Waves caused by earthquakes
There are two types of seismic waves:
On a seismogram - the displacement-time function of an earthquake recorded by instruments -, P(rimary) waves appear first, S(econdary) waves follow.
The fastest waves originating from the centrum of the quake are the so-called primary waves. When expanding, they periodically extend and press together the crust of the Earth.
They make the surface of the Earth displace literally.
These waves, expanding on the surface of the Earth are the most destructive. They make the surface of the Earth displace periodically vertically, as with the waves of the seas in storm.
Forecast on earthquakes
Though there are a number of researchers worldwide working on the realization of a reliable forecast, no breakthrough has been accomplished in the field so far, and it appears more and more presumable that complete solution will not be achieved in the near future, either. One reason for it is that in order to be able to get computations on the movement of the blocks of rocks, one should know exactly the tensions deep down. We can, though, measure these and many other parameters necessary to the predictions only on the surface with appropriate precision. As science stands at present, we can, at most, estimate the probable occurrence of a certain quake within a certain period of time in the area in question.
Our experiences say that if for example, a level 6 earthquake sets off on a certain scene of the earth, then around the same scene, quakes of around the same size are to be expected in a certain period of time. As this regularity cannot be regarded at as a prediction, today we still do not have a satisfying method for quake forecasting. We shall have to predict nothing less than the exact scene to the earthquake we expect to set off, the starting point of the quake in time (precisity should extend to at least the day, one or two days be within the margin of error) and, finally, the strength of the quake. Should we fail in even one figure, the method cannot be regarded at as a prediction.
Physical changes before earthquakes
According to observations carried out so far, earthquakes are generally preceded by different physical phenomena which bring along with them lesser or greater alterations in several physical parameters. Researchers relating to this are quite extensive and are performed in most diverse fields. Some of the more important ones are:
All phenomena are connected to diverse deformations of rocks that in most cases, precede earthquakes. Such deformations are mostly measurable even directly on the surface.
Similar alterations were detected in Hungary during the earthquake of Dunaharaszti in 1956.
Unfortunately, basic shortcomings of geodetic surveys are, concerning forecast of earthquakes, that these surveys cannot be repeated in any short periods can give reliable information on exact figures of crust deformations.
Additional physical phenomena
Yet, besides all these, there may be additional physical phenomena to help getting reliable forecast:
Lately, there were some important results born even in the field of complex examinations: Soviet and American scientists have, based on theoretic consideration, elaborated one model to forecast, each on a piece. The so-called dilatation-diffusion model (worked out by the American researchers) and the dilatation-instability model by the Soviets. Both give the alteration of the more important physical parameters during the development of earthquakes in ways that more or less diverge from the other:
Both models divide the proceedings of the quakes into five phases:
Apart from the phenomena listed above, there are examinations of several other phenomena and circumstances in progress. (For example, in Hungary and areas the like of Hungary in their seismicity, finding and studying of fault lines are intensively occupied with.)
Seismicity - seismo-tectonics
Map delineation gives a clear-cut overview on seismicity. Usually epicentres of the quakes, their distribution, frequency and the isoseists. A map made so can obviously indicate only the seismic condition of the past, and extrapolation is insecure. One can get to more reliable data if we take geological conditions (Location of break lines, displacements of the crust and its velocity etc.) into consideration as well. Seismotectonic maps gained from knowledge on the connection between earthquake zones and geologic structures can even be applied to pragmatic uses, such as choosing the location of a facility concerning its being exposed to the dangers of earthquakes.
Risk of earthquakes in the United States.
Most Americans believe that only people living in the northern parts of the United States, mainly California, have to take the risk of earthquakes into consideration. Computer-generated maps, though, claim that the chance to setting off of a grave earthquake cannot be neglected either in the eastern parts of the country, eithers. The map points out the probability or earthquakes occurring using colours and features of the ground. Risk is relatively low an dark green terrains while red peaks mark the terrains of grave danger.
Chinese scientist Chang Heng invented the first instrument suitable for detecting earthquakes in 132. Twelve dragonheads ornamented the side of the vase standing in the middle, around which there sat twelve toads. The dragons held balls of bronze in their mouths. When an earthquake occurred, one dragon dropped the ball into the mouth of the toad sitting beneath, the knocking of which was the signal warning for the earthquake. The instrument also indicated the direction of the quake, as it was always the toad opposite to the epicentre that caught the ball.
