Secular Earth Movements. Earthquakes
 The surface of the earth is undergoing constant change caused by powerful interior forces.
Fig. 18. Coast terraces chiselled by the waves
Inside the earth there is a continuous shifting of masses: the heavier gradually sink downward toward the centre of the earth, the lighter rise upward closer to the surface. These interior movements are reflected on the solid outer covering, or so-called crust of the earth.
Secular Earth Movements. The earth's crust experiences gradual, or "secular", movements — risings and sinkings (secular=lasting centuries).  Along the coasts of the Scandinavian Peninsula, at an altitude of 100—200 metres, we find clearly defined traces of former coastlines—terraces and ramparts of rock chiselled by the waves and masses of
sea shells.
Fig. 19. Earthquake vibrations and how they spread. 0 — the focus of the earthquake.
These bear witness to the rising of the land in that region. This phenomenon is still going on, at the rate of one centimetre a year. Other coastal regions experience sinkings. Evidence of this are the flooded ruins of ancient houses, the extensions of river valleys now under water, etc. Portions of the North Sea coast are gradually sinking (at the rate of 10 cm per hundred years).
 Movements of the earth's crust occur everywhere, as has been proved by precision instruments.
Fig. 20. Houses destroyed by the Messina earthquake.
Some regions of the Donets Hills and the Central Russian Upland in the East European Plain, for example, are gradually rising. Moscow and its neighborhood, on the other hand, are gradually sinking (at the rate of approximately several millimeters a year).
In the course of thousands of years, secular earth movements have greatly changed the shapes of the continents. When land rises and its area increases, islands are sometimes connected with the mainland. When land sinks and is flooded by the sea, the mainland is sometimes divided into parts and new islands are formed. Thus, several thousands of years ago the British Isles were part of the continent. Many regions where there is land today were formerly the bottoms of seas, e. g., the Central Danubian and Lower Danubian Plains.
 Earthquakes. Secular earth movements are so gradual as to be almost imperceptible. Earthquakes, on the other hand, are a terrible phenomenon of nature and sometimes result in great devastation and death.
Earthquakes are the result of a sudden breaking-up and displacement of layers of rock inside the earth at a depth of dozens and hundreds of kilometres.
Fig. 21. The effect of secular earth movements on the Apennine Peninsula.
At the point of origin, or focus of the earthquake, there occurs a shock that causes vibrations to spread in all
directions. These vibrations, or waves, pass through the earth, and upon reaching its crust, cause a sudden lifting and sinking of the ground and sea-floor, which leads to the formation of numerous cracks and fissures.
Earthquakes are most frequent and violent in young mountain areas with deep-sea depressions (so-called 'deeps' and 'trenches' ') near by. In Europe earthquakes occur most often in the young mountain region near the deep Mediterranean Sea.
A very violent earthquake occurred in the south of the Apennine Peninsula and on Sicily in 1908, when entire portions of the coast
A deep is a depression in the sea-floor of limited extent and great depth. A trench is a long narrow steep-sided submarine valley.
disappeared under water. Tremendous waves battered the coast causing enormous damage. Several towns were destroyed (Messina and others, see Fig. 20) and over 100,000 people were killed. |
