Observing earthquakes

Waves caused by earthquakes

There are two types of seismic waves:

  • P-waves that are longitudinal waves. Then, the direction of the impulse vector corresponds to that of the wave expansion. That is why they expand in solid, liquid and gaseous substances.
  • S-waves which are transverse waves, and therefore expand only in solid and no liquid substances.
  • Surface waves-expanding on the surface of the Earth, their energy exponentially diminishing with depth increasing.

On a seismogram - the displacement-time function of an earthquake recorded by instruments -, P(rimary) waves appear first, S(econdary) waves follow.

Primary waves

Primary waves

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.


Secondary waves

Secondary waves

They make the surface of the Earth displace literally.



Vertical surface waves

Surface waves

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:

  • examination of horizontal and vertical displacement of the Earth's surface,
  • observation of alteration in the inclination of the Earth's surface,
  • examination of alteration in the magnetic and gravitational force fields of the Earth,
  • examination of alteration in the velocity of the expansion of seismic waves, the electric conductivity of rocks, and the quantity of liquids and gases within the pores of rocks,
  • the alteration of seismic events (such as the number of the minor fore-shocks),
  • observation of the behaviour of certain animals.

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.
If a superior horizontal and altitudinal grid of points of reference are created in the area in danger and geodetic surveys are regularly repeated, then from these we may draw conclusions on the tensions accumulated in the rocks based on the main direction of displacements of the grid points. In certain areas, deformations can reach a magnitude of meters in the distance of 3040 kilometres within a couple of decades. For example, in the moment the earthquake of San Francisco in 1906 set off, the rocks on the two sides of the Saint Andreas fault line have been displaced from each other to even 3 to 5 metres, which means that the deformations preceding the earthquakes had been created by long years of slow displacement of about the same amount, but into an opposing direction.
The alterations in altitude detected between the points of the superior grid placed in the area south of Tokyo during the earthquake of Kanto in 1923 reached figures of about 80 centimetres and the southern part of the Boso Peninsula rose about 1,5 metres higher.

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.
The most appropriate instruments to the incessant observation of crust deformations are those that measure the inclination and extension of the ground. (Instruments measuring the inclination work on the principles of either the horizontal pendulum or of the communicating vessels; those measuring the extension, on the other hand, have a version called strain-meter, widely known as the seismograph.)
So the growth in the elastic tensions accumulated in the rocks can obviously be detected by measuring the altitude, inclination, extension or reduction of the surface of the Earth, which detections are of a decisive importance concerning forecast.

Additional physical phenomena

Yet, besides all these, there may be additional physical phenomena to help getting reliable forecast:
In the pores of rocks, there can liquid and gas or, better to say, the mixture of these be found. Rocks deform on the growth of tensions: rocks are pressed together or they extend, which press liquid and gas out of or into the pores of the rocks. When tensions grow further (even in the case when this process does not cumulate in the rock in concern to break alongside a greater piece of the surface), micro-crevasses develop into which liquid or gas can stream from the pores. This changes the conductivity of and the velocity of the waves expanding in the rock concern.

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:

  • displacements on the surface
  • velocity of the expansion of elastic waves
  • number of seismic events
  • emission of radon gas
  • electric resistance of rocks

Both models divide the proceedings of the quakes into five phases:
The first one is the introductory phase, when elastic tensions develop and accumulate in the rocks.
The second is the phase in which the inelastic deformation of the rocks start and more and more micro-crevasses develop in a cascade-like manner.
In the third one, uncertain deformations crop up and in certain areas, partial relaxation of tensions start.
Changes that may be suitable for an earthquake forecast happen during the second and third phases, so these to together can be regarded at as the phases of the events for forecast.
The fourth phase is the setting off of the earthquake.
The fifth phase is that of the "dying down" in which the remainders of the greater tensions dissolve fast during the minor aftershocks.

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.)
Unfortunately, the matter of forecasting earthquakes could get to the agenda, that is, could be seriously dealt with only in the last one or two decades as scientific and financial circumstances which made it possible to start researches into this direction arose only lately (and even those only in developed countries endangered by earthquakes).

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.

The 3 dimensional map of earthquake risks in the United States
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.



Measuring earthquakes

Chang Heng's earthquake measuring instrument

Tell-tale toads

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.



The seismograph

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:

log E = 1,5 M + 4,8 [Nm]

Seismograph, the device detecting and recording vibrations, has two basic types: the one is sensitive to horizontal vibrations, the other to vertical ones. In both ones, a bigger kind of a weight is applied to a frame by using a cord or a spring. When the ground vibrates, the weight remains at the same place because of its inertia, yet its surroundings start vibrating, together with the frame. The pen adjusted to the weight draws zigzag signals onto the tape of paper adjusted to the rotating cylinder. This so-called seismogram makes scientists able to compute when and where a quake sets off.

Horizontal seismographVertical seismograph

Two types of the seismograph can be seen in the two pictures (Horizontal: to the left, Vertical: on the top)

Experiences

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.
From the studies the geologists reason, that the Earth's central part is the core in liquid state, because the S-waves don't travel in it. With such a method it was discovered, that our Earth is a so-called belted structured planet.

More earthquake

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.

Transamerica Pyramid in San Francisco (Click to enlarge)




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:

  • while the quake is on, on should not leave the building or try to enter one, since most injuries originate from such actions (just think about chimneys that incline, mortar falling off the walls, and scrap glass)
  • One is to get as far as possible from windows, glasses, bookshelves and objects that may cause injuries when falling off. If possible, one should find shelter under a table or standing into a doorframe. In the open air, one should get as far as possible from built structures, trees, or electric lines.
  • following the quake, one is to close gas or electric devices and prepare oneselft to aftershocks that may follow. Buildings and structures heavily damages need to be left as soon as possible.

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

The mourning of Iranian 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.