An instrument indicating earthquakes that registrates the magnitude of the displacement of the ground in the function of time. Signals used to be written on tapes of paper, yet nowadays they have almost invariably been ousted by digital recording and computerized data processing. It helps in differentiating between S and P waves. Modern devices forecasting earthquakes can even detect displacements of 6-10 millimetres. Their spreading makes it able to compute kinetic energy in the focus. The dimensionless number (size or magnitude) can be defined using the data form the instrument. If the size of the earthquake is given in Richter magnitude (M), the amount of the energy released can be computed with the following formula:
Two types of the seismograph can be seen in the two pictures (Horizontal: to the left, Vertical: on the top)
Earthquakes really help scientists, to gain information about the Earth's internal structure. The reason for this is, that the quake-waves starting from one point of the earth's crust get through the inner layers to an other surface point of the Earth.
The statistics show, that the number of earthquakes hasn't increased. In the formation of that, that there are more earthquakes, probably takes part the information revolution of nowadays, namely there is much more advice got to people about the remote countries' destructive earthquakes.
Protection against the earthquakes
There are two possible protections against the seismic quakes, which are called active and passive control by the trade. In the countries, which are especially endangered, passive protection was applied: rubber-springs with a diameter of about 70 centimetres hold buildings, which have been vulcanized with steel-sheets. These are measured out for enormous load; they don't break up in case of displacement of 45 degrees, and support swing of 35 centimetres.
The most modern protection is the combination of satellite-based quake prediction and special foundations. This is used in America, Japan and Canada. Sensors under the surface of the earth in 8-10 kilometres depth are built in along the crust's fault and percieve the accumulation of energy. If these notice emission of energy, these automatically take fright at the computational system of the house. There are pistons moving on revolving discs built in several levels of the building, which are capable of moving the building in the opposite direction with the waves by coming into the earthquake's direction. For example the Citycorp Center in New York has foundations like this.
The buildings of the modern San Francisco, like the pyramid-like Transamerica Pyramid are resist earthquakes. The building had been drafted so, that the excursion of that be only 0,6 metres, when the earth is quaking. The traditional buildings, which have exactly the same height, have the excursion of 0,9 metres.
What to do when an earthquake occurs
Some advice worth considering:
Animals sense the coming of earthquakes
Many people survived the earthquake in Algeria because the animals had warned their masters to the quake motion that would demolish whole buildings. Several families living on Mount Zbarbar (known for its apiculture), which lies hardly 70 kilometres east of the city of Algiers, owe their lives only to the bees warning them. The mountain houses had been filled with stirred-up swarms of bees chasing the inhabitants out of their homes under the open skies on 21 May at around sunset-time, only minutes before the devastating quakes destroyed the buildings. According to the interpretation of a survivor at around the age of Metushelah, the bees carried out a divine mission by warning the villagers.
A shepherd owes his life to his sheep, as he, living on a farm 20 kilometres outside Algiers, came out of his house because of the hysteric frenzy performed by the sheep in the pen. The man thought that either a dog or a cat had raised panic among his animals that kept on attacking the walls of the structure as if wanting do demolish them. When the herdsman opened the doors of the sheep-pen, the livestock he used to look after stormed out of the structure, sweeping away their master, too, as a result of which they all escaped right before both the farm house and the pen collapsed.
On the coast at Algiers, home to the most violent quakes, 10 dwellers of a three-storey villa were crushed by the building going down like ninepins, leaving only a teen-aged boy who had lived on the top floor. His dog had broken into his room with unbearable barking and snarling and called his master out by running out and returning several times to the outdoors. They managed to get out of the building only a couple of moments before it collapsed.
Dozens of like stories evidence that animals sensed approaching disasters before they actual occurrence sooner than did humans. Some claim to have a clear recollection, after the evening finished, that the otherwise shy cockroaches crept forth out of slits on the walls in columns extending farther than the eye can reach in a has outwards minutes before the earthquakes that turned buildings upside down at a dreadful speed. And it was only later that kids playing in the streets realized why packs of stray dogs galloped away into directions unknown but far away from the houses.
In China a whole institute specialises in observing unusual animal behaviour. At least one man's life had been saved by a fish which sprang several times out of the aquarium on the table preceding the earthquake of 1976 near Tangshan. The owner found this behavior suspicious and went out to take a walk in the nearby park. He could tell his story because he survived.
The effects of earthquakes on people
There are such places on Earth, where the crustal plates slide towards each other, especially where many earthquakes occur. Such regions are Japan, San Francisco's surroundings, Mexico etc. In the people's intellectual world, there, in the myths this phenomenon of nature takes up expressive room. Moreover, a very big proportion of earthquake survivors require psychological treatment after, surely the experienced horror, loss of relatives or acquaintances result in sharp pain all through their life.
According to Islamic tradition, many people mourn by striking their faces and bodies.