur on the Atlantic coast also, but on a smaller scale. Ocean earthquakes are very quickly recorded by the Pacific Tsunami Warning Centre. There, their probable intensity is quickly calculated and a warning is sent immediately to the countries, which may be at risk. This warning means, that there are a few hours in which to evacuate the population. Another way is to build special dikes providing protection against these waves, but this has been implemented only in Japan.

An important tool for predicting earthquakes is the preparation of special maps indicating risk areas or areas where it has already occurred. In those regions, certain construction codes for constructing roads as well as buildings must be complied with in order to minimize potential damages caused by an earthquake.

In certain areas, where there is a risk of avalanches, or in seismically active regions such as San andreas in California, for example, the possibilities of new construction are limited. Homeowners in the risk area should have their houses firmly anchored in the ground. Rescue units must be organised from among the population, and provision and rescue plans should minimize resulting damages.

In recent years, seismologists have been working intensively on earthquake predictions. Scientists in various countries collect more and more information concerning the occurrence of earthquakes.

In the last few years, there have been six correct predictions of strong earthquakes in the regions of plate boundaries. This was done using the seismic gap method. It means the following: in principle, an earthquake is the release of energy generated by the stress produced by the movement of the plates. It is a continuous process. Based on precise calculations and in-depth study, it is possible to estimate when the time is "ripe" for a new earthquake. For example, the probability of an earthquake in southern California, the San andreas region, has been calculated for the next thirty years. At the initiative of the US government, a special programme has been developed, which has as its goal obtaining the most exact earthquake predictions in order to ensure sufficient protection of the population.

Great catastrophic earthquakes occur often in Japan. Records of the past 2000 years refer to repeated catastrophes. Japan is therefore concentrating its efforts on preparing a prediction programme which would help minimise the damages caused by earthquakes.

In other parts of the world, authorities often neglect this issue, stating that there is nothing that could be done. In the meantime it has been discovered, however, that slight foreshocks may be an indication that a great earthquake will follow. In those cases, animal behaviour suddenly changes, well-water levels change, and physical properties of minerals in the seismic zone also change. However, it is not possible to combine all these signs in such a way so as to be able to state that we can really predict earthquake.

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