Earthquakes are sudden movements orvibrations in the earth's crust. They cause the ground to shake violently, making the walls of buildings crack o r bulge or even tumble down. Whole have been destroyed, sometimes with great loss of life, and at times deep faults have appeared on the surface with the rocks on either side being displaced horizontally or vertically.
Earthquakes are caused by:
One tectonic plate sliding over or past another plate along the line of a fault ; Volcanic eruptions- the movement of molten rock below, or on to the earth's surface, which in turn is caused by the movement of plates.
Occurrence of earthquakes
The main types of regions where they occur are: the mid-ocean ridges; the ocean trenches (deeps) and volcanic islands; regions of crustal compression. Major earthquakes are caused by the movement of tectonic plates, e. g. the North American and the San Andreas Fault, in California and similar faults which extend northward into Alaska and the Aleutians
The nature of earthquakes
• The point at which an earthquakes originates is called the focus and sometimes it is several k ilomet r es below the surface. The point on the E art h 's surface immediately above the focus is called the epicentre. This is where the shock waves first hit the surface.
There are types of shock waves
• Body waves • These travel through the crust and are of two types: a) Primary waves which cause the crustal rock to move back and forth in the direction of wave movement
b) Secondary waves which cause the crustal rock to move from side to side, that is, at right angles to the direction of wave movement
• Surface waves • This travel through the surface rocks and are of two types: a) Love waves which cause the surface rocks to move from side to side, at right angles to the direction of wave movement
b) Rayleigh waves which cause the surface rocks to have a vertical circular movement very similar to that of water in a sea wave
|1.||Non perceptible quake||Detectable only with a seismograph.|
|2.||Very weak quake||Felt only indoors, by very sensitive persons chiefly in flats on upper floors.|
|3.||Weak quake||More people feel it in houses, but still not in the open. It is usually similar to the effects of a moving vehicle.|
|4.||Moderate quake||In the daytime, many people feel it indoors, but only a few outdoors. At night certain people wake up. Glasses and plates rattle. Furniture, doors and windows quiver. Walls creak. Stationary cars sway. Its effect is similar to that when a building is shaken by a heavy lorry.|
|5.||Fairly strong quake||Almost everybody feels it. It can also be felt in the open air. Many sleeping people wake up. Hanging objects start to move. Clocks with pendulums may stop. The glass in windows and doors break. Certain objects fall over. Trees shake.|
|6.||Strong quake||Everybody feels it. Many get frightened and run out of the house. Pieces of heavy furniture move from th eir places. Some chimneys collapse.|
|7.||Very strong quake||Everybody is frightened and runs of the houses. Minor damage to well-built houses, more serious damage to poorly- built ones. Many chimneys fall down. Drivers become aware of quake while driving.|
|8.||Fairly destructive quake||About a quarter of the houses are seriously damaged. Several of them collapse: most of them become uninhabitable. Chimneys of houses fall down: industrial chimneys topple, monuments collapse and are displaced. Muddy water is pressed out of wet soil. The quake disturbs drivers.|
|9.||Destructive quake||Half of the houses are seriously damaged. Many of them collapse: most of them become uninhabitable. Considerable cracks are formed in the ground. Underground cables break.|
|10.||Strongly destructive quake||Three-quarters of the buildings are seriously damaged. Most of them collapse. Even well-built houses are seriously damaged. Considerable cracks and landslides occur.|
|11.||Catastrophic quake||All of the stone houses collapse. Bridges fall down. Powerlines are destroyed completely. Rails are bent.|
|12.||Totally catastrophic quake.||All human constructions are ruined. Earthquake waves become observable on the surface. Certain objects lying on the ground fly into the air.|
The strongest earthquakes recorded in Hungary (grade 10.) took place in Komárom in 1763. 279 houses collapsed and 785 were seriously damaged. 63 persons were killed and 102 injured. q In the early morning of August 15, 1985, several strong earthquakes shook several regions in Transdanubia.The centre of the quakes was iv the vicinity of Berhida and Peremarton, where a number of houses and public buildings were damaged, and chimneys fell down. The quakes caused damage on the shores of Lake Balaton, and were also felt in Budapest. The first quake was observed at 6.13 a.m. The main quake (grade 5 on the Richter scale, and grade 7. on the Mercalli-Sieberg scale) took place at 6.28 a.m. The earthquake series was on average of grade 3.5 on the Richter scale, it lasted till the end of August. However, post-quake vibrations (aftershocks) were observed later too. q Early in the morning of September 19, 1985, an earthquake destroyed a part of Mexico-City. About 8000 people were killed in this catastrophe. The earthquake was of grade 8.1 on the Richter scale. Its energy was many thousand times bigger than that of the Berhida one. q It has been estimated that more than 14 million people have died as a result of earthquakes in the last 400 years.